# A re-emerging bright soft-X-ray state of the changing-look Active Galactic
Nucleus 1ES 1927+654: a multi-wavelength view

Ritesh Ghosh Center for Space Science and Technology, University of Maryland
Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA.  Astrophysics
Science Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA.
Center for Research and Exploration in Space Science and Technology,
NASA/GSFC, Greenbelt, Maryland 20771, USA  Sibasish Laha Center for Space
Science and Technology, University of Maryland Baltimore County, 1000 Hilltop
Circle, Baltimore, MD 21250, USA.  Astrophysics Science Division, NASA Goddard
Space Flight Center, Greenbelt, MD 20771, USA.  Center for Research and
Exploration in Space Science and Technology, NASA/GSFC, Greenbelt, Maryland
20771, USA  Eileen Meyer Department of Physics, University of Maryland,
Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA.  Agniva
Roychowdhury Department of Physics, University of Maryland, Baltimore County,
1000 Hilltop Circle, Baltimore, MD 21250, USA.  Xiaolong Yang Shanghai
Astronomical Observatory, Chinese Academy of Sciences, Shanghai 200030, China
Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing
100871, China  J. A. Acosta–Pulido Instituto de Astrofísica de Canarias (IAC),
E-38200 La Laguna, Tenerife, Spain  Universidad de La Laguna (ULL),
Departamento de Astrofísica, E-38206 La Laguna, Tenerife, Spain  Suvendu
Rakshit Aryabhatta Research Institute of Observational Sciences (ARIES),
Manora Peak, Nainital, 263002 India  Shivangi Pandey Aryabhatta Research
Institute of Observational Sciences (ARIES), Manora Peak, Nainital, 263002
India  Josefa Becerra González Instituto de Astrofísica de Canarias (IAC),
E-38200 La Laguna, Tenerife, Spain  Universidad de La Laguna (ULL),
Departamento de Astrofísica, E-38206 La Laguna, Tenerife, Spain  Ehud Behar
Department of Physics, Technion, Haifa 32000, Israel  Luigi C. Gallo
Department of Astronomy & Physics, Saint Mary’s University, 923 Robie Street,
Halifax, Nova Scotia, B3H 3C3, Canada  Francesca Panessa INAF - Istituto di
Astrofisica e Planetologia Spaziali, via Fosso del Cavaliere 100, I-00133
Roma, Italy  Stefano Bianchi Dipartimento di Matematica e Fisica, Università
degli Studi Roma Tre, Via della Vasca Navale 84, I-00146, Roma, Italy  Fabio
La Franca Dipartimento di Matematica e Fisica, Università degli Studi Roma
Tre, Via della Vasca Navale 84, I-00146, Roma, Italy  Nicolas Scepi School of
Physics and Astronomy, University of Southampton, Highfield, Southampton, SO17
1BJ, UK.  Mitchell C. Begelman JILA, University of Colorado and National
Institute of Standards and Technology, 440 UCB, Boulder, CO 80309-0440, USA.
Anna Lia Longinotti Instituto de Astronomía, Universidad Nacional Autónoma de
México, Circuito Exterior, Ciudad Universitaria, Ciudad de México 04510,
México  Elisabeta Lusso Dipartimento di Fisica e Astronomia, Università di
Firenze, Via G. Sansone 1, 50019, Sesto Fiorentino, Firenze, Italy  INAF -
Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125, Firenze,
Italy  Samantha Oates Birmingham Institute for Gravitational Wave Astronomy
and School of Physics and Astronomy, University of Birmingham, Birmingham B15
2TT, UK  Matt Nicholl Astrophysics Research Centre, School of Mathematics and
Physics, Queens University Belfast, Belfast BT7 1NN, UK  S. Bradley Cenko
Astrophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD
20771, USA.  Joint Space-Science Institute, University of Maryland, College
Park, MD 20742, USA  Brendan O’Connor Department of Physics, The George
Washington University, Washington, DC 20052, USA  Astronomy, Physics and
Statistics Institute of Sciences (APSIS), The George Washington University,
Washington, DC 20052, USA  Department of Astronomy, University of Maryland,
College Park, MD 20742-4111, USA  Astrophysics Science Division, NASA Goddard
Space Flight Center, Greenbelt, MD 20771, USA.  Erica Hammerstein Department
of Astronomy, University of Maryland, College Park, MD 20742-4111, USA
Astrophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD
20771, USA.  Center for Research and Exploration in Space Science and
Technology, NASA/GSFC, Greenbelt, Maryland 20771, USA  Jincen Jose Aryabhatta
Research Institute of Observational Sciences (ARIES), Manora Peak, Nainital,
263002 India  Krisztina Éva Gabányi Department of Astronomy, Institute of
Physics and Astronomy, ELTE Eötvös Loránd University, Pázmány Péter sétány
1/A, H-1117 Budapest, Hungary  Konkoly Observatory, ELKH Research Centre for
Astronomy and Earth Sciences, Konkoly Thege Miklós út 15-17, H-1121 Budapest,
Hungary  ELKH-ELTE Extragalactic Astrophysics Research Group, ELTE Eötvös
Loránd University, Pázmány Péter sétány 1/A, H-1117 Budapest, Hungary
Federica Ricci Dipartimento di Matematica e Fisica, Università degli Studi
Roma Tre, Via della Vasca Navale 84, I-00146, Roma, Italy  INAF - Osservatorio
Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone, Italy
Sabyasachi Chattopadhyay South African Astronomical Observatory, 1 Observatory
Rd, Observatory, Cape Town, 7925, South Africa Ritesh Ghosh
ritesh.ghosh<EMAIL_ADDRESS>
###### Abstract

1ES1927+654 is a nearby active galactic nucleus that has shown an enigmatic
outburst in optical/UV followed by X-rays, exhibiting strange variability
patterns at timescales of months-years. Here we report the unusual X-ray, UV,
and radio variability of the source in its post-flare state (Jan 2022- May
2023). Firstly, we detect an increase in the soft X-ray ($0.3-2\rm\,\,{\rm
keV}$) flux from May 2022- May 2023 by almost a factor of five, which we call
the bright-soft-state. The hard X-ray $2-10\rm\,\,{\rm keV}$ flux increased by
a factor of two, while the UV flux density did not show any significant
changes ($\leq 30\%$) in the same period. The integrated energy pumped into
the soft and hard X-ray during this period of eleven months is $\sim
3.57\times 10^{50}$ erg and $5.9\times 10^{49}$ erg, respectively. From the
energetics, it is evident that whatever is producing the soft excess (SE) is
pumping out more energy than either the UV or hard X-ray source. Since the
energy source presumably is ultimately the accretion of matter onto the SMBH,
the SE emitting region must be receiving the majority of this energy. In
addition, the source does not follow the typical disc-corona relation found in
AGNs, neither in the initial flare (in 2017-2019) nor in the current bright
soft state (2022-2023). We found that the core ($<1\,{\rm pc}$) radio emission
at 5 GHz gradually increased till March 2022 but showed a dip in August 2022.
The Güdel Benz relation ($L_{\rm radio}/L_{\rm X-ray}\sim 10^{-5}$), however,
is still within the expected range for radio-quiet AGN and further follow-up
radio observations are currently being undertaken.

galaxies: Seyfert, X-rays: galaxies, quasars: individual: 1ES 1927+654

††journal: ApJ

##  1 Introduction

Active Galactic Nuclei (AGN) are astrophysical systems hosting an accreting
super-massive black hole (SMBH) and emitting across all wavelength bands
through various physical processes. However, the exact physics of how matter
from larger distances ($>1\,{\rm pc}$) loses angular momentum and falls into
the accretion disk of the SMBH, thereby creating enormous luminosity, is still
not properly understood. Similarly, it is also not clear how the hot (T$\sim
10^{9}$ K) X-ray emitting plasma (corona), very commonly found in AGN, is
energetically supported by the accreting system. Ideally, one would understand
the physics behind these emission features when they switch on and off or show
extreme variability. However, our understanding of these systems is much
hindered by their long duty cycle ($\sim 10^{7}-10^{9}$ years, see, e.g.,
Marconi et al., 2004; Schawinski et al., 2015) compared to the human
timescale, thus preventing us from detecting an ignition or quenching event.

However, recent large-scale time-domain surveys, e.g., All-Sky Automated
Survey for Supernovae (ASAS-SN) (Shappee et al., 2014), Zwicky Transient
Facility (ZTF) Bellm et al. (2019) and others, have led to the identification
of new types of extreme variability in active galaxies, called “changing look"
or “changing state" active galactic nuclei (CL-AGNs, hereafter). These extreme
variations are characterized by order-of-magnitude changes in the optical, UV,
and X-ray luminosity of the source coupled with a rapid transition between
spectral states (type-1 to type-2 and vice-versa). For example, sources
initially exhibiting AGN type-2 characteristics with the optical/UV band
dominated by narrow emission lines only have transitioned to a type-1 state,
displaying prominent broad emission lines, and vice versa. There is often also
a change in the optical continuum slope from red to blue coinciding with the
appearance/disappearance of broad lines (LaMassa et al., 2015; Ruan et al.,
2019). We note here that there are two types of CL-AGNs, changing-obscuration
AGNs, and changing-accretion state AGNs. The CL-AGNs we talk about in this
work relate to the sources exhibiting rapid accretion state changes and not
obscuration changes (Ricci & Trakhtenbrot, 2022).

