# The 450 days X-ray monitoring of the changing-look AGN 1ES 1927+654

C. Ricci11affiliation: Núcleo de Astronomía de la Facultad de Ingeniería,
Universidad Diego Portales, Av. Ejército Libertador 441, Santiago, Chile
22affiliation: Kavli Institute for Astronomy and Astrophysics, Peking
University, Beijing 100871, China **affiliation<EMAIL_ADDRESS>,
M. Loewenstein33affiliation: Astrophysics Science Division, NASA Goddard Space
Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA 44affiliation:
Department of Astronomy, University of Maryland, College Park, MD 20742, USA ,
E. Kara55affiliation: MIT Kavli Institute for Astrophysics and Space Research,
70 Vassar Street, Cambridge, MA 02139, USA , R. Remillard55affiliation: MIT
Kavli Institute for Astrophysics and Space Research, 70 Vassar Street,
Cambridge, MA 02139, USA , B. Trakhtenbrot66affiliation: School of Physics and
Astronomy, Tel Aviv University, Tel Aviv 69978, Israel , I.
Arcavi66affiliation: School of Physics and Astronomy, Tel Aviv University, Tel
Aviv 69978, Israel 77affiliation: CIFAR Azrieli Global Scholars program,
CIFAR, Toronto, Canada , K. C. Gendreau33affiliation: Astrophysics Science
Division, NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD
20771, USA , Z. Arzoumanian33affiliation: Astrophysics Science Division, NASA
Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA ,
A. C. Fabian88affiliation: Institute of Astronomy, University of Cambridge,
Madingley Road, CB3 0HA Cambridge, UK , R. Li22affiliation: Kavli Institute
for Astronomy and Astrophysics, Peking University, Beijing 100871, China , L.
C. Ho22affiliation: Kavli Institute for Astronomy and Astrophysics, Peking
University, Beijing 100871, China 99affiliation: Department of Astronomy,
School of Physics, Peking University, Beijing 100871, China , C. L.
MacLeod1010affiliation: Center for Astrophysics, Harvard & Smithsonian, 60
Garden Street, Cambridge, MA 02138-1516, USA , E. Cackett1111affiliation:
Department of Physics & Astronomy, Wayne State University, 666 West Hancock
Street, Detroit, MI 48201, USA , D. Altamirano1212affiliation: Department of
Physics & Astronomy, University of Southampton, Southampton, Hampshire S017
1BJ, UK , P. Gandhi1212affiliation: Department of Physics & Astronomy,
University of Southampton, Southampton, Hampshire S017 1BJ, UK , P.
Kosec88affiliation: Institute of Astronomy, University of Cambridge, Madingley
Road, CB3 0HA Cambridge, UK 55affiliation: MIT Kavli Institute for
Astrophysics and Space Research, 70 Vassar Street, Cambridge, MA 02139, USA ,
D. Pasham55affiliation: MIT Kavli Institute for Astrophysics and Space
Research, 70 Vassar Street, Cambridge, MA 02139, USA , J.
Steiner55affiliation: MIT Kavli Institute for Astrophysics and Space Research,
70 Vassar Street, Cambridge, MA 02139, USA , C.-H. Chan1313affiliation: Racah
Institute of Physics, Hebrew University of Jerusalem, Jerusalem 91904, Israel
66affiliation: School of Physics and Astronomy, Tel Aviv University, Tel Aviv
69978, Israel

###### Abstract

1ES 1927+654 is a nearby active galactic nucleus (AGN) which underwent a
changing-look event in early 2018, developing prominent broad Balmer lines
which were absent in previous observations. We have followed up this object in
the X-rays with an ongoing campaign that started in May 2018, and that
includes 265 NICER (for a total of 678 ks) and 14 Swift/XRT (26 ks)
observations, as well as three simultaneous XMM-Newton/NuSTAR (158/169 ks)
exposures. In the X-rays, 1ES 1927+654 shows a behaviour unlike any previously
known AGN. The source is extremely variable both in spectral shape and flux,
and does not show any correlation between X-ray and UV flux on timescales of
hours or weeks/months. After the outburst the power-law component almost
completely disappeared, and the source showed an extremely soft continuum
dominated by a blackbody component. The temperature of the blackbody increases
with the luminosity, going from $kT\sim 80$ eV (for a 0.3–2 keV luminosity of
$L_{0.3-2}\sim 10^{41.5}\rm\,erg\,s^{-1}$) to $\sim 200$ eV (for
$L_{0.3-2}\sim 10^{44}\rm\,erg\,s^{-1}$). The spectra show evidence of ionized
outflows, and of a prominent feature at $\sim 1$ keV, which can be reproduced
by a broad emission line. The unique characteristics of 1ES 1927+654 in the
X-ray band suggest that it belongs to a new type of changing-look AGN. Future
X-ray surveys might detect several more objects with similar properties.

###### Subject headings:

galaxies: active — galaxies: evolution — quasars: general — quasars:
individual (1ES 1927+654)

##  1\. Introduction

Accretion onto supermassive black holes (SMBHs) is thought to play an
important role in the evolution of galaxies (e.g., Kormendy & Ho 2013), and to
be the mechanism allowing the SMBHs we observe in most massive galaxies (e.g.,
Magorrian et al. 1998) to reach masses of $M_{\rm BH}\sim
10^{6}-10^{10}M_{\odot}$. While most active galactic nuclei (AGN) in the local
Universe accrete at low values of the Eddington ratio111$\lambda_{\rm
Edd}=L\sigma_{\rm T}/4\pi GM_{\rm BH}m_{\rm p}c$, where $L$ is the source
bolometric luminosity, $\sigma_{\rm T}$ is the Thomson cross-section, G is the
Gravitational constant, $m_{\rm p}$ is the proton mass, $c$ is the speed of
light. ($\lambda_{\rm Edd}\lesssim 0.3$, e.g., Koss et al. 2017), periods of
very rapid accretion, possibly super-Eddington, are expected to be extremely
important to explain the growth of the first SMBHs (e.g., Trakhtenbrot et al.
2011; Volonteri 2012), and the vast majority of massive black holes might in
fact gain most of their mass through these episodes (King 2003).

Several aspects of the accretion process are still highly debated such as, for
example, the main mechanisms triggering accretion, and why some galaxies host
AGN while others do not (e.g., Alexander & Hickox 2012). Moreover some AGN
have been found to show very strong variability (e.g., Rumbaugh et al. 2018;
Trakhtenbrot et al. 2019b; Timlin et al. 2020), which could be associated with
changes in the inflow of matter or with instabilities of the accretion disk
(e.g., Kawaguchi et al. 1998). AGN variability in the optical/UV and X-rays
has been directly observed over a very wide range of time-scales (e.g.,
Sartori et al. 2018), ranging from minutes and hours (e.g., McHardy et al.
2014) up to several years (e.g., MacLeod et al. 2010). One of the most
interesting aspects of this strongly variable behaviour is associated with the
so-called changing-look AGN. Those are sources in which the optical/UV broad
emission lines, produced in the broad-line region (BLR), appear or disappear
(e.g., LaMassa et al. 2015; Ruan et al. 2016; Lawrence 2018). These objects
are different from the X-ray changing-look AGN, which display strong variation
of the line-of-sight column density ($N_{\rm H}$, e.g., Risaliti et al. 2009;
Bianchi et al. 2012a; Marinucci et al. 2016; Ricci et al. 2016), possibly
related to moving clouds in the broad-line region (e.g., Maiolino et al. 2010)
or in the torus (e.g., Markowitz et al. 2014; Buchner et al. 2019).

So far about 20-30 changing-look AGN have been discovered (e.g., McElroy et
al. 2016; Krumpe et al. 2017; Parker et al. 2016; Yang et al. 2018; Katebi et
al. 2019; Oknyansky et al. 2019; MacLeod et al. 2019), but the physical
mechanism behind these events is still poorly understood. Optical changing-
look AGN cannot be explained by variable obscuration, since the broad-line
region is too large to be obscured by the clouds of the dusty torus, except
maybe for a narrow range of inclination angles. Moreover, these objects do not
typically show any sign of obscuration in their X-ray spectra after
transitioning from type 1 to type 2 (e.g., Denney et al. 2014). It has been
argued that the appearance (disappearance) of the broad lines could be due to
an increase (decrease) of the accretion rate (e.g., Elitzur et al. 2014),
possibly related to instabilities in the accretion flow (e.g., Ross et al.
2018; Stern et al. 2018). In a few objects it has been proposed that the
increase in the AGN luminosity could be due to tidal disruption events (TDEs;
e.g., Merloni et al. 2015, Komossa 2015 and references therein).

1ES 1927+654 is the first accreting SMBH that was detected in the act of
changing phase (Trakhtenbrot et al. 2019a). This object is a rather well known
nearby ($z=0.019422$) AGN, showing peculiar characteristics, completely
unexpected within the framework of the AGN unification model (e.g., Antonucci
1993; Urry & Padovani 1995; Netzer 2015; Ramos Almeida & Ricci 2017; Hickox &
Alexander 2018). While its optical spectrum suggested a type-2 classification
(Bauer et al. 2000), in the X-rays the AGN appears completely unobscured
(i.e., $N_{\rm H}\simeq 10^{20}\rm\,cm^{-2}$; Boller et al. 2003b; Gallo et
al. 2013), making it a member of the true type 2 class (e.g., Panessa &
Bassani 2002; Brightman & Nandra 2008; Shi et al. 2010; Bianchi et al. 2012b).
Consistent with this idea, Tran et al. (2011) did not find any evidence of
hidden polarized broad H$\alpha$ or direct broad Pa$\beta$ and Br$\gamma$
lines. 1ES 1927+654 was found to have increased its flux by at least two
magnitudes in the optical V-band in March 2018 (AT2018zf/ASASSN-18el, Nicholls
et al. 2018; Stanek 2018) by the All-Sky Automated Survey for Supernovae
(ASAS-SN; Shappee et al. 2014). From the analysis of data obtained by the
Asteroid Terrestrial-impact Last Alert System (ATLAS; Tonry et al. 2018),
Trakhtenbrot et al. (2019a) found that the event started on December 23 2017.
Our optical spectroscopic follow-up campaign showed the appearence of strong
broad Balmer emission lines a few months after the optical flux increase
(Trakhtenbrot et al. 2019a). This implies that 1ES1927+654 underwent a
changing-look phase, in which the circumnuclear gas reacted to the strong
increase of the optical/UV flux, illuminating the broad line region, and
creating the broad lines.

Our ongoing X-ray monitoring campaign started in May 2018, after the detection
of broad optical lines, and consists of three joint XMM-Newton/NuSTAR
observations (158/169 ks in total), carried out in June (PI: N. Schartel/C.
Ricci), December 2018 (PI: N. Schartel/E. Kara) and on May 2019 (PI: C.
Ricci), 14 Swift/XRT observations (PI: I. Arcavi, C. Ricci, 26 ks), and 265
NICER observations (678 ks; see Kara et al. 2018 for a summary of the first
set of observations). In the X-rays the source shows an extremely peculiar
behaviour, with the X-ray corona, which is typcally the source of most of the
X-ray radiation in AGN (Haardt & Maraschi 1991; Merloni & Fabian 2001; Fabian
et al. 2009; Kara et al. 2016), disappearing after the optical/UV outburst
(Ricci et al. 2020). In this paper we report the detailed analysis of all
X-ray observations carried out in the first $\sim 450$ days of our monitoring
campaign of this puzzling object, which includes observations for a total
exposure time of $>1$ Ms. The paper is structured as follows: in §2 we
describe all the X-ray data sets used and the data reduction procedures
adopted. In §3 and §4 we illustrate the X-ray spectral and timing analysis of
XMM-Newton/NuSTAR and NICER data, respectively. In §5 we discuss our results,
while in §6 we summarize our findings and present our conclusions. Throughout
the paper we adopt standard cosmological parameters
($H_{0}=70\rm\,km\,s^{-1}\,Mpc^{-1}$, $\Omega_{\mathrm{m}}=0.3$,
$\Omega_{\Lambda}=0.7$). Unless otherwise stated, uncertainties are quoted at
the 90% confidence level.

##  2\. X-ray observations

A large number of X-ray observations were performed during 2018 and 2019 to
monitor 1ES 1927+654 using XMM-Newton, NuSTAR, Swift/XRT and NICER. The
distribution of these observations is shown in Fig. 1, while the observation
log is reported in Table A in Appendix A. These observations show very strong
variability, both on timescales of $<1$ day ($\sim 1-2$ orders of magnitude)
and of months (up to $\sim 4$ orders of magnitude, see Fig. 1 of Ricci et al.
2020). In the following we describe the procedure adopted for the data
reduction of XMM-Newton (EPIC, RGS and OM; §2.1), NuSTAR (§2.2), and NICER
(§2.3) observations. Given the large numbers of high-quality NICER
observations, we did not focus on the short ($\sim 2$ ks) Swift/XRT
observations here, and we report their reduction and spectral analysis in
Appendix LABEL:appendix:XRT_spectra.

Figure 1.— Schematic representation of the X-ray monitoring campaign of 1ES
1927+654 reported here, covering the period between 17 May 2018 and 5 August
2019. The time of the detection here corresponds to that reported by
Trakhtenbrot et al. 2019 (December 23 2017), based on observations from the
Asteroid Terrestrial-impact Last Alert System (ATLAS; Tonry et al. 2018). The
light grey stars and circles are the ATLAS/o and Swift/UVM2 magnitudes (see
Trakhtenbrot et al. 2019a and Ricci et al. 2020). The complete X-ray
observation log is reported in Table A in Appendix A.