1ES 1927+654 (alias AT2018zf, ASASSN-18el) is one such CL-AGN that has
undergone some dramatic changes (Trakhtenbrot et al., 2019; Ricci et al.,
2020, 2021; Laha et al., 2022; Li et al., 2022; Masterson et al., 2022) in the
recent past showing behaviour, unlike other known CL-AGNs. Previous optical
and X-ray studies (Boller et al., 2003; Gallo et al., 2013) have shown that
despite having only narrow emission lines in the optical spectrum, there is no
evidence of the line of sight absorption in any wavelength, thus exhibiting a
‘true type 2’ behaviour, meaning that the BLR is simply intrinsically absent
or too weak to be detected for this source. In Dec 2017, it showed a sudden
rise in the optical/UV flux by almost four magnitudes which peaked around
$\sim 200$ days after the start of the burst. The other interesting events
that followed in the next few months-years are; broad emission lines appeared
in this ‘true type-2 AGN’ after $\sim 200$ days of the initial flare, which
stayed on for another $\sim 300$ days and then gradually vanished; the X-rays
gradually dimmed after $\sim 3-4$ months of the optical flare and the
$2-10\rm\,\,{\rm keV}$ hard X-rays completely vanished in Aug-2018 for almost
$3$ months (Aug-2018- Oct-2018); the hard X-rays revived after Oct-2018 and
flared up $\sim 10$ times its pre-flare value and stayed there for about a
year; there was no correlation between the soft and the hard X-ray
variability, as well as between the X-rays and the UV during this flaring
event. This switching off the corona where the $2-10\rm\,\,{\rm keV}$ hard
X-ray flux was completely gone (Ricci et al., 2020, 2021) is particularly a
peculiar phenomenon that was first witnessed in this source. In the meantime,
after the initial flare, the UV flux monotonically dimmed with $t^{-0.91\pm
0.04}$ (Laha et al., 2022) and returned to a near-pre-flare value after $\sim
1200$ days of the initial flare. The core ($<1\,{\rm pc}$) radio flux density
at $5$ GHz showed a minimum (a factor of 4 below the pre-flare value) at the
time when the X-ray flux was low, and it gradually increased over the next 2
years. Some studies of this phenomenon claimed it to be a tidal disruption
event (TDE) (Ricci et al., 2020, 2021; Masterson et al., 2022). However, there
were already some concerns about this description, such as the optical
emission lines one would expect for a TDE like event were not present
(Trakhtenbrot et al., 2019), and the X-rays did not vary in the usual way a
TDE should do, that follows the UV flare with a time lag (Ricci et al., 2020).
We note that some TDEs emit X-rays but do not show the same UV/X-ray evolution
(see, e.g., ASASSN-15oi and AT2019azh; Gezari, 2021). However, the vanishing
and reemergence of the coronal emission of 1ES 1927+654 are uncommon in TDEs
in addition to a flatter ($t^{-0.91\pm 0.04}$) UV light-curve. This result is
also supported by high-cadence data from the Transiting Exoplanet Survey
Satellite (TESS) survey of the source (Hinkle et al., 2023). The completely
independent variations of the UV and the X-rays pointed to two or more
different physical processes (possibly involving magnetic field in the
accretion disk) apart from the accretion, which contributed to the X-ray
variations.

Our earlier study (Laha et al., 2022) encompassing the entire event (Dec 2017-
Dec 2021) suggested that this could be a case of magnetic pole inversion of
the SMBH. In such a case, matter exhibiting opposite magnetic polarity is
advected to the accretion disk, which cancels the existing magnetic field,
therefore temporarily switching off the corona, which is dependent on the
magnetic field. Further advection of matter with the opposite polarity builds
up a strong magnetic field (now with reversed poles), which re-creates the
corona. Finally, the accretion rate gets back to the pre-flare value, and so
does the X-ray emission (Scepi et al., 2021; Laha et al., 2022).

Other CL-AGNs such as Mrk 590 (Denney et al., 2014; Mathur et al., 2018; Ghosh
et al., 2022), Mrk 1080 (McElroy et al., 2016; Noda & Done, 2018), NGC 1566
(Parker et al., 2019; Tripathi & Dewangan, 2022) have also shown different
patterns of optical, UV and X-ray variability and different timescales, mostly
over several years. However, as it stands, the changing look phenomenon in 1ES
1927+654 is one of the fastest detected so far, with changes happening within
just a few months. The study of these interesting sources not only helps us
understand how the accretion disk and the corona get coupled (strong
correlation between UV and X-rays) and decoupled (no correlation) at different
phases of evolution but also how the soft X-ray emission in AGN (popularly
known as the soft X-ray excess) evolves with the other components of the
central engine, which is an ideal technique to understand its origin in the
first place. Thus these sources give us a unique view of how the soft, hard
X-ray and UV evolve with time, the origin of the soft X-ray excess, and the
relation between the emission in different energy bands as they vary (e.g.,
radio and X-rays).

We have followed up 1ES 1927+654 with multi-wavelength observations from
space-based and ground-based missions to track its behaviour in the post-flare
state. In this paper, we report another interesting phenomenon currently
ongoing in the source, using multi-wavelength data from Swift (X-ray and UV),
Himalayan Chandra Telescope or HCT (optical), Lowell Discovery telescope or
LDT (Optical), and Very Long Baseline Array or VLBA (radio). The paper is
arranged as follows: Section 2 discusses the observation, data reduction, and
analysis. Section 3 lists the most important results. Sec 4 discusses the
results and Sec 5 lists the main conclusions. Throughout this paper, we
assumed a cosmology with $H_{0}=71\rm\,\,{\rm km\,s^{-1}}\,{\rm
Mpc}^{-1},\Omega_{\Lambda}=0.73$ and $\Omega_{M}=0.27$. We regard a
correlation to be significant if the confidence level is $>99.99\%$.

Table 1: The details of multi-wavelength observations of 1ES 1927+654 used in
this work. Refer to (Laha et al., 2022) for all the previous Swift
observations. The XMM-Newton observation is used as a comparison for the pre-
flare X-ray state.

Observation band | Telescopes | observation date | observation ID | Net exposure | Short-id
---|---|---|---|---|---
|  | YYYY-MM-DD |  | (Sec) |
X-ray and UV | XMM-Newton EPIC-pn/OM | 2011-05-20 | 0671860201 | 28649 | X1
” | Swift-XRT/UVOT | 2018-05-17 | 00010682001 | 2190 | S01
” | ” | 2022-03-22 | 00010682033 | 2056 | S33
” | ” | 2022-04-23 | 00010682034 | 1276 | S34
” | ” | 2022-05-20 | 00010682035 | 563 | S35
” | ” | 2022-05-26 | 00010682036 | 884 | S36
” | ” | 2022-06-20 | 00010682037 | 1469 | S37
” | ” | 2022-07-20 | 00010682038 | 1412 | S38
” | ” | 2022-08-20 | 00010682039 | 652 | S39
” | ” | 2022-08-24 | 00010682040 | 1234 | S40
” | ” | 2022-08-26 | 00010682041 | 1810 | S41
” | ” | 2022-09-26 | 00010682042 | 2166 | S42
” | ” | 2022-10-28 | 00010682043 | 411 | S43
” | ” | 2022-11-20 | 00010682044 | 797 | S44
” | ” | 2022-11-26 | 00010682045 | 1768 | S45
” | ” | 2022-12-03 | 00010682046 | 988 | S46
” | ” | 2022-12-14 | 00010682047 | 3001 | S47
” | ” | 2022-12-17 | 00010682048 | 1441 | S48
” | ” | 2022-12-21 | 00010682049 | 1989 | S49
” | ” | 2023-01-03 | 00010682050 | 712 | S50
” | ” | 2023-01-07 | 00010682051 | 860 | S51
” | ” | 2023-01-14 | 00010682052 | 2958 | S52
” | ” | 2023-01-21 | 00010682053 | 817 | S53
” | ” | 2023-01-25 | 00010682054 | 1907 | S54
” | ” | 2023-01-28 | 00010682055 | 1926 | S55
” | ” | 2023-02-01 | 00010682056 | 1712 | S56
” | ” | 2023-02-04 | 00010682057 | 922 | S57
” | ” | 2023-02-08 | 00010682058 | 1883 | S58
” | ” | 2023-02-11 | 00010682059 | 974 | S59
” | ” | 2023-02-13 | 00010682060 | 1788 | S60
” | ” | 2023-02-25 | 00010682061 | 842 | S61
” | ” | 2023-02-27 | 00010682062 | 910 | S62
” | ” | 2023-03-02 | 00010682063 | 1780 | S63
” | ” | 2023-03-04 | 00010682064 | 891 | S64
” | ” | 2023-03-08 | 00010682065 | 1800 | S65
” | ” | 2023-03-11 | 00010682066 | 872 | S66
” | ” | 2023-03-14 | 00010682067 | 868 | S67
” | ” | 2023-03-18 | 00010682068 | 935 | S68
” | ” | 2023-03-20 | 00010682069 | 1065 | S69
” | ” | 2023-03-29 | 00010682071 | 809 | S71
” | ” | 2023-03-30 | 00010682072 | 1066 | S72
” | ” | 2023-04-16 | 00010682073 | 888 | S73
” | ” | 2023-04-22 | 00010682074 | 938 | S74
” | ” | 2023-04-29 | 00010682076 | 903 | S76
” | ” | 2023-05-01 | 00010682077 | 1168 | S77
” | ” | 2023-05-05 | 00010682078 | 902 | S78
Optical | HTC | 2022-10-31 | HCT-2022-C3-P10 | 2400 |
Radio | VLBA | 2022-03-05 | BM527 | 12600 |
” | VLBA | 2022-08-05 | BY177B | 10080 |

VLBA= Very Long Baseline Array.