###  2.1. XMM-Newton

After the optical/UV outburst, XMM-Newton (Jansen et al. 2001) carried out
three pointed observations of 1ES 1927+654: on June 5 2018 (46 ks), on
December 12 2018 (59 ks), and on May 2019 (52 ks). All observations were
performed simultaneously with NuSTAR, and in all of them the source was
clearly detected by the EPIC/PN (Strüder et al. 2001) and MOS (Turner et al.
2001) cameras, as well as by the Reflection Grating Spectrometer (RGS; den
Herder et al. 2001) and the optical monitor (OM, Mason et al. 2001). All
EPIC/PN observations were carried out in small-window mode, using a thin (June
and December 2018) or a thick (May 2019) filter. The June and December
EPIC/MOS observations were performed in large-window mode, while the one in
May 2019 was done in small-window mode. A thin filter was used for all the
EPIC/MOS exposures. During the June 2018 observation the 0.3–10 keV EPIC/PN
(MOS) net count rate of the source was $4.2\rm\,ct\,s^{-1}$
($0.72\rm\,ct\,s^{-1}$), while during the December 2018 and May 2019
observations it was significantly higher. In December 2018 the net count rates
were $18.9\rm\,ct\,s^{-1}$ ($1.4\rm\,ct\,s^{-1}$), and in May 2019 they were
$14\rm\,ct\,s^{-1}$ ($2.6\rm\,ct\,s^{-1}$). The EPIC/PN count rate of the May
2019 observation was lower than that of the December 2018 exposure, although
the flux was higher, because of the different filter used. For comparison,
during the 2011 observation (see Gallo et al. 2013), the EPIC/PN (thin filter)
count rate was $5.2\rm\,ct\,s^{-1}$.

The observation data files (ODFs) were reduced using the XMM-Newton Standard
Analysis Software (SAS) version 17.0.0 (Gabriel et al. 2004). The raw PN (MOS)
data files were then processed using the epchain (emchain) task. The analysis
of the background light curves in the 10–12 keV band (EPIC/PN), and above 10
keV (EPIC/MOS) of the June 2018 and May 2019 observations showed that no
strong background activity was present in neither PN nor MOS, so that we were
able to use the whole exposures. Some background activity was detected around
the end of the December 2018 observation, with the count rates rising to $\sim
1\rm\,ct\,s^{-1}$ in EPIC/PN. However, due to the very high flux of the source
during this observation, we did not filter out this last $\sim 6$ ks. As a
test, we compared the spectral parameters obtained by spectra with and without
this period, and found them to be in very good agreement. In our data
reduction only patterns that correspond to single and double events (PATTERN
$\leq 4$) were selected for PN, and patterns corresponding to single, double,
triple and quadruple events were selected for MOS (PATTERN $\leq 12$).

For the June 2018 observation, using circular regions centred on the object to
extract the source spectrum we detected pile-up in all EPIC cameras, so that
we adopted an annular region, with an inner (outer) radius of $6\arcsec$
($40\arcsec$) and $5\arcsec$ ($30\arcsec$), for PN and MOS, respectively. The
pile-up was considerably stronger in the December 2018 and May 2019
observations, and even using a large inner radius ($35\arcsec$) it was not
possible to completely remove its effect on the MOS spectrum. Therefore, for
these observations we used only the PN camera, extracting the spectrum using
an inner (outer) radius of $15\arcsec$ ($40\arcsec$). The background spectra
for the different observations were extracted from circular regions with a
radius of $50\arcsec$ and $60\arcsec$ for PN and MOS, respectively. These
regions were located on the same CCD as the source, where no other source was
present. The ARFs and RMFs were created using the arfgen and rmfgen tasks,
respectively. For the June 2018 observation the source and background spectra
of the two MOS cameras, together with the RMF and ARF files, were merged using
the addascaspec task. In the following we will refer to the combined spectra
as the MOS spectrum. The EPIC spectra were binned to have at least $20$ counts
per bin, and we applied the $\chi^{2}$ statistic.

NICER observed 1ES 1927+654 for $\sim 7$ ks during the December 2018 XMM-
Newton observation. This allowed us to compare the XMM-Newton and NICER
spectra, and to look for the presence of any spectral artefacts related to
pile-up and/or to X-ray loading in the EPIC/PN spectrum, since the observation
was carried out when the source was extremely bright. In Appendix
LABEL:appendix:simultaneousNICER we report the results of our analysis,
showing that the spectral parameters obtained by fitting the two datasets are
consistent (See Fig. LABEL:fig:xmmobs2018lcnicer and Table
LABEL:tab:fitXMMnicer_dec18). This allowed us to exclude any significant
impact of pile-up and/or to X-ray loading in the EPIC/PN spectrum.

The RGS data were reduced using the rgsproc task, and we used default source
and background extraction regions. The spectra from the two detectors were
combined into a single spectrum using the rgscombine task, for both the first
and second order. The RGS first order count rates were $0.26\rm\,ct\,s^{-1}$,
$1.6\rm\,ct\,s^{-1}$ and $1.9\rm\,ct\,s^{-1}$ for the June 2018, December 2018
and May 2019 XM-Newton observations, respectively. The RGS spectra were binned
to have at least $1$ counts per bin, and we applied Cash statistics (C-stat,
Cash 1979; Kaastra 2017).

The OM data were reduced using the standard guidelines. Details will be
reported elsewhere (Li et al. in prep.).

###  2.2. NuSTAR

The Nuclear Spectroscopic Telescope Array (NuSTAR, Harrison et al. 2013)
observed 1ES 1927+654 simultaneously with XMM-Newton, with exposures of $46$
ks (June 2018), $65$ ks (December 2018) and $58$ ks (May 2019). NuSTAR data
were processed using the NuSTAR Data Analysis Software nustardas v1.8.0 within
Heasoft v6.24, using the latest calibration files (Madsen et al. 2015). The
source was not detected in the 3–24 keV band during the June 2018 observation,
and we recovered upper limits on the flux of $F_{3-5}\leq 10^{-14}$,
$F_{5-10}\leq 6.3\times 10^{-15}$ and $F_{10-24}\leq 6.3\times 10^{-15}\,\rm
erg\,s^{-1}\,cm^{-2}$, for the 3–5 keV, 5–10 keV and 10–24 keV bands,
respectively. A clear detection up to 8 keV (11 keV) was obtained in December
2018 (May 2019), when the flux level of the source was considerably higher.
Given the strong variability of the source, we tested for differences in the
spectral shape and flux of the spectrum obtained from the whole observation
and that extracted from a time interval coincident with the XMM-Newton
observation. For the December 2018 observation we did not find any difference
in the spectral shape (see Appendix LABEL:sect:XMMvsNustar), so that for our
analysis we considered the spectrum obtained from the whole NuSTAR exposure.

The FPMA and FPMB spectra were extracted using the nuproducts task, selecting
circular regions with a radius of $50\arcsec$. The background spectra and
light-curves were obtained in a similar fashion, using a circular region of
$60\arcsec$ radius located where no other source was detected. The 3–8 keV net
count rate of the source in the December 2018 and May 2019 observations was
$2.3\times 10^{-2}\rm\,ct\,s^{-1}$ and $4\times 10^{-2}\rm\,ct\,s^{-1}$,
respectively. The NuSTAR FPMA/FPMB spectra were merged, as done for the
spectra of the two MOS cameras (in §3.1). The spectra were binned to have at
least $20$ counts per bin, and $\chi^{2}$ statistic was used for the fitting.

###  2.3. NICER

Our Neutron Star Interior Composition Explorer (NICER, Gendreau et al. 2012;
Arzoumanian et al. 2014; Gendreau et al. 2016) observational campaign
commenced on May 22, 2018 and is currently still ongoing. We report here on
our analysis of data obtained through August 5, 2019 (see Table A in Appendix
A for the full observation log). The median exposure during this interval was
$\sim 2$ ks, with a maximum of 20.5 ks. The typical observation cadence was
once per day, with the time spacing between successive observation sequences
varying between 5 hours and 8.7 days. The unfiltered NICER events were first
reprocessed to assure that the November 2018 energy scale update was applied
to all NICER observation sequences, and data from NICER focal plane detector
modules 14 and 34 that are known to undergo episodes of increased noise were
excluded. An ARF tailored to exclude these was utilized in the spectral
analysis. Filtered event files were constructed by applying the NICERDAS
Version 5.0 standard cleaning criteria that excludes times within the NICER-
specific definition of the SAA, and those with elevations with respect to the
earth’s limb and bright earth below their NICERDAS Version 5.0 default
values.222https://heasarc.gsfc.nasa.gov/docs/nicer/data_analysis
/nicer_analysis_guide.html Additional background-related filtering, described
below, was also applied.

Although the standard cleaning eliminates much of NICER internal background,
spectral analysis requires the modelling of the inevitable residual
background. The NICER internal background, due primarily to the presence of
optical light at low energy and energetic charged particles at high energy,
depends on the orbital parameters and space weather conditions at the time
when the observation was conducted, and is highly variable both in magnitude
and spectral shape. In order to model the background spectrum for each NICER
observation sequence, we utilize three background proxies extracted from the
unfiltered event list. One of these is the rate of focused background events
in an energy band (15–17 keV) where the telescope effective area is
effectively negligible (so all events will be due to particle background), and
another the rate of unfocused events identified as having a “ballistic
deficit” due to their having originated at the edges of individual detectors.
The latter metric considers the same ratio of slow-chain PI to fast-chain PI
values used to exclude background in standard screening, to instead select
background events. This rate is tightly correlated with the cut-off rigidity.
A two-dimensional library of spectra was constructed, indexed by the values of
these two rates, and extracted from the 2.7 Ms of NICER background field
accumulated as of January 28 2019. The background spectra for any time
interval is estimated by adopting the library spectrum corresponding to the
derived rates of these two proxies, and then rescaling by the ratio of the
15–17 keV rate to the average rate for that library entry to impose a “soft
landing” at the edge of the bandpass. The third background proxy accounts for
solar light-leak driven optical light loading noise events, and is constructed
from the slow-chain count rate in the 0–0.2 keV noise band that is below the
NICER nominal energy range for cleaned file events. The library described
above is restricted to “nighttime” conditions (noise band rate
$<200\rm\,ct\,s^{-1}$), and an additional library of “daytime” background
residuals is constructed from the background fields indexed by noise band rate
for rates $>200\rm\,ct\,s^{-1}$. The residual spectrum for any time interval
is estimated by adopting the library daytime residual spectrum corresponding
to the derived noised band rate, and then rescaling by the ratio of the 0–0.2
keV rate to the average such rate for that daytime library entry. The
resulting residual daytime spectrum is added to the nighttime spectra.

We subdivide the cleaned event file good time intervals into 120 s
subexposures, discarding any leftover intervals shorter than 10 s. Rates of
the three background proxies are then calculated for each new interval, and
daytime and nighttime background spectra identified accordingly. Intervals
that have rates that fall outside of the range in the nighttime library are
eliminated from the list of time intervals contributing to the source spectra.
The final background spectrum for each observation sequence is then the
exposure-weighted sum over these subexposures of the matched library spectra,
with a background exposure time that is similarly weighted. The highly
variable nature of both the source and background is reflected in the fact
that the NICER observations include sequences where the source is not
significantly detected above the background and those where the 0.3–3 keV
signal-to-noise ratio is $>1000$. The quality of the match between the
background spectrum estimated as described above and the total observed
spectrum at energies where the background is clearly dominant varies. We
eliminate some of the more poorly matched intervals from consideration by
excising those where the absolute value of the net 13–15 keV rate is
$>0.10\rm\,ct\,s^{-1}$. Nevertheless, the source and background spectra for
each observation sequence must be carefully scrutinized, and the analysis
details (number of components, bandpass used) tailored accordingly (§4). The
final sum of the 265 source spectra exposure times is $\sim 678$ ks.

##  3\. XMM-Newton/NuSTAR observations

In the following we report the results obtained by the spectral analysis of
the June 2018 (§ 3.1), December 2018 and May 2019 (§ 3.2) joint XMM-
Newton/NuSTAR observations of 1ES 1927+654. In § 3.3 we report on the results
obtained by applying partial covering and blurred reflection spectral models.
We also discuss the X-ray and UV variability of the source (§3.4) and the
time-resolved spectroscopy (§3.5) of the three observations, as well as the
origin of the broad feature at $\sim 1$ keV found in all XMM-Newton/NuSTAR
observations (§3.6).

Figure 2.— Panel A: June 2018 XMM-Newton EPIC/PN (blue) and MOS (red) spectra
of 1ES 1927+654. The blue and red shaded area represent the background spectra
of EPIC/PN and MOS, respectively. Panels B–E: ratio between models and the
data, sorted from the least (B) to most complex (E) model. The different
models used include a blackbody component (zbb), a power-law continuum (zpo),
a neutral absorber (ztbabs), a ionized absorber (xstar) and Gaussian lines
(zgauss). More details can be found in §3.1. Table 1Spectral parameters
obtained for the XMM-Newton June 2018 (column 2), December 2018 (column 3) and
May 2019 (column 4) observations (EPIC+RGS). The model used included a
blackbody component, a power law, two Gaussian absorption lines and two
emission lines, a neutral and an ionized absorber.