##  2 Observation, data reduction and data analysis

###  2.1 Swift XRT and UVOT

Observations of 1ES 1927+654 were carried out by The Neil Gehrels Swift
Observatory (Swift hereafter) initially at a monthly cadence from January 2022
to November 2022, and then at a weekly and bi-weekly cadence from December
2022 to May 2023 (See Table 1) under a Director’s Discretionary Time (DDT)
program (PI: S.Laha). The earlier Swift XRT (Burrows et al., 2005) and UVOT
(Roming et al., 2005) observations of this source (prior to 31st Dec 2021)
have been reported in our previous work Laha et al. (2022), and we use those
results in this work for comparison and to portray a complete picture of the
phenomenon.

We followed the automated XRT analysis approach via the online
tools111https://www.swift.ac.uk/user-objects (Evans et al., 2009) for the XRT
data in all our observations (S33-S78) as recommended for point sources by the
Swift help desk. Refer to Laha et al. (2022) for a full description of UVOT
data reprocessing and analysis, which we follow here. The UV flux densities
were corrected for Galactic absorption using the correction magnitude of $\rm
A_{\lambda}=0.690$ obtained from the NASA Extragalactic Database
(NED222https://ned.ipac.caltech.edu).

We used a simple absorbed power-law model in XSPEC (Arnaud, 1996) to fit the
$0.3-10\rm\,\,{\rm keV}$ XRT grouped spectra (a minimum of 10 counts per bin)
for all the Swift observations and added a black-body component to model the
soft X-ray excess emission below $2\rm\,\,{\rm keV}$. We note that adding a
black-body in the soft X-ray did not improve the fit statistics from
observations S33 to S48. However, in the latest observations (S49-S78), where
the soft X-ray flux increased notably, a black-body component improved the
fit-statistics significantly with a typical temperature $kT_{\rm e}\sim
0.2\rm\,\,{\rm keV}$. Table 2 quotes the best-fit parameters and fit
statistics.

We measured the UV monochromatic flux using the UVW2 filter for all Swift
observations and corrected it for Galactic reddening and extinction. We
created the UVOT source and background spectral files using the UVOT2PHA tool
and used the response files provided by the Swift team.

We estimated the bolometric luminosity ($L_{\rm\,bol}$) and hence the
accretion rate ($\lambda_{\rm\,Edd}$) of the source using the relation
$\lambda_{\rm\,Edd}=L_{\rm\,bol}/L_{\rm\,Edd}$, where $L_{\rm\,Edd}$ is the
Eddington rate of the source assuming a black hole of mass $\sim
10^{6}M_{\odot}$ (Ricci et al., 2020). The value is estimated using the
galaxy’s stellar mass inferred from K-band photometry relation from Kormendy &
Ho (2013). Here, we note that the estimates of the black hole mass of
1ES1927+654 varied up to $\sim 10^{7}M_{\odot}$ (Trakhtenbrot et al., 2019)
where the authors used the virial method of broad Balmer emission lines.
However, it is possible that due to the transient nature of the event, the
clouds emitting the broad Balmer lines might not have had the time to
virialize, as shown by their variable widths (Ricci et al., 2020). To estimate
$L_{\rm\,bol}$, we have used the simple approach $L_{\rm\,bol}=L_{\rm
UV}+L_{\rm X-ray}$, where $L_{\rm UV}$ is the integrated UV luminosity in the
band $0.001-100\,{\rm\,eV}$ obtained by fitting the Swift UVOT photometric
data, and $L_{\rm X-ray}$ is the X-ray luminosity in the band
$0.3-10\rm\,\,{\rm keV}$ measured from the best-fit model obtained from the
spectral fitting of each observation. The UVOT data (the 3 UV mono-chromatic
data points, UVM1, UVW1, and UVW2) for every Swift observation (S01-S78) have
been fitted with a disk black body (diskbb) in XSPEC. The model in XSPEC reads
as $tbabs\times REDDEN\times diskbb$ and the corresponding luminosity $L_{UV}$
is measured using the command $clumin$, over the energy range
$0.01\,{\rm\,eV}$ to $100\,{\rm\,eV}$.

Table 2: The spectral parameters obtained using Swift and XMM-Newton UV and
X-ray observations of 1ES 1927+654. For comparison with the pre-flare values,
we keep the XMM-Newton observation.

ID(DD/MM/YY) | $F_{0.3-2\rm\,\,{\rm keV}}^{\rm A}$ | $F_{2-10\rm\,\,{\rm keV}}^{\rm A}$ | $F_{1.5-2.5\rm\,\,{\rm keV}}^{\rm A}$ | kT (keV) | $\Gamma$ | UV filter | UV flux densityB | $\alpha_{\rm OX}$ | $\rm\chi^{2}/\chi^{2}_{\nu}$
---|---|---|---|---|---|---|---|---|---
X1 (20/05/11) | $9.41\pm 0.66$ | $3.92\pm 0.08$ | $1.64\pm 0.02$ | $0.20\pm 0.01$ | $2.21_{-0.02}^{+0.02}$ | UVM2 | $1.34\pm 0.03$ | 1.004 | $185/1.37$
S33 (22/03/22) | $18.26\pm 1.34$ | $4.66\pm 0.66$ | $2.49\pm 0.22$ | $-$ | $2.67_{-0.09}^{+0.10}$ | UVW2 | $2.00\pm 0.09$ | $1.002$ | $119.36/1.03$
S34 (23/04/22) | $14.46\pm 1.40$ | $4.59\pm 0.66$ | $2.25\pm 0.20$ | $-$ | $2.55_{-0.14}^{+0.14}$ | UVW2 | $2.04\pm 0.09$ | $1.022$ | $75.91/0.90$
S35 (20/05/22) | $8.70\pm 1.71$ | $2.81\pm 1.05$ | $1.37\pm 0.30$ | $-$ | $2.54_{-0.24}^{+0.27}$ | UVW2 | $1.83\pm 0.08$ | $1.087$ | $27.03/1.13$
S36 (26/05/22) | $10.04\pm 1.48$ | $5.02\pm 1.79$ | $2.12\pm 0.36$ | $-$ | $2.26_{-0.22}^{+0.22}$ | UVW2 | $1.91\pm 0.12$ | $1.021$ | $34.49/0.84$
S37 (20/06/22) | $12.42\pm 1.30$ | $4.03\pm 0.89$ | $1.96\pm 0.26$ | $-$ | $2.54_{-0.14}^{+0.15}$ | UVW2 | $1.91\pm 0.09$ | $1.034$ | $47.19/0.73$
S38 (20/07/22) | $13.66\pm 1.46$ | $5.22\pm 1.04$ | $2.37\pm 0.29$ | $-$ | $2.45_{-0.13}^{+0.14}$ | UVW2 | $2.04\pm 0.09$ | $1.013$ | $68.35/1.04$
S39 (20/08/22) | $18.23\pm 2.03$ | $6.35\pm 1.37$ | $2.99\pm 0.39$ | $-$ | $2.50_{-0.14}^{+0.15}$ | UVW2 | $2.28\pm 0.13$ | $0.993$ | $35.88/0.61$
S40 (24/08/22) | $20.39\pm 2.16$ | $5.37\pm 1.16$ | $2.83\pm 0.37$ | $-$ | $2.66_{-0.14}^{+0.15}$ | UVW2 | $2.02\pm 0.09$ | $0.982$ | $59.22/0.90$
S41 (26/08/22) | $14.14\pm 1.16$ | $4.62\pm 0.76$ | $2.24\pm 0.22$ | $-$ | $2.54_{-0.11}^{+0.11}$ | UVW2 | $2.24\pm 0.11$ | $1.038$ | $99.27/0.97$
S42 (26/09/22) | $17.71\pm 1.23$ | $4.79\pm 0.67$ | $2.50\pm 0.21$ | $-$ | $2.64_{-0.09}^{+0.09}$ | UVW2 | $1.98\pm 0.09$ | $0.999$ | $115.91/0.92$
S43 (28/10/22) | $19.71\pm 3.08$ | $6.55\pm 2.22$ | $3.15\pm 0.62$ | $-$ | $2.52_{-0.22}^{+0.23}$ | UVW2 | $1.96\pm 0.10$ | $0.959$ | $45.61/1.26$
S44 (20/11/22) | $25.55\pm 4.95$ | $6.35\pm 2.42$ | $3.43\pm 0.79$ | $-$ | $2.69_{-0.24}^{+0.24}$ | UVW2 | $2.02\pm 0.12$ | $0.950$ | $33.98/0.85$
S45 (26/11/22) | $26.02\pm 2.17$ | $7.18\pm 1.15$ | $3.72\pm 0.37$ | $-$ | $2.73_{-0.10}^{+0.11}$ | UVW2 | $2.30\pm 0.13$ | $0.958$ | $78.79/0.89$
S46 (03/12/22) | $42.71\pm 5.76$ | $10.52\pm 3.24$ | $5.70\pm 1.03$ | $-$ | $2.69_{-0.20}^{+0.21}$ | UVW2 | $2.00\pm 0.09$ | $0.864$ | $28.10/0.62$
S47 (14/12/22) | $26.01\pm 1.51$ | $6.14\pm 0.71$ | $3.38\pm 0.25$ | $-$ | $2.71_{-0.07}^{+0.08}$ | UVW2 | $2.02\pm 0.11$ | $0.952$ | $161.52/1.11$
S48 (17/12/22) | $25.92\pm 1.72$ | $5.41\pm 0.70$ | $3.14\pm 0.27$ | $-$ | $2.79_{-0.09}^{+0.09}$ | UVW2 | $1.86\pm 0.10$ | $0.951$ | $89.07/0.70$
S49 (21/12/22) | $34.81\pm 2.12$ | $10.12\pm 1.84$ | $5.10\pm 0.44$ | $0.20\pm 0.05$ | $2.47_{-0.19}^{+0.16}$ | UVW2 | $2.00\pm 0.10$ | $0.882$ | $126.62/0.86$
S50 (02/01/23) | $27.22\pm 3.05$ | $11.54\pm 4.85$ | $3.68\pm 0.78$ | $0.17\pm 0.04$ | $2.01_{-0.90}^{+0.50}$ | UVW2 | $1.76\pm 0.10$ | $0.915$ | $23.36/0.83$
S51 (07/01/23) | $39.74\pm 3.94$ | $8.01\pm 2.49$ | $4.31\pm 0.65$ | $0.17\pm 0.04$ | $2.56_{-0.41}^{+0.29}$ | UVW2 | $1.87\pm 0.11$ | $0.899$ | $66.94/0.96$
S52 (14/01/23) | $32.95\pm 1.68$ | $6.42\pm 1.06$ | $3.80\pm 0.27$ | $0.19\pm 0.03$ | $2.67_{-0.16}^{+0.14}$ | UVW2 | $1.91\pm 0.11$ | $0.924$ | $151.81/0.95$
S53 (21/01/23) | $30.75\pm 2.83$ | $7.41\pm 2.47$ | $3.70\pm 0.49$ | $0.19\pm 0.03$ | $2.29_{-0.49}^{+0.33}$ | UVW2 | $1.96\pm 0.11$ | $0.932$ | $74.32/1.08$
S54 (25/01/23) | $37.19\pm 2.26$ | $7.89\pm 1.51$ | $4.82\pm 0.46$ | $0.22\pm 0.05$ | $2.64_{-0.17}^{+0.16}$ | UVW2 | $1.89\pm 0.11$ | $0.882$ | $112.22/0.80$
S55 (28/01/23) | $42.53\pm 2.80$ | $8.30\pm 1.79$ | $5.19\pm 0.50$ | $0.20\pm 0.02$ | $2.54_{-0.22}^{+0.19}$ | UVW2 | $1.63\pm 0.22$ | $0.845$ | $119.84/0.92$
S56 (01/02/23) | $34.86\pm 2.24$ | $6.12\pm 0.99$ | $3.54\pm 0.24$ | $0.19\pm 0.03$ | $2.54_{-0.22}^{+0.27}$ | UVW2 | $1.96\pm 0.11$ | $0.940$ | $106.9/0.86$
S57 (04/02/23) | $35.95\pm 3.20$ | $6.22\pm 2.19$ | $3.86\pm 0.51$ | $0.19\pm 0.05$ | $2.68_{-0.39}^{+0.30}$ | UVW2 | $1.91\pm 0.11$ | $0.921$ | $74.79/0.95$
S58 (08/02/23) | $35.74\pm 1.98$ | $8.82\pm 1.39$ | $4.11\pm 0.31$ | $0.17\pm 0.02$ | $2.36_{-0.26}^{+0.21}$ | UVW2 | $1.62\pm 0.11$ | $0.883$ | $134.25/0.91$
S59 (11/02/23) | $38.15\pm 3.56$ | $6.54\pm 1.82$ | $3.59\pm 0.51$ | $0.15\pm 0.03$ | $2.66_{-0.36}^{+0.27}$ | UVW2 | $1.60\pm 0.08$ | $0.904$ | $66.18/0.89$
S60 (13/02/23) | $31.94\pm 2.09$ | $7.71\pm 3.20$ | $3.78\pm 0.35$ | $0.16\pm 0.03$ | $2.48_{-0.24}^{+0.20}$ | UVW2 | $2.02\pm 0.13$ | $0.934$ | $92.94/0.74$
S61 (25/02/23) | $37.50\pm 4.15$ | $6.83\pm 2.44$ | $5.14\pm 1.08$ | $0.25\pm 0.06$ | $2.96_{-0.37}^{+0.36}$ | UVW2 | $2.17\pm 0.15$ | $0.895$ | $58.62/0.95$
S62 (27/02/23) | $42.61\pm 3.56$ | $7.18\pm 2.31$ | $4.35\pm 0.53$ | $0.19\pm 0.03$ | $2.55_{-0.36}^{+0.38}$ | UVW2 | $1.76\pm 0.10$ | $0.887$ | $71.96/0.83$
S63 (02/03/23) | $38.06\pm 2.43$ | $8.31\pm 1.85$ | $4.08\pm 0.37$ | $0.18\pm 0.02$ | $2.34_{-0.28}^{+0.26}$ | UVW2 | $1.89\pm 0.11$ | $0.910$ | $146.87/1.15$
S64 (04/03/23) | $44.42\pm 3.53$ | $8.40\pm 2.15$ | $5.33\pm 0.68$ | $0.21\pm 0.05$ | $2.63_{-0.24}^{+0.24}$ | UVW2 | $1.81\pm 0.11$ | $0.858$ | $65.41/0.71$
S65 (08/03/23) | $33.07\pm 2.03$ | $6.60\pm 1.36$ | $3.43\pm 0.31$ | $0.17\pm 0.02$ | $2.51_{-0.29}^{+0.23}$ | UVW2 | $1.91\pm 0.11$ | $0.941$ | $122.24/0.91$
S66 (11/03/23) | $42.65\pm 4.15$ | $10.90\pm 3.15$ | $4.88\pm 0.63$ | $0.17\pm 0.03$ | $2.27_{-0.30}^{+0.40}$ | UVW2 | $1.67\pm 0.10$ | $0.860$ | $61.13/0.90$
S67 (14/03/23) | $39.96\pm 3.76$ | $9.35\pm 3.11$ | $4.96\pm 0.65$ | $0.19\pm 0.05$ | $2.52_{-0.40}^{+0.31}$ | UVW2 | $1.83\pm 0.11$ | $0.872$ | $58.54/0.81$
S68 (18/03/23) | $41.08\pm 3.53$ | $8.52\pm 2.64$ | $4.28\pm 0.53$ | $0.18\pm 0.02$ | $2.27_{-0.53}^{+0.37}$ | UVW2 | $2.02\pm 0.11$ | $0.913$ | $71.32/0.91$
S69 (20/03/23) | $43.01\pm 5.17$ | $9.53\pm 2.97$ | $5.71\pm 0.98$ | $0.24\pm 0.06$ | $2.70_{-0.44}^{+0.43}$ | UVW2 | $1.54\pm 0.09$ | $0.820$ | $56.39/1.01$
S71 (29/03/23) | $36.97\pm 3.29$ | $9.87\pm 2.95$ | $4.31\pm 0.55$ | $0.17\pm 0.02$ | $2.27_{-0.43}^{+0.32}$ | UVW2 | $2.18\pm 0.12$ | $0.925$ | $82.06/1.05$
S72 (30/03/23) | $39.48\pm 3.27$ | $4.49\pm 1.67$ | $2.86\pm 0.42$ | $0.17\pm 0.02$ | $2.69_{-0.55}^{+0.36}$ | UVW2 | $2.33\pm 0.15$ | $1.004$ | $69.94/0.82$
S73 (16/04/23) | $36.48\pm 2.89$ | $7.00\pm 1.93$ | $4.10\pm 0.45$ | $0.19\pm 0.05$ | $2.63_{-0.31}^{+0.26}$ | UVW2 | $1.80\pm 0.09$ | $0.901$ | $86.08/0.90$
S74 (22/04/23) | $42.50\pm 3.93$ | $7.95\pm 3.00$ | $4.80\pm 0.65$ | $0.20\pm 0.04$ | $2.63_{-0.49}^{+0.32}$ | UVW2 | $2.28\pm 0.12$ | $0.914$ | $66.04/0.85$
S76 (29/04/23) | $46.08\pm 6.15$ | $9.00\pm 4.87$ | $4.60\pm 0.97$ | $0.18\pm 0.04$ | $2.30_{-0.69}^{+0.66}$ | UVW2 | $2.28\pm 0.13$ | $0.921$ | $33.75/0.87$
S77 (01/05/23) | $40.69\pm 3.55$ | $4.32\pm 1.44$ | $3.29\pm 0.44$ | $0.19\pm 0.03$ | $2.90_{-0.34}^{+0.29}$ | UVW2 | $2.09\pm 0.09$ | $0.963$ | $83.03/1.00$
S78 (05/05/23) | $41.17\pm 3.68$ | $7.07\pm 2.41$ | $4.44\pm 0.59$ | $0.20\pm 0.04$ | $2.58_{-0.47}^{+0.32}$ | UVW2 | $2.11\pm 0.11$ | $0.914$ | $40.89/0.62$

A Flux in units of $10^{-12}\rm\,erg\,cm^{-2}\,s^{-1}{}$ corrected for
Galactic absorption.
B UV flux density in units of
$10^{-15}\rm\,erg\,cm^{-2}\,s^{-1}\mbox{\AA}^{-1}{}$
$\alpha_{\rm OX}=-0.385\log(\rm F_{2\rm\,\,{\rm keV}}/F_{2500\rm\AA})$
The UV flux density was corrected for Galactic absorption using the correction
magnitude of $\rm A_{\lambda}=0.690$ obtained from NED.