| (1) | (2) | (3) | (4)
---|---|---|---|---
|  | June 2018 | December 2018 | May 2019
a) |  $N_{\rm H}$ ($10^{20}\rm\,cm^{-2}$) |  $\cdots$ | $3.0\pm 0.1$ | $1.8_{-0.08}^{+0.06}$
b) |  $N_{\rm H}^{W}$ ($10^{20}\rm\,cm^{-2}$) | $2.2_{-0.5}^{+0.6}$ | $\leq 1.1$ | $\leq 1.1$
c) |  $\log\xi$ ($\rm\,erg\,cm\,s^{-1}$) | $2.9_{-0.8}^{+0.1}$ | $1.9_{-0.4}^{+0.7}$ | $2.7^{+0.2}_{-1.1}$
d) | $z$ | $-0.216_{-0.018}^{+0.017}$ | $-0.212_{-0.003}^{+0.006}$ | $-0.249^{+0.06}_{-0.013}$
e) | $\Gamma$ | $1.9_{-0.7}^{+0.5}$ | $3.02\pm 0.01$ | $2.98^{+0.08}_{-0.10}$
f) | $E_{\rm cut}$ |  $\cdots$ | $2.76_{-0.04}^{+0.05}$ | $3.3\pm 0.4$
g) |  $E_{1}$ (keV) | $1.01\pm 0.01$ | $1.074\pm 0.005$ | $1.01^{+0.03}_{-0.05}$
h) |  $\sigma_{1}$ (eV) | $103_{-7}^{+6}$ | $156\pm 4$ | $193^{+15}_{-25}$
i) |  $EW_{1}$ (eV) | $184_{-7}^{+10}$ | $94_{-22}^{+23}$ | $79^{+30}_{-20}$
j) |  $E_{2}$ (keV) | $1.73\pm 0.04$ | $1.92_{-0.06}^{+0.05}$ | $1.95^{+0.05}_{-0.06}$
k) |  $\sigma_{2}$ (eV) | $150_{-50}^{+70}$ | $261_{-46}^{+56}$ | $300^{+NC}_{-52}$
l) |  $EW_{2}$ (eV) | $460_{-142}^{+228}$ | $78_{-20}^{+18}$ | $149^{+31}_{-44}$
m) | $C_{\rm Nu}$ |  $\cdots$ | $0.78\pm 0.04$ | $0.79^{+0.04}_{-0.05}$
n) | $C_{\rm MOS}$ | $1.05\pm 0.01$ |  $\cdots$ |  $\cdots$
o) | $C_{\rm RGS}$ | $0.92\pm 0.01$ | $0.916\pm 0.004$ | $0.921\pm 0.004$
p) |  $E_{\rm abs1}$ (keV) |  $\cdots$ | $0.396\pm 0.002$ | $0.376^{+0.07}_{-0.06}$
q) |  $\sigma_{\rm abs1}$ (eV) |  $\cdots$ | $0.2_{-0.1}^{+0.2}$ | $25^{+7}_{-5}$
r) | Strength ($10^{-3}$) |  $\cdots$ | $1.4_{-0.6}^{+1.7}$ | $11\pm 3$
s) |  $E_{\rm abs2}$ (keV) |  $\cdots$ | $0.527\pm 0.001$ |  $\cdots$
t) |  $\sigma_{\rm abs2}$ (eV) |  $\cdots$ | $0.5\pm 0.1$ |  $\cdots$
u) | Strength ($10^{-3}$) |  $\cdots$ | $3.0_{-0.8}^{+1.0}$ |  $\cdots$
v) |  $kT$ (eV) | $102\pm 1$ | $142\pm 1$ | $160^{+4}_{-6}$
w) |  $F_{0.3-10}$ ($\rm 10^{-11}\,\,erg\,s^{-1}\,cm^{-2}$) | $1.0$ | $6.7$ | $7.9$
x) |  $L_{0.3-10}$ ($\rm erg\,s^{-1}$) | $8.5\times 10^{42}$ | $5.7\times 10^{43}$ | $6.7\times 10^{43}$
y) | Stat/DOF | 2943/2847 | 4137/3712 | 3916/3549

Note. — The table reports: the column density of the neutral absorber (a); the
column density (b), ionization parameter (c) and redshift (d) of the ionized
absorber; the photon index (e) and cutoff (f) of the power-law component; the
energy (g), width (h) and equivalent width (i) of the first Gaussian emission
line; the energy (j), width (k) and equivalent width (l) of the second
Gaussian line; the cross-calibration constant of the NuSTAR/FPM (m), XMM-
Newton MOS (n) and RGS (o) spectra; the energy (p), width (q) and strength (r)
of the first Gaussian absorption line; the energy (s), width (t) and strength
(u) of the second Gaussian absorption line; the temperature of the blackbody
(v), the 0.3–10 keV flux (w) and luminosity (x), and the value of the
statistic and the number of degrees of freedom (y).

###  3.1. The June 2018 observation

Our first joint XMM-Newton/NuSTAR X-ray observations were carried out on June
2018. The source an extremely soft continuum, very different from that
observed in the previous XMM-Newton observation, carried out in May 2011, and
from typical AGN. After the event in fact 1ES 1927+654 showed only a very weak
powerlaw component, which is found to typically dominate the X-ray emission of
AGN (e.g., Mushotzky et al. 1993). Because of its extreme softness, the source
was not detected by NuSTAR, and the spectral analysis was carried out only
using EPIC and RGS data. We used the EPIC PN (MOS) data in the 0.3–2.8 keV
(0.3–4.6 keV) range, and RGS data in the 0.35–1.3 keV interval. The spectral
analysis was carried out within XSPEC v12.10.0e (Arnaud 1996). In all models
we took into account Galactic absorption at the position of the source
($N_{\rm H}=6.87\times 10^{20}\rm\,cm^{-2}$, Kalberla et al. 2005) using the
tbabs spectral model (Wilms et al. 2000). A multiplicative constant (cons in
XSPEC) was added to all models to take into account any cross-calibration
offset between the different instruments.

The EPIC PN and MOS spectra of this observation are shown in panel A of Fig.
2. We started by fitting the X-ray spectrum with a simple blackbody model
(cons$\times$tbabs$\times$zbb), which resulted in a chi-squared of
$\chi^{2}=2483$ for 320 degrees of freedom (DOF), and left very strong
residuals around 1 keV and above $\simeq 1.5$ keV (see panel B). The addition
of a power-law component to the model [cons$\times$tbabs$\times$(zbb+zpo)]
improved the fit ($\chi^{2}=1918$ for 318 DOF), still leaving significant
residuals (panel C) between 1 and 2 keV. Similarly to what was done by Gallo
et al. (2013) we also added a neutral absorber (ztbabs in XSPEC), possibly
associated with gas in the host galaxy, leaving its column density free to
vary [cons$\times$tbabs$\times$ztbabs$\times$(zbb+zpo)], which improved the
fit ($\chi^{2}=1813$ for 317 DOF). Applying this model to the RGS spectrum
also shows significant residuals (see panel B of Fig. LABEL:fig:RGS_spec_18 in
Appendix LABEL:appendix:RGS_spectra), and in particular an excess around 1 keV
(clearly observed also in the second-order spectrum, see the top panels of
Fig. LABEL:fig:RGS_spec_order2).

To take into account the strong residuals at $\sim$1 keV, we added a Gaussian
line, leaving its width and energy free to vary. This greatly improved the fit
($\chi^{2}=405$ for 314 DOF, panel D), leaving only some residuals at $\simeq
1.8$ keV and around the absorption lines also observed in the RGS spectrum. We
therefore added a second Gaussian, leaving both the energy and the width free
to vary in the fit, which yielded a better fit ($\chi^{2}=375$ for 311 DOF).
To model the absorption lines resolved in the RGS spectra, we then added a
warm absorber using an xstar (Kallman & Bautista 2001; Bautista & Kallman
2001) table built for a blackbody with a temperature of $kT=100$ eV, fixing
the turbulent velocity to $v_{\rm turb}=10^{4}\rm\,km\,s^{-1}$
[cons$\times$tbabs$\times$ztbabs$\times$mtable{xstar}
$\times$(zbb+zpo+zgauss+zgauss)]. We left the redshift, ionization parameter
and column density of the ionized absorber free to vary in the fit. This
removed the need of a neutral absorber, which was not constrained by the fit
($N_{\rm H}\leq 2.3\times 10^{19}\rm\,cm^{-2}$), and resulted in a good fit
($\chi^{2}=346$ for 309 DOF, panel E). The ionized absorber provides a better
fit than a simple neutral absorber, providing a difference in the chi-squared
of $\Delta\chi^{2}=29$ for two additional DOF (i.e., ionization parameter and
redshift).

We then fitted the EPIC spectra together with RGS, using our best model, which
included a blackbody, a power-law, two Gaussian lines and a layer of ionized
material. This model can reproduce the data very well (Stat/DOF=2943/2847,
where Stat includes both $\chi^{2}$ and C-stat; see panel C of Fig.
LABEL:fig:RGS_spec_18 in Appendix LABEL:appendix:RGS_spectra), and resulted in
a temperature of the blackbody component of $kT=102\pm 1$ eV and a photon
index of $\Gamma=1.9^{+0.5}_{-0.7}$, consistent with what we found using only
EPIC data. The results of this final fit are reported in column 2 of Table 1,
while the spectrum and the different components used are illustrated in the
top panel of Fig. 3. We found that the ionized absorber has a redshift of
$z\sim-0.22\pm 0.02$, which implies a velocity of $v\sim 0.26\rm\,c$
comparable to those observed in some AGN (e.g., Tombesi et al. 2010) and
Ultra-luminous X-ray sources (e.g., Pinto et al. 2016; Walton et al. 2016). We
also created an absorption model with a lower turbulent velocity ($v_{\rm
turb}=300\rm\,km\,s^{-1}$), considering a blackbody with a temperature of
$kT=100$ eV for the X-ray continuum. We included this model in our fit to the
EPIC and RGS spectra, and found that it provides a worse fit
(Stat/DOF=2963/2847) than that obtained by considering an absorber with very
large turbulent velocity. While both the column density and the ionization
parameter were consistent with those obtained assuming $v_{\rm
turb}=10^{4}\rm\,km\,s^{-1}$, the redshift of the absorber was considerably
lower ($z=-0.101\pm 0.001$).

As a further test we replaced the xstar table with the xstar warmabs model,
which is not based on pre-calculated grids, but calculates the spectrum on the
fly. We produced specific population files using the observed X-ray spectrum
of 1ES 1927+654, rather than a single-component model. Fitting the X-ray
spectrum with this model
[cons$\times$tbabs$\times$ztbabs$\times$warmabs$\times$
(zbb+zpo+zgauss+zgauss)] resulted in a good fit (Stat/DOF=3021/2846), and
values of the temperature of the blackbody ($kT=102\pm 1$ eV) and photon index
($\Gamma=2.0^{+0.5}_{-0.7}$), consistent with what we found applying the XSTAR
table. With this approach we could also get some constraints on the turbulent
velocity of the absorber, which was found to be $v_{\rm turb}\geq
7418\rm\,km\,s^{-1}$.

We tested different X-ray continuum models to replace the blackbody component,
including a multicolor blackbody model and a Comptonized plasma (see Appendix
LABEL:appendix:XMM_alternativecontinuummodels), and we found that the data
appear to be best fitted by the blackbody model.

Figure 3.— Top panel: June 2018 XMM-Newton EPIC/PN (blue points) and MOS (red
points) spectra of 1ES 1927+654. The continuous line shows the best-fitting
model, which includes a blackbody (cyan dashed line), a power-law (black dot-
dashed line), and two Gaussian lines (dot-dot-dashed lines). These components
are absorbed by low column density neutral and ionized material (see §3.1 and
column 2 of Table 1 for details). The residuals of this fit are shown in panel
E of Fig. 2. Middle planel: same as the top panel for the December 2018 XMM-
Newton EPIC/PN (blue points) and NuSTAR (red points) observations, using a
cutoff power-law (black dot-dashed line). See §3.2 and column 3 of Table 1 for
details. The residuals of this fit are shown in panel E (left) of Fig. 4.
Bottom planel: same as the middle panel for the May 2019 XMM-Newton EPIC/PN
(blue points) and NuSTAR (red points) observations. (see §3.2 and column 4 of
Table 1 for details). The residuals of this fit are shown in panel E (right)
of Fig. 4.

Figure 4.— Panel A: December 2018 (left panels) and May 2019 (right panels)
XMM-Newton EPIC/PN (blue) and NuSTAR (red) spectra of 1ES 1927+654. The blue
and red shaded area are the background spectra of EPIC/PN and NuSTAR,
respectively. Panels B–E: ratio between the models and the data, sorted from
the least (B) to most complex (E) model. The different models used include a
blackbody component (zbb), a power-law continuum (zpo), a neutral absorber
(ztbabs), ionized absorbers (xstar) and Gaussian lines (zgauss). More details
can be found in §3.2.

###  3.2. The December 2018 and May 2019 observations

Two additional joint XMM-Newton/NuSTAR observations of the source were carried
out on December 2018 and May 2019 (panels A of Fig. 4). During these
observations the source was $\simeq 7-8$ times brighter than in June, and the
maximum luminosity was reached during the May 2019 observation. While the
spectra were found to be still extremely soft, the emission was significantly
harder than in the previous XMM-Newton observation. This, combined with the
higher overall flux, allowed NuSTAR to detect the source in both observations.
For the December 2018 (May 2019) we used the EPIC/PN data in the 0.3–3 keV
(0.3–3 keV) range, and RGS data in the 0.35–1.85 keV (0.35–1.85 keV) interval;
NuSTAR data were used in the 3–8 keV (3–11 keV) range.

We started by applying a model that includes a blackbody plus a powerlaw
component [cons$\times$tbabs$\times$ztbabs$\times$(zbb+zpo)]. This resulted in
a poor fit ($\chi^{2}$/DOF=1637/567 and $\chi^{2}$/DOF=1340/667 for December
2018 and May 2019, respectively), with clear residuals in the 1–2 keV region,
similarly to what we found studying the June 2018 XMM-Newton observation (see
$\S\ref{sect:xmmobs_spec_june}$). Moreover, the fit also fails to reproduce
the spectrum above 3 keV (panels B of Fig. 4). Adding a Gaussian line
[cons$\times$tbabs$\times$ztbabs$\times$(zbb+zpo+zgauss)], panels C] to this
model removes the residuals at $\simeq 1$ keV, leaving however very strong
residuals above 3 keV ($\chi^{2}$/DOF=820/564 and $\chi^{2}$/DOF=1079/664) and
a cross-calibration constant for the NuSTAR spectrum that is rather
unrealistic ($C_{\rm Nu}=0.41\pm 0.03$ and $C_{\rm Nu}=0.53\pm 0.03$),
considering that the observations were carried out simultaneously.

####  3.2.1 Cutoff powerlaw continuum

Since a power-law continuum cannot reproduce the 3–10 keV region, we used a
cutoff power law model to account for the spectral curvature observed in
NuSTAR data [cons$\times$tbabs$\times$ztbabs$\times$(zbb+zcut+zgauss)]. This
reproduces much better the spectra ($\chi^{2}$/DOF=647/563 and
$\chi^{2}$/DOF=1040/663, panels D). An ionized absorber appears to be still
present, as shown by the RGS spectra (see panels B of Fig.
LABEL:fig:RGS_spec_18 and Fig. LABEL:fig:RGS_spec_19 in Appendix
LABEL:appendix:RGS_spectra), with absorption features around $0.7-0.8$ keV. We
therefore used the same xstar table adopted for the June observation to take
into account this absorber. Some residuals are also evident in the $1.7-2$ keV
region, consistent with what we found for the June 2018 observation, so that
we added also a second Gaussian component, which improved significantly the
fit ($\chi^{2}$/DOF=621/557 and $\chi^{2}$/DOF=726/657, panels E).

As we did for the June 2018 observation we used this model to fit the NuSTAR,
XMM-Newton EPIC/PN and RGS data (Stat/DOF=4363/3718 and Stat/DOF=3949/3552).
The RGS spectrum of the December 2018 observation shows two additional
absorption features at $\sim 0.4$ keV and $\sim 0.53$ keV that are not
reproduced by the ionized absorber. Only the feature at $\sim 0.4$ keV was
found in the May 2019 spectrum. We therefore added two (one) Gaussian
absorption lines to the December 2018 (May 2019) spectrum, leaving both the
energy and the width free to vary, which improved the fit (Stat/DOF=4137/3712
and Stat/DOF= 3916/3549, see panels C of Figs. LABEL:fig:RGS_spec_18 and
LABEL:fig:RGS_spec_19). The results of this fit are reported in columns 3 and
4 of Table 1, while the spectra and the different components used are
illustrated in the middle and bottom panels of Fig. 3.

The temperature of the blackbody component is higher in December 2018 (May
2019) than it was in June 2018 ($kT=142\pm 1$ eV and $kT=160^{+4}_{-6}$ eV,
respectively). The velocity of the outflow is slightly higher in May 2019 than
in the previous two observations. The photon index is consistent in the two
observations, and the power-law component is significantly stronger with
respect to the blackbody component than in December 2018.

####  3.2.2 Thermal Comptonization continuum

In order to describe the hard X-ray component produced by the corona, which
was only weakly detected in the first XMM-Newton observation, we have thus far
used a cutoff power-law, consistent what is typically done for AGN (e.g.,
Dadina 2007; Ricci et al. 2017). However, if the seed photons of the
Comptonization process are the soft X-ray photons from the thermal component,
this approach might not be physically self consistent. In order to test this
scenario we used the nthcomp model (Zdziarski et al. 1996; Życki et al. 1999),
which provides an accurate description of the X-ray continuum arising from
thermal Comptonisation. The parameters of this model are the temperature of
the Comptonizing electrons ($kT_{\rm e}$), the temperature of the blackbody
providing the seed photons ($kT_{\rm bb}$), the asymptotic power-law photon
index ($\Gamma_{\rm nth}$) and the normalization. In XSPEC our model is
cons$\times$tbabs$\times$ztbabs$\times$mtable{xstar}
$\times$(zbb+nthcomp+zgauss+zgauss). In our fits of the December 2018 and May
2019 XMM-Newton observations we substituted nthcomp to the cutoff power-law
component, fixed the temperature of the seed photons to that of the thermal
component ($kT_{\rm bb}=kT$), and let $kT_{\rm e}$ and $\Gamma_{\rm nth}$ free
to vary. In both cases this model provided a worse ($\chi^{2}/DOF=631.1/557$
for the December 2018 observation) or similar ($\chi^{2}=727/657$ for May
2019) fit to the cutoff power-law model. Both temperature of the electrons in
the Comptonizing plasma and the photon index are higher in the May 2019
observation ($kT_{\rm e}=4.4_{-1.5}^{+7.3}\rm\,keV$ and $\Gamma=4.4\pm 0.1$)
than in the December 2018 one ($kT_{\rm e}=1.8_{-0.5}^{+1.0}\rm\,keV$ and
$\Gamma=3.7\pm 0.2$). Interestingly the blackbody temperature obtained is
consistent between the two observations, and is significantly lower than when
using a cutoff power-law model: $kT=124\pm 1$ eV and $kT=122\pm 4$ eV for the
December 2018 and May 2019 observation, respectively. The temperature is
however significantly higher than in our first XMM-Newton observation
($kT=102\pm 1$ eV).

Figure 5.— XMM-Newton EPIC/PN 0.3–10 keV light curve of 1ES 1927+654 for the
June 2018 (top panel), December 2018 (middle panel) and May 2019 (bottom
panel) observations (200 s bins). The figure also shows the intervals used for
the time-resolved spectroscopy, which are denoted by the vertical red dashed
lines (see §3.5). Figure 6.— Variability spectra of the 2011 (blue filled
diamonds), June 2018 (red filled circles), December 2018 (green empty circles)
and May 2019 (grey filled stars) XMM-Newton/EPIC PN observations on timescales
of 200 s. After the event, in June 2018 the source became much more variable
at all the energies probed here. In December 2018 and May 2019, when the
source luminosity was significantly higher the peak of the variability is
found at higher energies.

###  3.3. Comparison with the 2011 XMM-Newton observation and alternative
modelling

The 2011 XMM-Newton observation of 1ES 1927+654 was studied in detail by Gallo
et al. (2013), who applied several different models, including a power-law
absorbed by two partial covering neutral or ionized obscurers, as well as
blurred ionized reflection. The authors found that all models could well
reproduce the X-ray spectrum of the source, and they concluded that the X-ray
emission in this source was consistent with being produced by a corona
surrounding a standard accretion disc. In this section we test neutral and
ionized partial covering models, as well as blurred ionized reflection, on the
June 2018 and December 2018 XMM-Newton/NuSTAR observations.

####  3.3.1 Partial covering

We first tested a model that includes a power law absorbed by two layers of
partially covering neutral material
(tbabs$\times$ztbabs$\times$zpcfabs$\times$zpcfabs$\times$zpo). This model
leaves very strong residuals across most of the spectrum, and results in a
very high reduced chi-squared ($\sim 39$ and $\sim 18$ for the June and
December observations, respectively) and in a very steep power law
($\Gamma\sim 7.3$ and $\Gamma\sim 5.5$, respectively). Including a blackbody
component [tbabs$\times$ztbabs$\times$zpcfabs$\times$zpcfabs$\times$(zpo+zbb)]
improves the fit, but it still results in a poor reduced chi-squared ($\sim
4.5$ and $\sim 2$ for the June and December observations, respectively). We
then considered a model that includes partial covering ionized obscuration. We
started by considering only a power-law continuum, as done by Gallo et al.
(2013; tbabs$\times$ztbabs$\times$zxipcf$\times$zxipcf$\times$zpo). This
cannot reproduce well the continuum, leaving strong residuals and resulting in
a reduced $\chi^{2}$ of $\sim 1.4$ for both the June and December 2018
observations. Similarly, simultaneously fitting these two observations and the
2011 observation, leaving all the parameters of the absorbers free to vary
while tying the photon index and normalization of the power law component,
results in a poor fit ($\chi^{2}$/DOF=6609/1550). This model fails to
reproduce the spectrum even if we untie both the normalization and the photon
index ($\chi^{2}$/DOF=6539/1548 and $\chi^{2}$/DOF=6098/1546, respectively).
The addition of a blackbody component and of a third partially covering
ionized absorber
[tbabs$\times$ztbabs$\times$zxipcf$\times$zxipcf$\times$zxipcf$\times$(zpo+zbb)]
provides a good fit for both the June ($\chi^{2}\rm/DOF=346/306$) and the
December ($\chi^{2}\rm/DOF=617/556$) observations. However, several of the
parameters obtained from the fit are quite extreme: the photon indices would
be $\Gamma\sim 5-6$, the velocity of the absorbers would be up to $\sim
0.55$c, while the unobscured luminosity of the June 2018 observation would be
$\sim 2\times 10^{47}\rm\,erg\,s^{-1}\,cm^{-2}$. This would make 1ES 1927+654
the brightest AGN in the local Universe by several orders of magnitude. The
strong change in the ratio between power-law and blackbody emission between
the two observations (see Ricci et al. 2020) would be even more extreme than
what we found including two Gaussian lines (see Fig. 13), with the fractional
contribution of the power-law component to the flux increasing of $\sim 2500$
times between the June and December 2018 observations.

####  3.3.2 Relativistic reflection

We also tested several relativistic reflection models (Dauser et al. 2010,
2013; García & Kallman 2010; García et al. 2013, 2014), such as relxilllp and
relxilllpD (which considers a high-density accretion disk), and found that
they also leave very strong residuals through the whole spectrum. The
relxilllpD fit in particular results in a steep continuum ($\Gamma\sim 3.4$),
a high inclination angle with respect to the disk ($i\sim 89^{\circ}$,
similarly to Gallo et al. 2013), and in a reduced chi-squared of $\sim 2.7$.

Figure 7.— Top panel: X-ray flux (in the 0.3–3 keV range) versus UVW2 flux
(5.8 eV) for the simultaneous EPIC/PN and OM exposures during the December
2018 observation. Bottom panel: 0.3–3 keV flux versus UVW2 flux for the
Swift/XRT and UVOT observations, as well as for the June 2018 and May 2019
XMM-Newton observations.

###  3.4. X-ray and UV variability

####  3.4.1 X-rays

1ES 1927+654 shows very strong variability in the 0.3–10 keV band during the
three XMM-Newton observations (Fig. 5), though the variability characteristics
are clearly different during each exposure. During the June 2018 observation
the AGN varies by a factor of 28 in $\sim 27$ ks (top panel), while the ratio
between the minimum and maximum flux in December 2018 and May 2019 (middle and
bottom panels, respectively) was only $\sim 4-5$, and the source varied more
rapidly on shorter timescales.

In panels A and B of Fig. LABEL:fig:XMMlc_18_bands (Appendix
LABEL:appendix:XMM_lightcurves) we show the variability in four different
energy bands for the June 2018 observation. The light curves show a very
similar pattern, although with different ratios between the maximum and the
minimum flux, ranging between $\sim 18$ for the 0.3–0.5 keV band to a factor
of $\sim 159$ in the $1-1.3$ keV range. The light curves in six energy bands
for the December 2018 observation are illustrated in panels A, B and C of Fig.
LABEL:fig:XMMlc_18_december_bands. Also in this case the variability is
similar in all bands, but has a different ratio between the maximum and
minimum flux, ranging from $\sim 3$ in the 0.3–0.5 keV band to $\sim 70$ in
the 2–3 keV range. During the May 2019 observation we also found that the
variability is similar in all bands (panels A, B and C of Fig.
LABEL:fig:XMMlc_18_december_bands), with the ratio between the maximum and
minimum flux going from $\sim 4$ in the 0.3–0.5 keV band to $\sim 59$ in the
1–1.3 keV band. Interestingly, for all observations the hardness ratio varies
following the flux of the source, suggesting that the radiation becomes harder
when the flux increases (see panels C–E of Fig. LABEL:fig:XMMlc_18_bands and
panels D–H of Fig. LABEL:fig:XMMlc_18_december_bands).

The XMM-Newton EPIC/PN variability spectrum of 1ES 1927+654, calculated
considering timescales of 200 s, is shown in Fig. 6 for the 2011 (blue
diamonds, see Gallo et al. 2013) observation, as well as for the June 2018
(red filled circles), the December 2018 (green empty circles) and the May 2019
(cyan filled circles) observations. The fractional root mean square
variability amplitude ($F_{\rm var}$) was calculated following Edelson et al.
(1990) and Vaughan et al. (2003, see Eqs. 10 and B2 in their paper). In the
2011 observation the source shows a peak at $\sim 1.5$ keV, and a bell-shaped
variability amplitude, similar to other well known unobscured AGN (e.g., Gallo
et al. 2004; Gierliński & Done 2006). After the optical/UV outburst the source
is much more variable than in 2011, and in particular in June 2018, only a few
months after the detection of the optical/UV brightening, when $F_{\rm var}$
peaks at $96\pm 2\%$ in the 1–1.1 keV band, and then decreases to $71\pm 5\%$
in the 1.2–2 keV range. In December 2018 the peak of the variability is found
at higher energies ($E\geq 1.5$ keV), and is $F_{\rm var}=73\pm 1\%$, a value
comparable to what was observed in the same band six months before.
Interestingly, in December 2018 the variability below $\sim 0.7$ keV is back
to a level comparable with what was found in the May 2011 observation. During
the May 2019 observations the shape of the variability spectrum of the source
is similar to that observed in the December 2018 observation, with a peak in
the $1.5-2$ keV bin ($F_{\rm var}=75\pm 1\%$). During this last observation
the variability below 1 keV is significantly higher than in December ($F_{\rm
var}\simeq 30-50\%$). In 2018/2019 the value of $F_{\rm var}$ above $\sim 1.2$
keV remains unchanged, and all of the changes occur at lower energies.

####  3.4.2 UV

During the December 2018 XMM-Newton observation OM carried out 11 exposures of
the source in the UVW2 band (5.8 eV). In order to compare the UV and X-ray
flux we extracted EPIC/PN spectra using the same intervals adopted for the
UVW2 exposures, and we fitted them using the same models we applied for the
time-resolved spectroscopy (§3.5). No clear correlation is found neither
between the 0.3–3 keV and the UVW2 flux (top panel of Fig. 7), nor between the
UVW2 flux and the emission in different X-ray bands or cosidering different
spectral components. Similarly, no clear trend is found when studying the
relation between the UVW2 and the X-ray flux for the 14 Swift observations and
the June 2018 and May 2019 XMM-Newton observations (bottom panel of Fig. 7).

Figure 8.— Top panels: XMM-Newton EPIC/PN spectra of the periods of the June
2018 (left panel), December 2018 (middle panel) and May 2019 (right panel)
observations with the highest (intervals 2/16/13) and lowest (intervals 8/5/4)
fluxes (see Fig. 5). The spectral results obtained for all intervals of these
three observations are reported in Tables
LABEL:tab:fitXMM18epochs_Jun–LABEL:tab:fitXMM19epochs_May of Appendix
LABEL:appendix:exposuresintervals. Central panels: ratio between the spectra
and a powerlaw plus blackbody model, which shows the persistent presence of a
feature at $\sim 1$ keV, except at the highest luminosity (intervals 16 and 13
of the December 2018 and May 2019 observations, respectively). Bottom panels:
ratio between the best-fitting spectral models of the highest and lowest flux
intervals, after having normalized their fluxes at 0.3 keV. The plots show
that the difference arises both from a higher-temperature blackbody (1) and a
stronger power-law component (2).