###  2.2 VLBA

1ES 1927+654 was observed by the Very Long Baseline Array (VLBA) on 05 March
2022 under DDTs proposals BM527 (PI: E. Meyer) and as part of project BY177B
(PI: X. Yang) on 05 August 2022. A standard dual-polarization 6 cm frequency
setup was used, with central channel frequencies of 4868 MHz, 4900 MHz, 4932
MHz, 4964 MHz, 4996 MHz, 5028 MHz, 5060 MHz, and 5092 MHz, and a total
bandwidth of 32 MHz. As the source was expected to be too faint for self-
calibration, a relatively fast-switching cadence between the target and a
bright calibrator source was used in both projects for phase-referencing
(Beasley & Conway, 1995). The observations were $3.5$ hours and $2.8$ hours
resulting in acceptable $uv$ coverage for imaging.

The data were checked for radio frequency interference and then calibrated
using VLBARUN (Astronomical Image Processing System or AIPS VLBA pipeline;
Greisen (1990)). The imaging and analysis of source structure were done using
a custom version of DIFMAP Shepherd et al. (1994) ngDIFMAP (Roychowdhury et
al., submitted). The best model (as discussed in Laha et al. 2022 for a
previous VLBA observation), as determined by the reduced chi-squared, is a
point source centered on a uniformly bright disk. The details of the procedure
are as given in Laha et al. (2022), and Table 3 lists the details of the point
source and extended disk that were found to describe the source best. We used
Monte Carlo simulations (e.g., Briggs, 1995; Chael et al., 2018; Roychowdhury
et al., submitted) to verify that the extended emission around the point
source is intrinsic to the source and is not an artefact of interferometric
errors. In Table 4, we list the ratio of the core radio flux density to the
$2-10\rm\,\,{\rm keV}$ flux (also known as the Güdel Benz relation, Güdel &
Benz, 1993).

We find that the point source flux density has increased by a factor of $\sim
1.8$ in a year from March 2021 to March 2022, followed by a decrease by a
similar factor by August 2022. The flux density of the uniform disk has
remained mostly unchanged, while its structure has shrunk by $\sim 20\%$
between March 2021 and August 2022.

Table 3: Details of the radio observations, with corresponding flux density
measurements. In cases where only an unresolved core is observed, the total
flux density equals the core flux density. For cases where we detect resolved
extended emission, the central point source (PS) flux density is noted
alongside the extended flux density and the semi-major and semi-minor axes of
a best-fitting uniform disk model. Note that “flux$"$ in the table headers
refers to flux density. The brightness temperature $T_{B}$ (lower limit)
(Kovalev et al., 2005) has been calculated for the point source at 5 GHz

.  Obs. Freq. Date Total flux Central PS flux Extended flux Disk Dimensions
RMS Resolution $T_{B}$ (GHz) (MM/YY) (mJy/beam) (mJy) (mJy) (mas$\times$mas)
(Jy/beam) (mas$\times$mas) ($\times 10^{6}$ K) VLBA 4.98 03/22 6.6$\pm$0.5
2.4$\pm$0.1 4.2$\pm$0.4 4.4$\pm$0.1$\times$4.2$\pm$0.1 1.0$\times 10^{-4}$
3.42$\times$1.72 $>9.7$ VLBA 4.98 08/22 5.6$\pm$0.5 1.5$\pm$0.1 4.1$\pm$0.4
4.0$\pm$0.1$\times$3.2$\pm$0.1 6.8$\times 10^{-5}$ 3.59$\times$1.36 $>5.7$

Table 4: Semi-contemporaneous X-ray and Radio fluxes (either from 1.5 or 5 GHz
VLBI) and the Güdel-Benz Relation.

X-ray Epoch | Mean 2-10 keV X-ray flux ($F_{X}$) | VLBI epoch | VLBI flux ($F_{R}$) | Ratio of mean fluxes $F_{R}/F_{X}$ |
---|---|---|---|---|---
(MM/YY) | ($\rm\,erg\,cm^{-2}\,s^{-1}{}$) | (MM/YY) | ($\rm\,erg\,cm^{-2}\,s^{-1}{}$) |  |
∗05/11 | $3.70\pm 0.07\times 10^{-12}$ | 08/13 | $5.00\pm 0.50\times 10^{-16}$ | $1.35\pm 0.11\times 10^{-4}$ |
∗05/11 | $3.70\pm 0.07\times 10^{-12}$ | 03/14 | $1.80\pm 0.20\times 10^{-16}$ | $4.86\pm 0.05\times 10^{-5}$ |
12/18 | $1.70\pm 0.50\times 10^{-12}$ | 12/18 | $3.50\pm 0.30\times 10^{-17}$ | $2.05\pm 0.42\times 10^{-5}$ |
03/21 | $4.40\pm 1.04\times 10^{-12}$ | 03/21 | $6.40\pm 0.50\times 10^{-17}$ | $1.45\pm 0.23\times 10^{-5}$ |
03/22 | $4.66\pm 0.66\times 10^{-12}$ | 03/22 | $1.10\pm 0.05\times 10^{-16}$ | $2.36\pm 0.07\times 10^{-5}$ |
08/22 | $5.80\pm 0.90\times 10^{-12}$ | 08/22 | $6.40\pm 0.50\times 10^{-17}$ | $1.10\pm 0.09\times 10^{-5}$ |

∗ There are no contemporary X-ray observations of this source along with VLBI
in 2013 and 2014. Hence we used the $2-10\rm\,\,{\rm keV}$ flux from the 2011
XMM-Newton observation.

###  2.3 Optical spectroscopy

####  2.3.1 The Himalayan Chandra Telescope observation

We observed the source on October 31, 2022, using the 2m Himalayan Chandra
Telescope (HCT) located at Hanle, India. We used grism 7 covering a wavelength
range of 4000Å to 7000Å and 2 arcsec slit for observation. Flux calibration
was done using a spectroscopic standard star BD+28 4211. Standard
spectroscopic reduction using the Image Reduction and Analysis Facility (IRAF)
package is used to extract the spectra, including flat subtraction,
wavelength, and flux calibration.

To estimate the spectral properties of the optical emission line, we first
modeled the host galaxy using PPXF code (Cappellari, 2017) and MILES stellar
templates (Falcón-Barroso et al., 2011). We subtracted the best-fit stellar
model from the spectrum leading to the pure AGN spectrum. Prominent narrow-
emission lines such as [O III], N II, [S II] are clearly present in the
optical spectrum; however, the H$\beta$ line is only detected in the host-
subtracted spectrum. This is similar to our earlier findings in Laha et al.
(2022). To estimate the emission line flux and width, we performed a multi-
component modeling of the host-subtracted spectrum using Gaussian functions
separately for the H$\beta$ and H$\alpha$ regions. We model all the narrow
components using a single Gaussian. In the H$\beta$ complex, narrow H$\beta$
and the [O III]4959, 5007 doublets were modeled while keeping the flux ratio
fixed at the theoretical values. In the H$\alpha$ region, the narrow
H$\alpha$, [NII]6549, [NII]6568, [S II]6717, [S II]6732 were modeled where the
flux ratio of NII doublets was fixed to their theoretical values, and SII
doublets ratio was fixed to unity. During the fit, velocity shift and width
were kept as free parameters. The best-fit model was obtained via the chi-
square minimization method, and the uncertainty on the model parameter was
obtained by the Monte Carlo approach (see Rakshit et al. 2020). We noticed
excess emission in the residual spectrum around the H$\alpha$ line while
modeling with a single Gaussian (narrow component). Therefore, we have added
another Gaussian profile with a line full-width-at-half-maximum (FWHM) larger
than 900 km/s to represent the broad component for H$\alpha$ regions. We find
a statistically improved fit with this addition of the broad component.

####  2.3.2 The Lowell Discovery Telescope observation

We obtained optical spectroscopy on October 30, 2022, using the DeVeny
spectrograph mounted on the 4.3m Lowell Discovery Telescope (LDT) in Happy
Jack, AZ, for a total exposure of 600 s. DeVeny was configured with the 300 g
mm-1 grating and a 1.5″slit width. The spectrum covers wavelengths $3600\,\AA$
\- $8000\,\AA$ at a dispersion of $2.2\,\AA$ pix-1. The data were processed
using methods described in Prochaska et al. (2020), that is, to perform bias
subtraction, flat fielding, cosmic ray removal, trace extraction, and telluric
corrections, with Ne, Ar, Hg, and Cd arc lamps used for wavelength
calibration. The spectrum was flux calibrated using observations of the
spectrophotometric standard star G191-B2B obtained on the same night.

The LDT spectrum does not have a good absolute flux calibration; hence, the
spectrum was re-scaled to have the same level as the GTC spectra in 2021, as
indicated by the photometry obtained by ZTF (minimal change $<0.03$
magnitude). That is, the r and g magnitudes did not vary by more than $3\%$.
First, the spectrum was corrected of galactic interstellar extinction. The
stellar population contribution was subtracted using pPXF. The line
measurements from the LDT observation are reported in Table 6 and are
consistent with the results of HCT.

Table 5: Optical emission line properties without (the values with broad components are given in the parenthesis) the broad component.  Lines | Center wavelength | FWHM | Flux
---|---|---|---
| (Å) | (km s-1) | (erg s-1 cm-2)
|  |  | $\times 10^{-15}$
H$\beta$(NC) | 4857.5$\pm$3.9 | 899 $\pm$ 100 | 3.67 $\pm$ 0.30
$\rm[OIII]4959$ | 4953.6$\pm$3.9 | 706$\pm$71 | 3.78$\pm$0.10
$\rm[OIII]5007$ | 5001.4$\pm$3.9 | 721$\pm$23 | 11.42$\pm$0.30
$\rm[NII]6549$ | 6548.5$\pm$4.1 (6551.17$\pm$4.14) | 421$\pm$63 (673$\pm$170) | 0.72$\pm$0.09 (0.50$\pm$0.18)
H$\alpha$(BC) | (6563.5$\pm$5.5) | (1172$\pm$327) | (5.06$\pm$1.30)
H$\alpha$(NC) | 6564.0$\pm$4.1 (6564.8$\pm$4.1) | 673$\pm$ 63 (389$\pm$66) | 5.94$\pm$0.42 (1.98$\pm$0.91)
$\rm[NII]6585$ | 6585.5$\pm$4.1 (6587$\pm$4.1) | 640$\pm$ 106 (491$\pm$149) | 2.23$\pm$0.30 (1.48$\pm$0.53)
$\rm[SII]6718$ | 6718.1$\pm$4.1 | 478$\pm$ 113 | 0.68$\pm$0.08
$\rm[SII]6732$ | 6727.8$\pm$4.1 | 520$\pm$ 133 | 0.69$\pm$0.09
Table 6:  Optical emission line properties measured in the the Lowell (LDT)
spectrum.

Line ID | Center Wave. | Fluxobs | Fluxdered | FWHM | $\rm Comments^{a}$
---|---|---|---|---|---
| (Å) | (erg s-1 cm-2) | (erg s-1 cm-2) | (km s-1) |
|  | $\times 10^{-15}$ | $\times 10^{-15}$ |  |
$\rm[OII]3727$ | 3727.49$\pm$0.26 | 2.52$\pm$0.28 | 3.48$\pm$0.39 | 710.6$\pm$60.4 | higher (48%)
HeII4686 | 4687.99$\pm$0.85 | 0.74$\pm$0.19 | 0.96$\pm$0.25 | 676.1$\pm$135.2 | same
Hb | 4862.10$\pm$0.72 | 1.25$\pm$0.29 | 1.61$\pm$0.37 | 821.6$\pm$153.0 | same
$\rm[OIII]4959$ | 4960.92$\pm$0.06 | 2.33$\pm$0.05 | 2.97$\pm$0.07 | 556.5$\pm$9.4 | higher (52%)
$\rm[OIII]5007$ | 5008.87$\pm$0.06 | 7.07$\pm$0.16 | 9.00$\pm$0.20 | 556.5$\pm$9.4 | higher (52%)
$\rm[NII]6548$ | 6550.10$\pm$0.26 | 0.57$\pm$0.07 | 0.68$\pm$0.08 | 390.5$\pm$41.4 | same
Ha | 6565.23$\pm$0.15 | 5.13$\pm$0.21 | 6.03$\pm$0.25 | 537.3$\pm$17.2 | same
$\rm[NII]6584$ | 6585.51$\pm$0.26 | 1.73$\pm$0.22 | 2.04$\pm$0.26 | 390.5$\pm$41.4 | same
$\rm[SII]6716$ | 6718.03$\pm$1.81 | 0.30$\pm$0.11 | 0.35$\pm$0.13 | 374.3$\pm$75.0 | same
$\rm[SII]6731$ | 6735.27$\pm$0.77 | 0.56$\pm$0.15 | 0.65$\pm$0.18 | 374.3$\pm$75.0 | same

a compared to 2021 Gran Telescopio CANARIAS (GTC) observation (Laha et al.,
2022).

Figure 1:  The light curves of the X-ray, UV, and Radio parameters of the
central engine of the AGN 1ES1927+654, as observed by Swift and VLBA (see
Tables 2 and Table 3 for details). The shaded region corresponds to the
observations discussed in our earlier work Laha et al. (2022), but we have
included them here for the sake of completeness and to track the long-term
nature of this source. The rest of the figure refers to the new observations
used in this work. The start date of the light curve is 2017-12-23,
corresponding to the burst date reported by Trakhtenbrot et al. (2019). The
X-axis is in the units of days elapsed from the start date. The dotted
horizontal lines in every panel refer to their pre-flare values (in 2011). The
inverted triangles are the upper limits. The X-ray flux in units of
$10^{-12}\rm\,erg\,cm^{-2}\,s^{-1}{}$ corrected for Galactic absorption. From
top to bottom are panels: (1) The $2-10\rm\,\,{\rm keV}$ X-ray flux, (2) The
$0.3-2\rm\,\,{\rm keV}$ X-ray flux, (3) The hardness ratio:
$F_{2-10\rm\,\,{\rm keV}}/F_{0.3-2\rm\,\,{\rm keV}}$, (4) The UV (UVW2) flux
density (in units of $10^{-15}\rm\,erg\,cm^{-2}\,s^{-1}\mbox{\AA}^{-1}$), (5)
the $\alpha_{\rm OX}$, and (6) the Core radio flux ($<1\,{\rm pc}$ spatial
resolution) NOTE: The vertical line corresponds to the observation S8, where
the X-ray corona jumps back (created) after being destroyed, and there is a
dip in the UV flux by a factor of 2, and also the X-ray spectra become harder
(panel 3). Figure 2:  The zoomed-in version of the X-ray and UV light curves
plotted in Fig 1 encompassing the observations S33-S78 to highlight the rise
of the bright-soft-state of the source 1ES1927+654. The vertical dashed blue
line corresponds to observation S35 when both the soft and the hard X-ray
fluxes exactly matched their pre-flare values in 2011. See Table 2 for
details. The start date (day zero) of the light curve is 2017-12-23,
corresponding to the burst date reported by Trakhtenbrot et al. (2019), and
the X-axis is in the units of days elapsed from the start date. The dotted
horizontal lines in every panel refer to their pre-flare values (in 2011).
From top to bottom are panels: (1) The X-ray $2-10\rm\,\,{\rm keV}$ flux (in
units of $10^{-12}\rm\,erg\,cm^{-2}\,s^{-1}$), (2) The X-ray $0.3-2\rm\,\,{\rm
keV}$ flux (in units of $10^{-12}\rm\,erg\,cm^{-2}\,s^{-1}$), (3) The hardness
ratio: $F_{2-10\rm\,\,{\rm keV}}/F_{0.3-2\rm\,\,{\rm keV}}$, (4)The evolution
of the UV (UVW2) flux density along with the mean (1.96) denoted by a solid
line and standard deviations (0.19) denoted by dashed-dotted lines (in units
of $10^{-15}\rm\,erg\,cm^{-2}\,s^{-1}\mbox{\AA}^{-1}$), (5) the $\alpha_{\rm
OX}$ values during observations S33-S78 are plotted.

##  3 Results

In our earlier work Laha et al. (2022), we reported the ‘initial flare’ (May
2018 to December 2021) of the source 1ES 1927+654. Here we report the results
from our continuing multi-wavelength campaign from January 2022 to March 2023.
We detect a significant rise in the soft X-ray flux (by a factor of 5 in a
matter of $\sim 11$ months), which we call the bright-soft-state. Figure 1
shows the time evolution of the fluxes in $0.3-2\rm\,\,{\rm keV}$,
$2-10\rm\,\,{\rm keV}$, UVW2 flux density and the core radio flux. In Figure
2, we have zoomed into the time period when the soft X-ray flux has shown a
continuous rise. Below we discuss the results obtained from our continuing
multi-wavelength observational campaign.

###  3.1 Evolution of $0.3-2\rm\,\,{\rm keV}$, $2-10\rm\,\,{\rm keV}$ X-ray
fluxes, the UV flux density, and the photon index

The soft X-ray flux showed considerable variability in months to week
timescale in its post-flare state (after Oct 2021, when it reached the pre-
flare state for the first time). It showed a rise and then a drop after that.
On 20th May 2022, it reached its pre-flare value of $\sim 9\times
10^{-12}\rm\,erg\,cm^{-2}\,s^{-1}$ again. After this date, the flux showed a
steady rise (See Fig 2 second panel) for eleven straight months. Nevertheless,
the biweekly Swift observations have captured significant variability at days
time-scale. As of May 5th, 2023, the soft X-ray flux level is $\sim 5$ times
that of its pre-flare value.