###  3.5. Time-resolved spectroscopy

Given the very strong X-ray variability of 1ES 1927+654 on short ($\lesssim
5-10$ ks) timescales, we studied its spectral evolution in different intervals
using EPIC/PN for all XMM-Newton observations (§3.5.1). This was done by
considering different good time intervals for the spectral extraction. For the
June 2018 observation we divided the exposure into 11 intervals, while for the
December 2018 observation, given the higher flux and more rapid variability,
we divided the observation into 19 periods. For the May 2019 observation we
considered 13 periods. The intervals used for the spectral extraction are
shown in Fig. 5. The exposures of the different intervals vary between 1.2 ks
and 8 ks (see Appendix LABEL:appendix:exposuresintervals). To compare the
spectral variations after the optical/UV outburst with the previous X-ray
observations, we performed also time-resolved spectroscopy for the 2011 XMM-
Newton observation (§ 3.5.2). Due to the low flux of some of the intervals, we
fitted all spectra using Cash statistics, after binning the spectra to have at
least one count per bin, to avoid possible issues related to empty bins in
XSPEC.

Figure 9.—  Top panel: Temperature of the blackbody component versus the 0.3–2
keV luminosity for the June 2018 (blue circles), December 2018 (red diamonds)
and May 2019 (black stars) XMM-Newton observations (see §3.5). The black
dotted line represents the $L\propto(kT)^{4}$ relation. The X-ray spectral
model used included a power-law, a blackbody component and a Gaussian line
(see Appendix LABEL:appendix:exposuresintervals). Middle panel: Same as top
panel for the individual NICER observations (see §4.2.1). Bottom panel: Same
as top panel for the stacked NICER observations (see §4.2.2). The cyan
diamonds and the red stars are the values obtained by using a power law or a
thermally Comptonized continuum, respectively.

####  3.5.1 June/December 2018 and May 2019

Strong spectral variability was detected in all XMM-Newton observations
carried out after the optical/UV outburst. We fitted all the spectra with a
model that included neutral absorption, a power law and a blackbody component,
plus a Gaussian line [tbabs$\times$ztbabs$\times$(zpo+zbb+zgauss)]. The
spectral change between the highest and lowest flux intervals can be clearly
seen in the top panels of Fig. 8: the spectral shape changes significantly
with flux, with the source becoming harder at higher fluxes, in good agreement
with what is discussed in §3.4.1. The middle panels of the figure show the
residuals obtained by fitting the spectra with a model that did not include an
emission line [tbabs$\times$ztbabs$\times$(zpo+zbb)], highlighting the fact
that the broad feature at 1 keV is present both at the highest and the lowest
flux level, with the exception of the highest luminosity intervals of the
December 2018 and May 2019 observations. The bottom panels show the ratio
between the best-fitting model of the highest and lowest-flux intervals,
normalized at 0.3 keV. The plot illustrates that the spectral difference
arises due to both a higher-temperature blackbody (1) and a stronger power-law
component (2) during the higher flux periods.

The results of the spectral fitting for all intervals using the emission line
model are reported in Tables
LABEL:tab:fitXMM18epochs_Jun–LABEL:tab:fitXMM19epochs_May in Appendix
LABEL:appendix:exposuresintervals. The model reproduces the X-ray spectra of
the source well in all the intervals. The temperature of the blackbody
component ranges between $kT=79^{+7}_{-7}$ eV and $kT=183^{+4}_{-3}$ eV. A
very tight relation between the flux level and the temperature of the
blackbody is found for all intervals (top panel of Fig. 9), with $kT$
increasing as the source gets brighter. Fitting this trend with a function of
the type $\log(L_{0.3-2}/\rm erg\,s^{-1})=a+b*\log(kT/\rm eV)$ we find a very
steep slope ($b=7.9\pm 1.1$) for the June 2018 observation, while in December
2018 and May 2019 the trend is flatter ($b=2.0\pm 0.2$ and $b=3.1\pm 0.4$,
respectively). The median photon index is $\Gamma=3.52\pm 0.13$, and spans a
wide range ($\Gamma=0.8-5.5$) at the lowest luminosities. No clear relation is
found between the photon index and the luminosity.

The emission line is well constrained in all intervals except the most
luminous ones (see central panels of Fig. 8). The energy of the line does not
show any significant trend with luminosity for the June 2018 observation,
while in December 2018, at the highest luminosities, the energy of the line
appears to increase, up to $\sim 1.15$ keV. No clear trend is observed in the
May 2019 observation. During the June 2018 observation the flux of the
Gaussian component increases as the source gets brighter (blue filled circles
in the middle left panel of Fig. 10), while in the December 2018 and May 2019
observations the line flux shows a flat trend with luminosity, and a possible
decrease at the highest luminosities. A possible increase of the line width
with luminosity is observed, with the Gaussian line being broader in December
2018 and May 2019 than in June 2018.

To further examine the relation between spectral changes and luminosity, we
also divided the June 2018 RGS observation into two intervals, based on their
flux. The high-flux interval covers the first 13 ks of the observation
(intervals 1–5 in Fig. 5), while the low-flux interval covers the 21–39 ks
range (intervals 7–9). We then fitted both intervals separately using our best
model for the continuum, which includes a blackbody and a power law
[tbabs$\times$(zpo+zbb)]. The ratio between the model and the data is shown in
Figure LABEL:fig:RGS_highlowflux_june18 for the low (red squares) and high
(black diamonds) flux intervals. No strong change is evident between the two
spectra below 1.2 keV, with the possible exception of the absorption feature
at $E\simeq 0.7-0.8$ keV, which appears to be broader in the low-flux
interval.

Figure 10.— Left panels: Relation between the properties of the broad line at
$\simeq 1$ keV and the luminosity for the June 2018 (blue circles), December
2018 (red diamonds) and May 2019 (black stars) XMM-Newton observations (see
§3.5). Right panel: Same as left panels for the NICER observations (see §4).

####  3.5.2 May 2011

In order to compare the relation between flux ratios in different bands and
the 0.3–10 keV flux with that before the outburst, we analyzed the light
curves of the 2011 observations (see Appendix LABEL:sect:2011xmmobs for
details). The ratios of different energy bands and the 0.3–0.5 keV flux versus
the 0.3–10 keV flux (in bins of 1 ks) are shown in Fig.
LABEL:fig:XMMlc_11_bands_ratios for the 0.75–1, 1–1.3, 1.3–2, 2–3, 3–5, 5–7
and 7–10 keV bands. We fitted fourteen different intervals of this observation
(see bottom panel of Fig. LABEL:fig:xmmobs2011eeufs) with a model that
includes both a power-law and a blackbody component
[tbabs$\times$ztbabs$\times$(zpo+zbb)], leaving $\Gamma$, $N_{\rm H}$ and $kT$
free to vary. We did not find any significant relation between the blackbody
temperature and the luminosity (Table LABEL:tab:fitXMM11epochs).

###  3.6. The broad 1 keV feature

As discussed in §3.5.1, we found clear evidence in the XMM-Newton observations
of a broad feature at 1 keV, consistent with an emission line. The RGS spectra
(see panels A and B of Figs. LABEL:fig:RGS_spec_18 and LABEL:fig:RGS_spec_19
in Appendix LABEL:appendix:RGS_spectra) show that the line might be
asymmetric, with a shape that is reminiscent of the relativistically broadened
profile typically observed in Fe K (e.g., Fabian et al. 2000; Walton et al.
2013) and L (e.g., Fabian et al. 2009) lines. We therefore fitted the line
using the relline model (Dauser et al. 2010, 2013) instead of the symmetric
Gaussian line. We fixed the inner radius in relline to $R_{\rm in}=-1$, so
that it would self-consistently scale with the spin ($a$) of the SMBH, and the
outer radius to $R_{\rm out}=400\rm\,r_{\rm g}$, where $\rm r_{\rm g}=GM_{\rm
BH}/c^{2}$ is the gravitational radius. The two indices of the emissivity
profile were set to have the same value and were left free to vary, while the
distance from the SMBH at which the indices change was fixed to an arbitrary
value. The normalization and energy of the line, as well as the spin of the
SMBH and the inclination angle of the system, were also left free to vary.

We studied the feature at $\sim 1$ keV by fitting the two 2018 XMM-Newton
observations simultaneously (PN, MOS and RGS for the June 2018 observation; PN
and RGS for the December 2018 observation). We did not use the May 2019
observations because the line was less prominent. We substituted the $\sim 1$
keV Gaussian line with a relativistic line in our best model (see §3.1 and
§3.2), which includes a power-law and a blackbody component, as well as
neutral absorption and a second Gaussian line at $\sim 1.8$ keV
[cons$\times$tbabs$\times$ztbabs$\times$mtable{xstar}
(zbb+zpo+relline+zgauss)]. We also included two absorption lines for the
December 2018 observation (see § 3.2). We left all the parameters of the
continuum, of the lines and of the warm absorbers free to vary between the two
observations, setting instead the energy of the broad line, the spin of the
SMBH and the inclination angle to have the same values for both datasets. The
normalization and the index of the emissivity profile was left free in both
observations. The model fits the data well (Stat/DOF=7752/7021), and results
in a spin of $a=0.41_{-0.05}^{+0.07}$, an inclination of $16.5_{-1.5}^{+2}$
degrees, and an energy of the line of $1.37_{-0.03}^{+0.05}$ keV, consistent
with emission from Mg XI. During the first and second observation the index of
the emissivity profile was $8.0_{-0.7}^{+1.2}$ and $4.6_{-0.3}^{+0.5}$,
respectively. The temperature of the blackbody component obtained for the June
observation is consistent with the one we found using a Gaussian line ($102\pm
1$ eV), while that of the December observation is slightly higher ($152\pm 2$
eV). It should be noted however that this fit does not result in a significant
improvement of the chi-squared with respect to a model that includes a
Gaussian line instead of the relativistic line (Stat/DOF=7754/7022).

##  4\. The NICER monitoring campaign

NICER has been observing 1ES 1927+654 for about 450 days, starting from $\sim
150$ days after the beginning of the event. This very intense monitoring,
together with very large effective area of NICER, has allowed us to monitor
the behaviour of the source within great detail. In the following, we discuss
the results obtained by studying the X-ray variability (§4.1) and spectroscopy
(§4.2) of 1ES 1927+654 with NICER observations.

###  4.1. X-ray Variability

The first NICER observations, carried out in May 2018, $\sim 150$ days after
the beginning of the event, found 1ES 1927+654 at a 0.5–10 keV flux level
similar to that observed in the 2011 XMM-Newton observation (see Fig. 1 of
Ricci et al. 2020). However, the X-ray spectral shape of the source was at the
time already very different from that of a typical AGN, with the continuum
being dominated by a very soft component, as also seen in our Swift/XRT
observations. Around the end of May 2020 the flux of the source started to
decline, reaching a minimum of $\sim 10^{40}\rm\,erg\,s^{-1}$ in July/August
2018. In the following 100 days the average X-ray emission increased rapidly,
going even above the initial luminosity level and peaking at $\sim
10^{44}\rm\,erg\,s^{-1}$ in some intervals $\sim 300$ days after the outburst.
In the following 300 days the median luminosity continued increasing. Between
300 and 400 days from the event it was $\log(L_{0.5-10}/\rm
erg\,s^{-1})=43.27$, in the 400–500 days range it increased to
$\log(L_{0.5-10}/\rm erg\,s^{-1})=43.89$ and finally in the last 100 days of
our monitoring campaign it reached $\log(L_{0.5-10}/\rm erg\,s^{-1})=44.25$.
As discussed in Ricci et al. 2020 the X-ray luminosity appears to have an
asymptote at $L_{0.5-10}/\rm erg\,s^{-1}\sim 44.25$. Interestingly, the very
strong intraday X-ray variability that was observed in the first year after
the event, when the source was found to vary of up to two orders of magnitude
in a few hours, decreases strongly for $t\gtrsim 400$ days. In the last few
months of our campaign 1ES 1927+654 is in fact found to vary only of $\sim
0.3-0.5$ dex in luminosity.

Figure 11.— Chi-squared versus degrees of freedom for the NICER spectra of our
X-ray monitoring campaign. The red dashed line shows $\chi^{2}=\rm DOF$.

Figure 12.— Examples of two fits of NICER spectra obtained during a low
($L_{0.3-2}=9.1\times 10^{41}\rm\,erg\,s^{-1}$, left panel) and a high
($L_{0.3-2}=9.7\times 10^{43}\rm\,erg\,s^{-1}$, right panel) flux interval.
The shaded area in the top panels represent the background, while the bottom
panels illustrate the ratio between the best-fitting model (continuous lines
in the middle panel) and the data. During the low-luminosity periods no power-
law component is needed to reproduce the spectra, which is well fit by a
blackbody component (dashed line) and a Gaussian line (dot-dashed line). In
the high-luminosity intervals the power-law component (dot-dot dashed line)
reappears.

###  4.2. X-ray spectroscopy

In the following we first analyze the spectra obtained from the individual
NICER exposures (§4.2.1), and then focus on the spectra obtained by stacking
different observations carried out within a few days to a few weeks from each
other (§4.2.2).

####  4.2.1 Individual spectra

We fitted a total of 232 NICER spectra, excluding 33 exposures for which the
signal-to-noise ratio was not sufficient to perform spectroscopy. The fits
were carried out in an energy range that went from 0.3 keV to $E_{2}$, where
$E_{2}$ is the energy where the source and background rates are equal. Given
the different signal to noise ratios of the observations, due to the very
strong variability of the source, $E_{2}$ went from 0.5 keV up to 5 keV. We
therefore used four different models to reproduce the X-ray emission of 1ES
1927+654.

  * •

The 11 observations in which the maximum energy was $E_{2}<1$ keV were fitted
with Model 1, which included photo-electric intrinsic and foreground
absorption, a blackbody and a power-law component
[tbabs$\times$ztbabs$\times$(zbb+zpo)]. As we did for the XMM-Newton and Swift
observations, foreground absorption was fixed at the Galactic value, and all
redshifts were fixed to $z=0.019422$. Below 1 keV the spectra of these
observations were dominated by the blackbody component, therefore the photon
index of the power law was fixed to $\Gamma=3$, while the normalization was
left free to vary.

  * •

All the exposures for which $1\leq E_{2}/\rm keV<1.8$ were fitted with Model
2, which includes the same component as Model 1 plus a Gaussian line
[(tbabs$\times$ztbabs$\times$(zbb+zpo+zgauss)]. The energy, width and
normalization of the line were left free to vary while, as for Model 1, the
photon index of the power law was fixed to $\Gamma=3$. This model was used for
78 observations.

  * •

The 57 exposures in which $1.8\leq E_{2}/\rm keV<3$ were fitted with Model 3.
The only difference between this model and Model 2 is that the photon index of
the power law component was left free to vary.

  * •

The remaining 86 exposures were fit with Model 4, which considers a cutoff
power-law instead of a simple power-law
[(tbabs$\times$ztbabs$\times$(zbb+zcut+zgauss)].

The models provide typically a good fit, with the median reduced chi-squared
being $\chi^{2}$/DOF=1.03 (see Fig. 11). The NICER observations encompass a
very broad range of 0.5–10 keV luminosities, from $\sim
10^{40}\rm\,erg\,s^{-1}$ to $\sim 10^{44}\rm\,erg\,s^{-1}$. In Fig. 12 we
illustrate two of the NICER spectra, during a low (left panel) and a high
(right panel) luminosity period. The harder-when-brighter behaviour discussed
in the previous sections is very evident when examining the NICER spectra. The
blackbody temperature varies between $\sim 70$ eV and $\sim 200$ eV, and is
found to correlate with the luminosity of the source (middle panel of Fig. 9).
Fitting this trend with the same relation we used for the time-resolved XMM-
Newton observations [$\log(L_{0.3-2}/\rm erg\,s^{-1})=a+b*\log(kT/\rm eV)$,
see §3.5] we find a slope of $b=3.85\pm 0.01$. A break in this $kT-L_{0.3-2}$
relation is observed at $\log(L_{0.3-2}/\rm erg\,s^{-1})\simeq 43.7$, with the
trend being steeper (flatter) at lower (higher) luminosities. Fitting the two
luminosity intervals separately we find $b=7.4\pm 0.3$ for $\log(L_{0.3-2}/\rm
erg\,s^{-1})<43.7$ and $b=0.9\pm 0.2$ for $\log(L_{0.3-2}/\rm erg\,s^{-1})\geq
43.7$.

The properties of the $\sim 1$ keV Gaussian line versus the X-ray luminosity
are illustrated in the right panel of Fig. 10. The energy of the line (top
panel) does not vary significantly up to $\log(L_{0.3-2}/\rm
erg\,s^{-1})\simeq 43.7$, above which it appears to increase up to $\sim 1.15$
keV. Similarly to what we have found in the XMM-Newton observations, we see a
clear increase in the flux of the line (middle panel) with the luminosity.
Interestingly, above $\log(L_{0.3-2}/\rm erg\,s^{-1})\simeq 43.7$ such a trend
disappears, and the flux of the line appears to decrease with the luminosity.
A similar behaviour is observed for the width of the line (bottom panel),
which increases up to $\simeq 200$ eV at $\log(L_{0.3-2}/\rm
erg\,s^{-1})\simeq 43.7$ for then decreasing.

####  4.2.2 Stacked spectra

We also analyzed the spectral variations of 1ES 1927+654 by stacking NICER
data in sixteen different and successive epochs, spanning intervals between a
few days and a few weeks (see Appendix LABEL:sect:stackednicerspecappendix).
We started by using the same approach to the spectral modelling that we used
for the individuals spectra (§4.2.1), based on the energy range in which the
spectra were not dominated by the background. Our spectral models include a
blackbody component plus a Gaussian line [bb;
tbabs$\times$ztbabs$\times$(zbb+zgauss)], a blackbody, a Gaussian line and
either a power law [po; tbabs$\times$ztbabs$\times$(zpo+zbb+zgauss)] or a
cutoff power-law [cut; tbabs$\times$ztbabs$\times$(zcut+zbb+zgauss)]. All the
spectra for which a power-law or a cutoff power-law model was used were also
fitted with a thermally Comptonized plasma [nth;
tbabs$\times$ztbabs$\times$(nthcomp+zbb+zgauss)]. In the latter model the
temperature of the seed photons was fixed to the temperature of the blackbody
component. The results of our analysis are reported in Table
LABEL:tab:fitNICERstackedSpec in Appendix LABEL:sect:stackednicerspecappendix.
For four spectra we used the bb model, while for the remaining twelve spectra
we used the po or cut models, as well as the nth models. For the first
interval the model left strong residuals, suggestive of absorption features,
and resulted in a chi-squared of $\chi^{2}$/DOF=400/98. Adding two ionized
absorbers, using a xstar table that assumes $v_{\rm turb}=300\rm\,km\,s^{-1}$
improved the fit significantly ($\chi^{2}$/DOF=113/92). The other spectra are
well fit by the baseline models, and do not require any additional components.
The temperature of the blackbody component obtained by our fitting changes
when considering the nth model with respect to the po or cut models, with $kT$
being typically lower for the former model. However, as shown in the bottom
panel of Fig. 9, regardless of the model adopted for the hard X-ray component,
we find that the blackbody temperature increases for increasing luminosities.

##  5\. Discussion

Figure 13.— Left panels: Relation between the contribution of the different
spectral components to the 0.3–2 keV flux and the total luminosity for the
June 2018 (filled blue circles), December 2018 (filled red diamonds) and May
2019 (empty black stars) XMM-Newton observations. The top and middle panel
shows the flux of the power-law ($F_{\rm powerlaw}$) and blackbody ($F_{\rm
blackbody}$) component, respectively, while the bottom panel shows their
ratio. Right panels: same as left panels for the NICER observations.

###  5.1. X-ray spectral properties 1ES 1927+654

The X-ray spectral shape of 1ES 1927+654 inferred from our X-ray monitoring
campaign is very different from what has been observed in typical AGN; the
main peculiarities of this object are the supersoft continuum, dominated by a
blackbody component ($kT\sim 80-200$ eV), the disappearance of the power law
component (Ricci et al. 2020), the broad emission features at $\sim 1$ keV and
$\simeq 1.8$ keV, and the extremely low cutoff energy of the power-law
($E_{\rm C}\sim 3$ keV). In the following we discuss first the X-ray
characteristics of 1ES 1927+654 inferred by previous studies (§5.1.1), and
then the exceptional properties observed over the past $\sim 2$ years
(§5.1.2).

####  5.1.1 Previous X-ray observations of 1ES 1927+654

1ES 1927+654 was first detected in the X-ray band by the Einstein slew survey,
and classified as an AGN (Elvis et al. 1992, see also Perlman et al. 1996 and
Simcoe et al. 1997). The source was later detected by the ROSAT mission in the
0.1–2.4 keV band, and was reported in the all-sky survey bright source
catalogue (Voges et al. 1999). ROSAT observations showed that 1ES 1927+654 is
very variable, and a PSPC pointed observation in December 1998 detected a
strong X-ray flare. The 0.1–2.4 keV luminosities inferred by the all-sky
survey and by the pointed observation were $4.6\times 10^{43}\rm\,erg\,s^{-1}$
and $\sim 9\times 10^{43}\rm\,erg\,s^{-1}$, respectively. Boller et al.
(2003b) studied the ROSAT and Chandra spectra of this AGN, and found a rapid
and large-amplitude variation, together with a very steep continuum:
$\Gamma=2.6\pm 0.3$ in the 0.1–2.4 keV band and $2.7\pm 0.2$ in the 0.3–7 keV
band. During the ROSAT all-sky survey the source showed significant spectral
variability, and the spectral variations did not display any clear correlation
with the flux. The pointed PSPC observation showed hardness ratio variability,
although also in this case the spectral changes were not correlated with the
flux variations.

Gallo et al. (2013) analyzed the 2011 XMM-Newton observation of 1ES 1927+654
(28 ks), together with a $\sim 70$ ks Suzaku observation, also carried out in
2011. The luminosity of the source in the 0.3–10 keV band was
$8.63^{+0.05}_{-0.05}\times 10^{42}\rm\,erg\,s^{-1}$, very similar to that of
the June 2018 XMM-Newton observation333$L_{0.3-10}=8.54^{+0.10}_{-0.34}\times
10^{42}\rm\,erg\,s^{-1}$. Gallo et al. (2013) applied several spectral models
to the data, all taking into account a power law component, plus different
features, including a blackbody, two neutral partial covering absorbers, two
ionized partial covering absorbers and blurred reflection. During these
observation the narrow Fe K$\alpha$ feature, which is ubiquitously found in
AGN (e.g., Nandra & Pounds 1994; Shu et al. 2010), was not detected, and only
an upper limit on the EW could be obtained ($EW\lesssim 30$ eV). This is
considerably lower than what would be expected for an AGN with a 2–10 keV
luminosity of $3.1\times 10^{42}\rm\,erg\,s^{-1}$: considering the well known
anti correlation between the X-ray luminosity and the Fe K$\alpha$ EW one
would in fact expect $EW\simeq 100$ eV (e.g., Iwasawa & Taniguchi 1993;
Bianchi et al. 2007; Ricci et al. 2013a, b, 2014; Boorman et al. 2018; Matt &
Iwasawa 2019). Using a power-law plus blackbody model, similar to our baseline
model, resulted in $\Gamma\simeq 2.3$, a value significantly different from
the median value inferred for AGN in the local ($z\lesssim 0.05$) Universe
($\Gamma=1.8-1.9$; e.g., Nandra & Pounds 1994; Piconcelli et al. 2005; Winter
et al. 2009; Lubiński et al. 2016; Ricci et al. 2011, 2017). Most unobscured
AGN show a strong component below $\simeq 1$ keV dubbed the soft excess, which
can be typically reproduced by a blackbody component with a temperature of
$\sim 100$ eV (e.g., Gierliński & Done 2004; Winter et al. 2009; Boissay et
al. 2016; Ricci et al. 2017; García et al. 2019), and is believed to be either
due to relativistic reflection from the innermost regions of the disk (e.g.,
Crummy et al. 2006) or to emission from a cooler Comptonizing region, possibly
in the form of an atmosphere of the accretion disk (e.g., Done et al. 2012).
In the case of 1ES 1927+654 the temperature of the blackbody was higher
($kT\simeq 170$ eV) than what is typically found in local AGN.

Ionized absorbers have been systematically found in the X-ray spectra of
nearby AGN (e.g., Costantini et al. 2007; Pounds & Vaughan 2011), in some
cases showing a high degree of complexity (e.g., Kaastra et al. 2014, 2018)
and reaching very high velocities (e.g., Tombesi et al. 2010; Kosec et al.
2018; Boissay-Malaquin et al. 2019; Walton et al. 2019 ). Gallo et al. (2013)
found that both a ionized partial covering model and a blurred reflection
model are able to well reproduce the data. In the case of the partially
covering ionized absorber two outflows with a velocity of $\sim 0.3$c and
column densities of $3\times 10^{22}\rm\,cm^{-2}$ and $6.6\times
10^{23}\rm\,cm^{-2}$ were needed, while the blurred model required the system
to be observed edge-on.

####  5.1.2 The X-ray spectral shape after the optical/UV outburst

After the optical/UV outburst (late December 2017) the X-ray spectral shape of
1ES 1927+654 drastically changed with respect to the 2011 observation. Our
analysis showed that the spectrum is dominated by a blackbody component, and
that the power-law was extremely faint in May/June 2018, was undetected in
July–August 2018, and re-appeared only a few months later, when the flux of
the source increased (Ricci et al. 2020). In § 3.3 we discussed how neither
partial covering or blurred reflection models can explain the X-ray properties
of 1ES 1927+654 observed after the optical event.

A supersoft X-ray continuum has so far been observed only in handful of
objects classified as AGN (e.g., Terashima et al. 2012; Miniutti et al. 2013,
2019; Sun et al. 2013; Lin et al. 2017a; Shu et al. 2017, 2018; Giustini et
al. 2020), and never before was the power-law observed in the process of
disappearing. The XMM-Newton and the NICER observations show that the flux of
the power-law and that of the blackbody component increases with the 0.3–2 keV
luminosity of the source (top and middle panels of Fig. 13, see also Ricci et
al. 2020). While for the same luminosity interval covered by the 2011 XMM-
Newton observation [$\log(L_{0.3-2}/\rm erg\,s^{-1})\sim 42.5-43$] the flux of
the power-law component after the event is considerably lower than in 2011
(green empty squares), that of the blackbody component is $\sim 0.7$ dex
higher. Interestingly, the ratio between the flux of the power law and that of
the blackbody shows a clear increase with the luminosity (see Fig. 3 of Ricci
et al. 2020), going from $\sim 10^{-2}$ for $\log(L_{0.3-2}/\rm
erg\,s^{-1})\lesssim 43$ to $\sim 10^{0.4}$ for $\log(L_{0.3-2}/\rm
erg\,s^{-1})\sim 44$ (Fig. 13). The transition between having a blackbody-
dominated flux to a power-law dominated one is found at $\log(L_{0.3-2}/\rm
erg\,s^{-1})\sim 43.7$, and at the highest luminosities the source goes back
to having a ratio comparable to that observed in May 2011 (Ricci et al. 2020).