The hard X-ray flux had reached its pre-flare value much earlier than the soft
flux and had shown some variability throughout our campaign. But we do not
detect any significant continuous rise of the flux above the pre-flare value
like the soft. From Fig 2 top panel we note that the $2-10\rm\,\,{\rm keV}$
flux showed a rise (by a factor of $\sim 2$) in January 2023. We note that the
flux variation and the overall increase in the soft band over the last year,
is more significant compared to the hard band (See Fig 1)

The UVW2 flux density of 1ES 1927+654 was monitored using the Swift UVOT, and
we quote the UVW2 band for consistency with the literature (although we use
all the wavelength bands when we estimate $L_{\rm\,bol}$). The UVW2 flux
density shows minimal variations in the post-flare state, in particular during
the soft X-ray rise from 20th May 2022. The UV flux density values show a
$\leq 30\%$ variability over a year-long timescale with a mean value of
$1.96\times 10^{-15}$ and a standard deviation of $0.19\times 10^{-15}$ (in
units of $\rm\,erg\,cm^{-2}\,s^{-1}\mbox{\AA}^{-1}$) respectively and are
shown in the fourth panel (from top) of Figure 2. Here, we also note that the
UVW2 flux level has not yet reached the pre-flare value of $1.34\pm 0.03\times
10^{-15}\rm\,erg\,cm^{-2}\,s^{-1}\mbox{\AA}^{-1}$. See Table 2 for details. As
discussed earlier, we estimated the UV band luminosity in the range
$(0.01-100)\,{\rm\,eV}$ using the multi-band photometric data from Swift-UVOT.

After the coronal reappearance, when the source reached its pre-flare state,
the power-law photon index $\Gamma$ has been more or less stable around the
pre-flare value ($\Gamma\sim 2.2-2.6$) and remained consistent within
$3\sigma$ errors, even during the current bright-soft-state.

###  3.2 Evolution of $\lambda_{\rm\,Edd}$

We find that the Eddington ratio
($\lambda_{\rm\,Edd}=L_{\rm\,bol}/L_{\rm\,Edd}$) for this source was as high
as $\lambda_{\rm\,Edd}\sim 0.53$ (five times that of the pre-flare state) when
the X-ray flux was the highest during the ‘initial flare’ in Nov 2019 (See
Table LABEL:Table:lambdaedd), and then it dropped to the pre-flare value
($\lambda_{\rm\,Edd}\sim 0.09$) and stayed the same until the soft X-ray flux
started to rise in May 2022. Over the period of months, we see a gradual rise
in the Eddington ratio with the current value of $\lambda_{\rm\,Edd}\sim
0.25$, half of the bright flux state in the ‘initial flare’. This increase in
the Eddington ratio is solely due to the rise in the soft X-ray flux and not
due to the contribution from UV.

Figure 3 shows the relation between $\lambda_{\rm\,Edd}$ and the $\Gamma$. We
do not see any strong correlation between these two parameters if we consider
the observations during the post-flare bright-soft-states (S33-S78). We find
that the $\Gamma$ varies within a narrow range of values with a considerable
spread in $\lambda_{\rm\,Edd}$.