The power-law is very steep ($\Gamma\sim 3$), and our joint XMM-Newton/NuSTAR
observations showed that the addition of a cutoff with a very low energy
($E_{\rm cut}\sim 3$ keV, see Table 1) is required (see panels B and C of Fig.
4). This value is considerably lower than what is typically observed in nearby
accreting SMBHs. From the analysis of the broadband X-ray properties of the
$\sim 840$ AGN detected by Swift/BAT during the first 70 months of operations,
Ricci et al. (2017) found that the median cutoff energy is $200\pm 29$ keV
($210\pm 36$ keV for the unobscured AGN). Previous BeppoSAX observations found
consistent results ($230\pm 22$ keV for unobscured AGN, see Dadina 2007,
2008). Recent NuSTAR observations have also confirmed that the cutoff energy
is typically found between $\sim 40$ keV (Kara et al. 2017) and $\sim 200-300$
keV (e.g., Tortosa et al. 2017, 2018; Kamraj et al. 2018; Baloković et al.
2020), with the highest recorded value thus far being for NGC 5506 ($E_{\rm
cut}=720^{+130}_{-190}$ keV, Matt et al. 2015). A relation between the cutoff
energy and the Eddington ratio was recently found by Ricci et al. (2018), who
showed that sources with higher $\lambda_{\rm Edd}$ tend to have cooler
coronae. This could be explained by the fact that AGN typically tend to avoid
the region in the temperature-compactness parameter space where runaway pair
production dominates (Bisnovatyi-Kogan et al. 1971; Svensson 1984; Fabian et
al. 2015, 2017). Extending the relation of Ricci et al. (2018) to higher
Eddington ratios, assuming that the trend does not change at $\lambda_{\rm
Edd}\gg 1$, one would expect to find such low values of $E_{\rm cut}$ only for
$\lambda_{\rm Edd}\sim 30$. Assuming instead the relation between the photon
index and the Eddington ratio (e.g., Shemmer et al. 2006; Brightman et al.
2013; Trakhtenbrot et al. 2017), and in particular the one obtained by
Trakhtenbrot et al. (2017) considering a cutoff power-law model
($\Gamma=2.00+0.167\log\lambda_{\rm Edd}$), one would need $\lambda_{\rm
Edd}\sim 6$ to obtain the photon index found by our observations. It should be
however remarked that the physical properties of the corona at these extreme
Eddington ratios, where the accretion flow might be very different (e.g.,
Abramowicz et al. 1988), are still unclear.

The black hole mass of 1ES 1927+654, inferred by Trakhtenbrot et al. (2019a)
by analyzing broad Balmer lines, is $1.9\times 10^{7}\rm\,M_{\odot}$. As
discussed in Ricci et al. (2020), the black hole mass could however be quite
different, due to the fact that the BLR clouds might not have had the time to
virialize. This is also shown by the variable FWHM of both H$\alpha$ and
H$\beta$ (Li et al. in prep.; see Appendix A of Ricci et al. 2020). The
stellar mass of the galaxy ($2.3\times 10^{9}M_{\odot}$) suggests $M_{\rm
BH}\sim 10^{6}M_{\odot}$ (Kormendy & Ho 2013). At $\gtrsim 300$ days after the
optical outburst the optical/UV flux had decreased strongly (Trakhtenbrot et
al. 2019a), and most of the emission is observed in the X-rays (Ricci et al.
2020). Assuming that the X-ray bolometric correction is $\kappa_{\rm X}\sim
1$, and $M_{\rm BH}\sim 10^{6}M_{\odot}$, the maximum observed X-ray
luminosity would correspond to $\lambda_{\rm Edd}\sim 1$. Therefore, we can
conclude that the maximum Eddington ratio would be $\lambda_{\rm
Edd}\simeq\kappa_{\rm X}\gtrsim 1$, and it is likely that the source was
super-Eddington for a considerable fraction of its time during our monitoring
campaign. In a forthcoming publication (Li et al. in prep.) we will discuss in
detail the spectral energy distribution of this source. It should be noted
that some of the properties of 1ES 1927+654 after the optical outburst, such
as its softness, are similar to those of some of the most extreme narrow-line
Seyfert 1s, e.g. 1H 0707$-$495 and IRAS 13224$-$3809 (e.g., Boller et al.
2003a; Fabian et al. 2004). However none of these sources shows a power-law
component as steep and faint as 1ES 1927+654. Interestingly, both 1H
0707$-$495 and IRAS 13224$-$3809 show an excess due to a relativistically-
broadened Fe L line at 0.9 keV (Fabian et al. 2009, 2013). The energy of this
line is however lower than that of the feature observed in the X-ray spectrum
of 1ES 1927+654 ($E=1.01\pm 0.01$ keV, see Table 1).

Similarly to the 2011 observations, we do not see any sign of a Fe K$\alpha$
line at 6.4 keV. This is not surprising, due to the very low flux of the
source at 7.1 keV. However, we could in principle expect to see an Fe
K$\alpha$ echo from more distant material (e.g. the BLR, or the torus), which
is not found in any of our X-ray observations. This, together with the absence
of a Compton hump, suggests that the circumnuclear environment of 1ES 1927+654
is particularly poor in gas and dust. The two broad emission lines observed at
$\sim 1$ keV and $\simeq 1.8$ keV are also extremely puzzling, and they have
not been observed in any other AGN thus far. The line at $\sim 1$ keV could be
associated to Ne x (1.02 keV) or to the ionized Fe-L emission from Fe xx-xiv.
In the latter case the increase in the energy of the line observed at the
highest luminosities (top panels of Fig. 10) might be related to the increase
in the ionization state of the circumnuclear material. The second line is
found at $E\sim 1.7$ keV during the first XMM-Newton observation and at $\sim
1.9$ during the second and third observations. While the line at $\sim 1.7$
keV could be Si K$\alpha$ (1.74 keV) or to Al xiii ($1.73$ keV), it is unclear
to what transition the line at $\sim 1.9$ could be associated. It should be
stressed that we cannot exclude that these two broad features are produced by
strong and very turbulent ionized outflows, and we refer to a future
publication (Masterson et al. in prep.) for a detailed analysis.

###  5.2. The peculiar X-ray spectral variability

Our X-ray monitoring campaign (§3–4) showed a clear relation between the
spectral shape and the flux of the source. This was observed in all X-ray
observations, with the spectrum becoming harder as the flux increased. This
effect was first illustrated, in a model-independent way, by the fact that: i)
the flux ratios between different bands and the softest (0.3–0.5 keV) band
follow the same pattern as the overall variability (see panels C, D and E of
Fig. LABEL:fig:XMMlc_18_bands and panels D, E, F, G, H of Fig.
LABEL:fig:XMMlc_18_december_bands); ii) when we plot the flux ratios in each
band as a function of the total flux (see Fig. LABEL:fig:XMMlc_18_ratios in
Appendix LABEL:appendix:XMM_lightcurves and Fig. LABEL:fig:NICER_HR in
Appendix LABEL:appendix:HRnicer for XMM-Newton and NICER, respectively) we see
a very clear correlation in all bands, up to the 2–3 keV bin for XMM-Newton
(i.e., the limit of our EPIC/PN detections); iii) comparing the spectra of the
highest and lowest flux intervals (top panels of Fig. 8), and looking at their
ratios (bottom panels of Fig. 8) we see a clear hardening. This effect is due
to both the increase of the temperature of the blackbody with the X-ray
luminosity (Fig. 9), and to the increase of the ratio between the flux from
the power-law and blackbody component with the luminosity (see Fig. 13 and
Ricci et al. 2020). The harder-when-brighter behaviour does not appear to be
associated with variations of the outflowing material. Fitting the X-ray
spectra with a blackbody component in a range that is not strongly affected by
the ionized absorbers ($0.3-0.5$ keV) we obtain a significant difference in
the temperature of the blackbody of low and high-luminosity intervals. In
particular for the June 2018 XMM-Newton observation we obtain
$kT=122_{-6}^{+7}\rm\,eV$ and $kT=88_{-6}^{+7}\rm\,eV$ for the brightest
(interval 2) and faintest (interval 8) periods (see Fig. 5). Moreover, no
clear difference is observed in the RGS spectra of two intervals with very
different fluxes during the June 2018 observation (Fig.
LABEL:fig:RGS_highlowflux_june18 in Appendix
LABEL:appendix:exposuresintervals).

Unobscured AGN typically show a softer-when-brighter behaviour (e.g.,
Sobolewska & Papadakis 2009) in the X-rays. This is likely related to the
steeping of the power-law component for increasing luminosities (e.g., Shemmer
et al. 2006), which is also found in Galactic black hole binaries (e.g., Wu &
Gu 2008). On the other hand, the temperature of the blackbody component,
adopted to reproduce the soft excess, does not vary with the source luminosity
(e.g., Gierliński & Done 2004). At low Eddington ratios ($\lambda_{\rm
Edd}\lesssim 10^{-2}$) it has been shown that AGN can present a harder-when-
brighter behaviour (e.g., Gu & Cao 2009; Constantin et al. 2009; Younes et al.
2011; Emmanoulopoulos et al. 2012; Connolly et al. 2016), which is however
entirely related to the power-law component, and that results in a negative
correlation between $\Gamma$ and $\lambda_{\rm Edd}$. Such a trend has also
been observed in black hole binaries in the low/hard state (e.g., Kalemci et
al. 2004; Yamaoka et al. 2005; Yuan et al. 2007), and it has been argued that
such behaviour might be related to the fact that accretion in these objects
happens through an advection-dominated accretion flow (ADAF; Ichimaru 1977; Ho
2008; Yuan & Narayan 2014). Such a trend has been observed also in blazars
(e.g., Krawczynski et al. 2004), where shocks and Synchrotron Self-Compton
processes in the jet are thought to be responsible for the hardening of the
X-ray radiation (Ghisellini et al. 2009). There are however important
differences between the case of 1ES 1927+654 and that of low Eddington ratio
AGN, black hole binaries in the low/hard state and blazars, which suggest that
the physical mechanisms responsible for the spectral variability might be very
different. The spectral shape of 1ES 1927+654 is in fact extremely soft, with
a very strong blackbody component, while low Eddington ratio AGN and blazars
have significantly harder continua. Moreover, the power-law component does not
show any spectral variation with luminosity, contrary to what is found in ADAF
and strongly jetted AGN.

###  5.3. The origin of the blackbody component

In Ricci et al. (2020) we discuss how the event that created the broad lines,
and drastically transformed the X-ray properties of 1ES 1927+654, might be
related to a TDE in an AGN (e.g., Merloni et al. 2015; Blanchard et al. 2017;
Liu et al. 2020). The blackbody component, observed in all X-ray spectra of
1ES 1927+654 after the optical event, could be produced in an accretion disk.
However, assuming $M_{\rm BH}\sim 10^{6}\rm M_{\odot}$, one would expect the
maximum temperature to be $\sim 35$ eV ($\sim 100$ eV) at the highest
luminosity for a non-rotating (maximally rotating) black hole, assuming
$\kappa_{\rm X}\simeq 1$ (see Eq. LABEL:eq:tempmax in Appendix
LABEL:sect:xmmobs_spec_june_diskbb). This is considerably lower than that
recovered from our fitting ($kT\sim 200$ eV or $kT\sim 235$ eV, for a
blackbody or a disk blackbody, respectively). As discussed in §5.1.2, it is
likely that $\kappa_{\rm X}\geq 1$, which could lead to a disk temperature
consistent with our observations (particularly if $\kappa_{\rm X}\gtrsim 15$).
Moreover, the colour temperature correction for $M_{\rm BH}\sim 10^{6}\rm
M_{\odot}$ could be $\gtrsim 2$ (Done et al. 2012), which would also
contribute to increasing the observed blackbody temperature.

The blackbody radius varies between $\sim 4\times 10^{9}\rm\,cm$ (at $\sim
10^{40}\rm\,erg\,s^{-1}$) and $\sim 7\times 10^{10}\rm\,cm$ (at $\sim
10^{44}\rm\,erg\,s^{-1}$), assuming $M_{\rm BH}\sim 10^{6}\rm M_{\odot}$ and
$\kappa_{\rm X}\sim 1$. This is lower than the gravitational radius of the
source, $R_{\rm g}=GM_{\rm BH}/c^{2}\sim 1.5\times 10^{11}\rm\,cm$ (assuming
$M_{\rm BH}\sim 10^{6}\rm M_{\odot}$), which might suggest that the blackbody
emission cannot be easily explained by a standard accretion disk, unless a
high inclination angle with respect to the normal to the disk is assumed. Even
if the black hole mass was considerably lower (e.g., $\sim 10^{5}\rm
M_{\odot}$), the gravitational radius ($\sim 1.5\times 10^{10}\rm\,cm$) would
still be larger than the blackbody radius at the lowest luminosities observed
here. Only a black hole mass of $\lesssim 5\times 10^{4}M_{\odot}$ would be
consistent with the observations. In fact, while the blackbody radius would be
larger than the gravitational radius at the highest luminosities for
$\kappa_{\rm X}\gtrsim 5$, at the lowest luminosities one would need a rather
extreme value of the X-ray bolometric correction ($\kappa_{\rm X}\gtrsim
1200$). However, at this time (i.e. $\sim 200$ days after the event) the
optical emission was remarkably stronger than in the X-rays (Trakhtenbrot et
al. 2019a; Ricci et al. 2020), and the $5100\AA$ luminosity was $\sim
10^{43}\rm\,erg\,s^{-1}$, which could correspond to a bolometric luminosity of
$L\sim 10^{44}\rm\,\,erg\,s^{-1}$ (e.g., using the bolometric corrections of
Kaspi et al. 2000). This would lead to a blackbody radius of $\sim 4\times
10^{11}\rm\,cm>R_{\rm g}$, resolving this discrepancy. Therefore, we conclude
that the soft X-ray emission could be associated to an accretion disk,
provided that the X-ray bolometric corrections are high enough.

1ES 1927+654 shows very strong spectral variability during our campaign, with
the temperature of the blackbody component increasing with the luminosity
(Fig. 9). This behaviour is very different from what is typically observed in
TDEs, which show little evolution in the temperature of the blackbody
component. Interestingly, a similar behaviour has been observed also in the
narrow-line Seyfert 1 IRAS 13224$-$3809 (Chiang et al. 2015), and it was
explained as being possibly related to the heating of the inner disc by the
X-ray corona and by the effect of light bending, which allows radiation
emitted and/or reflected by the disk to illuminate its surface. If the event
were due to a TDE in a pre-existing AGN, part of the X-ray radiation could be
produced in the shocks between the debris and the disk, which would produce
the observed harder-when-brighter behaviour. The latter scenario will be
tested in details in a forthcoming publication (Masterson et al. in prep.).
Interestingly, such behaviour has been recently observed also in a candidate
TDE with peculiar properties (Lin et al. 2017b), as well as in recent
observations of supersoft X-ray AGN (e.g., Miniutti et al. 2019; Shu et al.
2017; Giustini et al. 2020), objects which could also be associated to TDEs
(e.g., Shu et al. 2018; King 2020).

###  5.4. 1ES 1927+654 and true type-2 AGN

Before becoming a type-1 AGN, 1ES 1927+654 was considered to be one of the
clearest cases of true type-2 AGN (e.g., Tran et al. 2011), i.e. unobscured
sources lacking the BLR (e.g., Tran 2001; Panessa & Bassani 2002). A large
fraction of these objects are found to accrete at $\lambda_{\rm Edd}\lesssim
10^{-2}$ (e.g., Bianchi et al. 2012b; Panessa et al. 2009). It has been argued
that this would be in agreement with models that predict the disappearance of
the BLR at low Eddington ratios (e.g., Nicastro 2000), such as the disc-wind
scenario (e.g., Elitzur & Ho 2009; Elitzur et al. 2014). In some cases,
however, these sources have been found to have high accretion rates (e.g., Ho
et al. 2012), which led Elitzur & Netzer (2016) to argue that the dependence
on black hole mass might relax the limits on the accretion rate in the disk-
wind model. A number of obscured AGN were also thought to lack a BLR, as
suggested by the non-detection of polarized broad lines (e.g., Tran 2003).
Marinucci et al. (2012) found that most these objects have low Eddington
ratios ($\lambda_{\rm Edd}<10^{-2}$), possibly also in agreement with the
disk-wind scenario. However, thanks to VLT/FORS2 optical spectropolarimetry
observations, Ramos Almeida et al. (2016) were able to find polarized broad
lines in several of these objects, questioning whether the lack of detection
of broad lines was only related to dilution due to stellar emission in the
host galaxy. Recently, using Hubble Space Telescope (HST) spectroscopy,
Bianchi et al. (2019) found that the archetypal true type-2 AGN, NGC 3147,
actually has very broad lines, which show an asymmetric relativistic profile.
The fact that the BLR was not detected before in this objects is due to: i)
the very broadened emission profile, caused by the high mass ($2.5\times
10^{8}\rm\,M_{\odot}$) of the black hole and by the small distance ($\sim
80\rm\,r_{\rm g}$) between the SMBH and the BLR (due to the low luminosity of
the object, e.g. Kaspi et al. 2005); ii) the intrinsic weakness of the optical
emission with respect to the host galaxy. NGC 3147 has in fact a 2–10 keV
luminosity of $3.3\pm 0.1\times 10^{41}\rm\,erg\,s^{-1}$ ($\lambda_{\rm
Edd}\simeq 10^{-4}$, Bianchi et al. 2017).

These results seem to suggest that most true type-2 AGN are just type-1 AGN in
which the broad lines are too broad and/or too faint to be detected. This
might also be important for changing-look AGN (e.g., Dexter et al. 2019;
Frederick et al. 2019; Yan et al. 2019). So far no object has been found to
show changes in both the level of obscuration and in the optical type.
Therefore, at the beginning/end of the changing-look event, a fraction of
sources would be classified as unobscured type-2 AGN. In 2011, before the
outburst, 1ES 1927+654 had a 2–10 keV luminosity an order of magnitude larger
than NGC 3147 ($3.3\times 10^{42}\rm\,erg\,s^{-1}$, Gallo et al. 2013). Using
the relation between the radius of the BLR and the X-ray luminosity inferred
by Kaspi et al. (2005), we find that the BLR would lie at $\sim 5$ light days
from the SMBH, which corresponds to $\sim 4,500\rm\,r_{\rm g}$ for $M_{\rm
BH}=1.9\times 10^{7}M_{\odot}$ ($\sim 88,000\rm\,r_{\rm g}$ for $M_{\rm
BH}=10^{6}M_{\odot}$), a factor of $\sim 60$ ($\sim 1200$) larger than NGC
3147. One would therefore not expect to see extremely broad lines in this
object. From the relation between the luminosity of the broad H$\alpha$ line
($L_{\rm bH\alpha}$) and that in the X-ray band (Stern & Laor 2012), we would
expect $L_{\rm bH\alpha}\simeq 10^{42}\rm\,erg\,s^{-1}$. As shown by Stern &
Laor (2012; see their Fig. 14) this is considerably higher than the upper
limit on the broad H$\alpha$ line ($L_{\rm bH\alpha}\lesssim 3\times
10^{39}\rm\,erg\,s^{-1}$) inferred by Tran et al. (2011) for 1ES 1927+654.
This, together with the lack of Fe K$\alpha$ emission and of a Compton hump,
suggests that before the event 1ES 1927+654 might have had accretion
properties rather different from those of typical AGN.

###  5.5. 1ES 1927+654 and changing-look AGN

Over the past years several dozens of changing-look AGN have been discovered
at different wavelengths (e.g., Yang et al. 2018; MacLeod et al. 2019; Sheng
et al. 2020; Hon et al. 2020). A few of these objects have been followed up
and studied in detail in the X-ray band. 1ES 1927+654 shows several extreme
characteristics that have not been observed in other changing-look AGN. In
particular, 1ES 1927+654 is the only changing-look AGN that has been found to
undergo a complete transformation of its X-ray spectral properties, and to
show X-ray variability of over four orders of magnitude on timescales of
months. Moreover, another peculiarity of 1ES 1927+654 is that the broad
optical lines were found to disappear a few months after they were created
(Trakhtenbrot et al. 2019a).

Several changing-look AGN show little X-ray spectral variability, and
characteristics consistent with those of typical AGN. NGC 1566 was found to
transition from Sy 1.8-19 to Sy 1.2 (Oknyansky et al. 2019), following an
increase by a factor of $\sim 40-70$ in the X-rays. After the changing-look
event, at the peak of the X-ray luminosity, its overall X-ray spectral
properties however are not remarkably different from those of nearby AGN
(Parker et al. 2019). HE 1136$-$2304 brightened in the X-rays by a factor of
$\sim 30$, which was accompanied by an increase in the flux of the Balmer
lines, causing a transition from a Sy 1.9 to a Sy 1.5 (Parker et al. 2016).
The X-ray properties of this source are also not very different from those of
typical AGN, with the source showing a well defined power-law component. Wang
et al. (2020) found that the X-ray luminosity decreases (increases) as UGC
3223 transitions from Sy 1.5 to Sy 2 (Sy 2 to Sy 1.8), but did not find
evidence of strong X-ray spectral variations.

On the other hand, Noda & Done (2018) showed that the appearance/disappearance
of broad optical lines in Mrk 1018 (Husemann et al. 2016; McElroy et al. 2016;
Krumpe et al. 2017) could be directly associated to the presence/absence of a
strong soft excess in the X-rays, which extends to the UV and is responsible
for the ionizing photons responsible for the emission lines. Noda & Done
(2018; see also Ruan et al. 2019a, b) discussed that these variations are
similar to the soft-to-hard state transition in black hole binaries, where the
inner disc is replaced by an advection dominated accretion flow as the
Eddington ratio decreases. This behaviour is remarkably different from what
was observed in 1ES 1927+654, which showed an extremely strong soft X-ray
component even after the disappearance of the broad lines.

The properties of 1ES 197+654 seem to suggest that the changing-look event is
very different from those observed in the aforementioned sources, and that it
could be associated to a TDE (Ricci et al. 2020). An alternative explanation
for the origin of the event in 1ES 1927+654 has been recently proposed by
Scepi et al. (2021), who argued that the inversion of magnetic flux in a
magnetically arrested disk could explain its X-ray and optical light-curves,
as well as the observed timescales.

##  6\. Summary and conclusions

We reported here the results obtained by our 2018/2019 X-ray monitoring
campaign of the changing-look AGN 1ES 1927+654. The source, which was
previously classified as an unobscured type 2 AGN ($N_{\rm H}\simeq
10^{20}\rm\,cm^{-2}$), was found to have significantly increased its optical
flux in March 2018. Pre-discovery detections from ATLAS showed that the event
started on December 23 2017. Our optical spectroscopic follow-up campaign
showed, for the first time, evidence of strong broad Balmer emission lines in
this source, $\sim 1-3$ months after the optical flux rise (Trakhtenbrot et
al. 2019a). Our X-ray monitoring campaign includes 265 NICER (for a total of
678 ks) and 14 Swift/XRT (26 ks) observations, as well as three simultaneous
XMM-Newton/NuSTAR (158/169 ks) exposures.

In the X-rays, 1ES 1927+654 shows a behaviour unlike any other AGN. The main
characteristics of the source are the following.

  * •

After the optical/UV outburst the source is found to show a super soft X-ray
continuum, with the emission being almost entirely due to a blackbody
component ($kT\simeq 100$ eV). The power-law component, ubiquitously found in
AGN, and which was present in previous X-ray observations, had almost
completely disappeared (top panel of Fig. 3 and left panel of Fig. 12).

  * •

When the flux of the source increases a steep power law component re-appears.
The ratio between the power-law and blackbody flux increases with the
luminosity, with the power-law dominating at $\log(L_{0.3-2}/\rm
erg\,s^{-1})\gtrsim 43.7$ (Fig. 13 and Ricci et al. 2020).

  * •

The source shows extremely strong X-ray variability, with an amplitude of two
orders of magnitude on timescales of a few hours (Fig. 5), and of up to four
orders of magnitudes on timescales of a few months (Fig. 1 of Ricci et al.
2020).

  * •

The variability spectrum inferred from XMM-Newton observations (Fig. 6) shows
that the peak of the variability moves to higher energies when the X-ray
luminosity increases. The X-ray variability is found to be completely
disconnected from the UV variability (§3.4.2).

  * •

A simple power-law component cannot reproduce the curvature above a few keV,
and an extremely low energy cutoff ($E_{\rm cut}\sim 3$ keV) is needed when
NuSTAR data are included (see panels B and C of Fig. 4).

  * •

The spectral shape shows a clear relation with the luminosity, becoming harder
when brighter. This is due to: i) relative increase of the flux from the
power-law component (bottom panels of Fig. 13); ii) a higher temperature of
the blackbody component (see bottom panels of Fig. 8). We find that $kT$ shows
a tight dependence on the luminosity in all our observations (Fig. 9). The
harder-when-brighter behaviour is also clearly observed in a model-independent
fashion when looking at the hardness ratios (Fig. LABEL:fig:XMMlc_18_ratios in
Appendix LABEL:appendix:XMM_lightcurves and Fig. LABEL:fig:NICER_HR in
Appendix LABEL:appendix:HRnicer for XMM-Newton and NICER, respectively).

  * •

Two prominent features at $\sim 1$ keV and $\sim 1.8$ keV were found in the
spectra (see panels B of Figs. 2 and 4), and could be reproduced by using
broad Gaussian lines. The broad feature at $\sim 1$ keV is found in all
observations, with the exception of the ones in which the source was most
luminous (central panels of Fig. 8).

The unique characteristics of 1ES 1927+654 in the X-ray band suggests that it
belongs to a new type of changing-look AGN, which underwent some catastrophic
event that restructured its accretion flow. The lack of X-ray flux above 2 keV
right after the detection of the optical/UV event implies that the event
destroyed the X-ray corona, and possibly also the innermost regions of the
accretion flow. The corona appears to be in the process of being reformed as
the X-ray luminosity of the source increases. In Ricci et al. (2020) we
speculated that the characteristics of 1ES 1927+654 might match what would be
expected from the interaction between a tidally disrupted star with an
accretion disk around a SMBH, which would empty the innermost regions of the
accretion flow (Chan et al. 2019, 2020, 2021). Future observational studies
and simulations will be able to better understand the link between TDEs and
flaring AGN such as 1ES 1927+654.

In a forthcoming paper (Masterson et al. in prep.) we will test alternative
X-ray spectral models, with the goal of understanding the origin of the broad
features at $\sim 1$ keV and $\sim 1.8$ keV. Soft X-ray surveys, such as the
one that is currently being carried out by eROSITA (Merloni et al. 2012), or
those that will be performed by the future mission Einstein Probe (Yuan et al.
2015) could potentially detect several objects like 1ES 1927+654 in the next
few years.

We acknowledge XMM-Newton, NuSTAR and Swift for the DDT observations they
kindly guaranteed us. We thank the referee for their report, which helped us
improve the paper. LH acknowledges financial support from the National Key R&D
Program of China grant No. 2016YFA0400702, and the National Science Foundation
of China grants No. 11721303 and 11991052. CR acknowledges support from the
Fondecyt Iniciacion grant 11190831. BT acknowledges support from the Israel
Science Foundation (grant No. 1849/19). IA is a CIFAR Azrieli Global Scholar
in the Gravity and the Extreme Universe Program and acknowledges support from
that program, from the European Research Council (ERC) under the European
Union’s Horizon 2020 research and innovation program (grant agreement number
852097), from the Israel Science Foundation (grant number 2752/19), from the
United States - Israel Binational Science Foundation (BSF), and from the
Israeli Council for Higher Education Alon Fellowship. PG acknowledges support
from STFC and a UGC-UKIERI Thematic Partnership. DA acknowledges support from
the Royal Society. CHC is partially supported by ERC advanced grant “TReX”. PK
acknowledges support from the Science and Technology Facilities Council. CR
acknowledges T. Kallman, F. Bauer, C.S. Chang and the Santiago AGN community
for useful discussion. Based on observations with the NASA/ESA/CSA Hubble
Space Telescope obtained [from the Data Archive] at the Space Telescope
Science Institute, which is operated by the Association of Universities for
Research in Astronomy, Incorporated, under NASA contract NAS5-26555. Support
for Cycle 25 Program GO-15604 was provided through a grant from the STScI
under NASA contract NAS5-26555. Facilities: NICER, NuSTAR, Swift, XMM-Newton

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