Table 7: The evolution of the disk temperature and $\lambda_{\rm\,Edd}$ estimated from the Swift and XMM-Newton UV and X-ray observations of 1ES 1927+654. The Eddington rate for an SMBH with a mass of $10^{6}M_{\odot}$ is $1.3\times 10^{44}\rm\,erg\,s^{-1}$. The X-ray luminosity is calculated from the $0.3-10\rm\,\,{\rm keV}$ energy band by adding the soft and hard X-ray flux. During the lowest luminosity phase (S03-S07), we assumed an upper limit of $10^{41}\rm\,erg\,s^{-1}$ for the X-ray flux. Obs | Diskbb | $L_{UV}$ | $L_{X}$ | $L_{Bol}$ | $\lambda_{\rm\,Edd}$
---|---|---|---|---|---
Id |  in $\,{\rm\,eV}$ | $10^{43}\rm\,erg\,s^{-1}$ | $10^{43}\rm\,erg\,s^{-1}$ | $L_{UV}+L_{X}$ | $L_{Bol}/L_{Edd}$
X1 | $03\pm 01$ | $0.28_{-0.05}^{+0.05}$ | $0.86_{-0.01}^{+0.01}$ | $1.14_{-0.06}^{+0.06}$ | $0.09\pm 0.01$
S01 | $08\pm 01$ | $2.10_{-0.07}^{+0.07}$ | $1.70_{-0.14}^{+0.14}$ | $3.80_{-0.21}^{+0.21}$ | $0.29\pm 0.02$
S02 | $08\pm 02$ | $1.92_{-0.07}^{+0.08}$ | $0.54_{-0.09}^{+0.09}$ | $2.46_{-0.16}^{+0.17}$ | $0.19\pm 0.01$
S03 | $06\pm 02$ | $1.81_{-0.06}^{+0.05}$ | $0.01_{-0.01}^{+0.01}$ | $1.82_{-0.07}^{+0.06}$ | $0.14\pm 0.01$
S04 | $08\pm 01$ | $1.98_{-0.04}^{+0.05}$ | $0.01_{-0.01}^{+0.01}$ | $1.99_{-0.05}^{+0.06}$ | $0.15\pm 0.01$
S05 | $04\pm 02$ | $1.56_{-0.05}^{+0.05}$ | $0.01_{-0.01}^{+0.01}$ | $1.57_{-0.06}^{+0.06}$ | $0.12\pm 0.01$
S06 | $08\pm 03$ | $1.38_{-0.05}^{+0.06}$ | $0.01_{-0.01}^{+0.01}$ | $1.39_{-0.06}^{+0.07}$ | $0.11\pm 0.01$
S07 | $07\pm 04$ | $1.28_{-0.06}^{+0.05}$ | $0.01_{-0.01}^{+0.01}$ | $1.29_{-0.07}^{+0.06}$ | $0.10\pm 0.01$
S08 | $03\pm 02$ | $1.00_{-0.07}^{+0.09}$ | $2.01_{-0.30}^{+0.30}$ | $3.01_{-0.37}^{+0.39}$ | $0.23\pm 0.03$
S09 | $10\pm 04$ | $1.11_{-0.04}^{+0.05}$ | $3.42_{-0.28}^{+0.28}$ | $4.53_{-0.32}^{+0.33}$ | $0.35\pm 0.03$
S10 | $13\pm 07$ | $1.08_{-0.04}^{+0.04}$ | $2.31_{-0.18}^{+0.18}$ | $3.39_{-0.22}^{+0.22}$ | $0.26\pm 0.02$
S11 | $12\pm 06$ | $0.99_{-0.02}^{+0.03}$ | $2.15_{-0.16}^{+0.16}$ | $3.16_{-0.18}^{+0.19}$ | $0.24\pm 0.01$
S12 | $12\pm 06$ | $1.06_{-0.03}^{+0.03}$ | $1.29_{-0.12}^{+0.12}$ | $2.35_{-0.15}^{+0.15}$ | $0.18\pm 0.01$
S13 | $08\pm 03$ | $1.03_{-0.03}^{+0.03}$ | $2.82_{-0.18}^{+0.18}$ | $3.85_{-0.21}^{+0.21}$ | $0.30\pm 0.02$
S14 | $07\pm 04$ | $0.82_{-0.04}^{+0.04}$ | $3.92_{-0.20}^{+0.18}$ | $4.74_{-0.24}^{+0.22}$ | $0.37\pm 0.02$
S14A | $07\pm 04$ | $0.60_{-0.05}^{+0.05}$ | $6.23_{-0.92}^{+0.88}$ | $6.83_{-0.97}^{+0.93}$ | $0.53\pm 0.07$
S33 | $05\pm 02$ | $0.28_{-0.02}^{+0.03}$ | $1.47_{-0.13}^{+0.13}$ | $1.75_{-0.16}^{+0.15}$ | $0.14\pm 0.01$
S34 | $04\pm 03$ | $0.30_{-0.02}^{+0.03}$ | $1.23_{-0.13}^{+0.13}$ | $1.53_{-0.16}^{+0.15}$ | $0.12\pm 0.01$
S35 | $05\pm 03$ | $0.27_{-0.04}^{+0.05}$ | $0.74_{-0.18}^{+0.18}$ | $1.01_{-0.23}^{+0.22}$ | $0.08\pm 0.02$
S36 | $06\pm 02$ | $0.28_{-0.03}^{+0.03}$ | $0.97_{-0.20}^{+0.20}$ | $1.25_{-0.23}^{+0.22}$ | $0.10\pm 0.02$
S37 | $04\pm 03$ | $0.29_{-0.03}^{+0.04}$ | $1.06_{-0.14}^{+0.14}$ | $1.35_{-0.17}^{+0.18}$ | $0.10\pm 0.02$
S38 | $05\pm 03$ | $0.31_{-0.03}^{+0.03}$ | $1.21_{-0.16}^{+0.16}$ | $1.52_{-0.18}^{+0.19}$ | $0.12\pm 0.02$
S39 | $08\pm 05$ | $0.29_{-0.03}^{+0.03}$ | $1.58_{-0.22}^{+0.22}$ | $1.87_{-0.24}^{+0.25}$ | $0.14\pm 0.02$
S40 | $05\pm 02$ | $0.28_{-0.05}^{+0.05}$ | $1.66_{-0.21}^{+0.21}$ | $1.94_{-0.25}^{+0.26}$ | $0.15\pm 0.02$
S41 | $08\pm 04$ | $0.28_{-0.03}^{+0.03}$ | $1.21_{-0.12}^{+0.12}$ | $1.49_{-0.14}^{+0.15}$ | $0.12\pm 0.01$
S42 | $03\pm 01$ | $0.32_{-0.03}^{+0.02}$ | $1.45_{-0.12}^{+0.12}$ | $1.77_{-0.15}^{+0.14}$ | $0.14\pm 0.01$
S43 | $03\pm 01$ | $0.33_{-0.03}^{+0.03}$ | $1.69_{-0.29}^{+0.30}$ | $2.02_{-0.32}^{+0.33}$ | $0.16\pm 0.02$
S44 | $04\pm 02$ | $0.29_{-0.04}^{+0.04}$ | $1.92_{-0.35}^{+0.37}$ | $2.21_{-0.39}^{+0.41}$ | $0.17\pm 0.03$
S45 | $03\pm 02$ | $0.33_{-0.05}^{+0.05}$ | $2.14_{-0.21}^{+0.20}$ | $2.47_{-0.26}^{+0.25}$ | $0.19\pm 0.02$
S46 | $05\pm 03$ | $0.30_{-0.03}^{+0.04}$ | $3.22_{-0.55}^{+0.54}$ | $3.52_{-0.58}^{+0.58}$ | $0.27\pm 0.04$
S47 | $03\pm 01$ | $0.30_{-0.04}^{+0.04}$ | $2.07_{-0.13}^{+0.14}$ | $2.37_{-0.17}^{+0.18}$ | $0.18\pm 0.02$
S48 | $03\pm 01$ | $0.30_{-0.04}^{+0.05}$ | $2.01_{-0.16}^{+0.16}$ | $2.31_{-0.20}^{+0.21}$ | $0.18\pm 0.02$
S49 | $03\pm 02$ | $0.37_{-0.05}^{+0.05}$ | $2.89_{-0.24}^{+0.23}$ | $3.26_{-0.29}^{+0.28}$ | $0.25\pm 0.02$
S50 | $04\pm 02$ | $0.30_{-0.05}^{+0.07}$ | $2.49_{-0.40}^{+0.40}$ | $2.79_{-0.45}^{+0.47}$ | $0.22\pm 0.03$
S51 | $05\pm 03$ | $0.27_{-0.04}^{+0.06}$ | $3.07_{-0.41}^{+0.40}$ | $3.34_{-0.45}^{+0.45}$ | $0.26\pm 0.03$
S52 | $08\pm 04$ | $0.27_{-0.05}^{+0.05}$ | $2.53_{-0.17}^{+0.16}$ | $2.80_{-0.22}^{+0.21}$ | $0.22\pm 0.02$
S53 | $06\pm 03$ | $0.27_{-0.03}^{+0.05}$ | $2.45_{-0.34}^{+0.34}$ | $2.72_{-0.37}^{+0.39}$ | $0.21\pm 0.03$
S54 | $03\pm 02$ | $0.30_{-0.05}^{+0.05}$ | $2.90_{-0.24}^{+0.24}$ | $3.20_{-0.29}^{+0.29}$ | $0.25\pm 0.02$
S55 | $04\pm 02$ | $0.30_{-0.05}^{+0.05}$ | $3.39_{-0.31}^{+0.29}$ | $3.69_{-0.36}^{+0.34}$ | $0.28\pm 0.03$
S56 | $03\pm 01$ | $0.28_{-0.03}^{+0.03}$ | $2.37_{-0.17}^{+0.15}$ | $2.65_{-0.20}^{+0.18}$ | $0.20\pm 0.02$
S57 | $03\pm 01$ | $0.33_{-0.05}^{+0.05}$ | $2.71_{-0.35}^{+0.36}$ | $3.04_{-0.40}^{+0.41}$ | $0.23\pm 0.03$
S58 | $03\pm 01$ | $0.32_{-0.06}^{+0.08}$ | $2.87_{-0.24}^{+0.22}$ | $3.19_{-0.30}^{+0.30}$ | $0.25\pm 0.02$
S59 | $02\pm 01$ | $0.30_{-0.05}^{+0.06}$ | $2.87_{-0.34}^{+0.36}$ | $3.17_{-0.39}^{+0.42}$ | $0.24\pm 0.03$
S60 | $04\pm 02$ | $0.44_{-0.04}^{+0.04}$ | $2.55_{-0.32}^{+0.34}$ | $2.99_{-0.36}^{+0.38}$ | $0.23\pm 0.03$
S61 | $06\pm 03$ | $0.30_{-0.03}^{+0.04}$ | $2.85_{-0.42}^{+0.40}$ | $3.15_{-0.45}^{+0.44}$ | $0.24\pm 0.03$
S62 | $03\pm 01$ | $0.32_{-0.06}^{+0.06}$ | $3.20_{-0.36}^{+0.38}$ | $3.52_{-0.42}^{+0.44}$ | $0.27\pm 0.03$
S63 | $04\pm 02$ | $0.30_{-0.03}^{+0.04}$ | $2.98_{-0.27}^{+0.29}$ | $3.28_{-0.30}^{+0.33}$ | $0.25\pm 0.02$
S64 | $03\pm 02$ | $0.28_{-0.04}^{+0.04}$ | $3.40_{-0.38}^{+0.36}$ | $3.68_{-0.42}^{+0.40}$ | $0.28\pm 0.03$
S65 | $04\pm 01$ | $0.31_{-0.04}^{+0.03}$ | $2.55_{-0.22}^{+0.22}$ | $2.86_{-0.25}^{+0.26}$ | $0.22\pm 0.02$
S66 | $02\pm 01$ | $0.32_{-0.05}^{+0.06}$ | $3.24_{-0.37}^{+0.37}$ | $3.56_{-0.43}^{+0.42}$ | $0.27\pm 0.03$
S67 | $05\pm 03$ | $0.28_{-0.04}^{+0.05}$ | $3.17_{-0.44}^{+0.44}$ | $3.55_{-0.48}^{+0.49}$ | $0.27\pm 0.04$
S68 | $04\pm 02$ | $0.30_{-0.05}^{+0.05}$ | $3.19_{-0.40}^{+0.40}$ | $3.49_{-0.45}^{+0.45}$ | $0.27\pm 0.04$
S69 | $02\pm 02$ | $0.29_{-0.04}^{+0.05}$ | $3.38_{-0.52}^{+0.50}$ | $3.67_{-0.56}^{+0.55}$ | $0.28\pm 0.04$
S71 | $05\pm 03$ | $0.33_{-0.04}^{+0.05}$ | $3.01_{-0.39}^{+0.41}$ | $3.31_{-0.43}^{+0.46}$ | $0.26\pm 0.03$
S72 | $03\pm 01$ | $0.34_{-0.04}^{+0.05}$ | $2.83_{-0.30}^{+0.32}$ | $3.15_{-0.34}^{+0.37}$ | $0.24\pm 0.03$
S73 | $03\pm 01$ | $0.29_{-0.04}^{+0.05}$ | $2.80_{-0.32}^{+0.30}$ | $3.07_{-0.36}^{+0.35}$ | $0.24\pm 0.03$
S74 | $08\pm 04$ | $0.28_{-0.01}^{+0.02}$ | $3.24_{-0.44}^{+0.46}$ | $3.50_{-0.45}^{+0.48}$ | $0.27\pm 0.04$
S76 | $05\pm 02$ | $0.30_{-0.03}^{+0.04}$ | $3.54_{-0.70}^{+0.72}$ | $3.81_{-0.73}^{+0.76}$ | $0.29\pm 0.06$
S77 | $04\pm 02$ | $0.30_{-0.02}^{+0.02}$ | $2.89_{-0.32}^{+0.30}$ | $3.17_{-0.34}^{+0.32}$ | $0.24\pm 0.03$
S78 | $04\pm 02$ | $0.33_{-0.04}^{+0.04}$ | $3.10_{-0.40}^{+0.38}$ | $3.40_{-0.44}^{+0.42}$ | $0.26\pm 0.03$
Figure 3:  The evolution of $\log(\lambda_{\rm\,Edd})$ and the power-law slope
$\Gamma$ of the changing-look AGN 1ES 1927+654 during the initial flare (red
squares) and the post-flare state (blue circles). See Table
LABEL:Table:lambdaedd for details). We do not see any correlation between
these parameters either during the initial flare or the post-flare states.

Figure 4:  Left: The relation between the soft X-ray ($0.3-2\rm\,\,{\rm keV}$)
and hard X-ray ($2-10\rm\,\,{\rm keV}$) fluxes during the initial flare. The
red circles denote the X-ray flaring period covering 2018-2019 (observations
S01-S14A). The upper limits denote the lowest state in X-rays in both axes in
the lower left corner of the figure. The soft and the hard X-rays do not show
any significant correlation. Right: Same as left, but for the new observations
(S33-S78) encompassing the bright-soft-state shows a significant correlation
between the soft and hard X-ray solely driven by the power-law flux alone due
to the soft nature of the source. The best fit linear regression fit gives us:
$y=6.47+3.52x$, with a Spearman rank correlation coefficient=$0.65$, and a
statistical significance $>99.99\%$ .

Figure 5:  Left: The powerlaw ($0.3-10\rm\,\,{\rm keV}$) flux vs the blackbody
($0.3-10\rm\,\,{\rm keV}$) flux plot during the bright soft state (i.e.,
observations S49-S78) exhibits no significant correlation. The bright-soft-
state plotted in green, and the pre-flare XMM data plotted in magenta for
comparison. Middle: The powerlaw $0.3-10\rm\,\,{\rm keV}$ flux vs UVW2 flux
density plot and Right: The blackbody $0.3-10\rm\,\,{\rm keV}$ flux vs UVW2
flux density plot during the same period as in the left panel similarly shows
no statistically significant correlation.

###  3.3 Correlations between soft X-ray excess, the coronal and the UV flux

We find a significant correlation between $F_{0.3-2.0\rm\,\,{\rm keV}}$ and
$F_{2-10\rm\,\,{\rm keV}}$ in the post-flare bright-soft-state of the source.
Figure 4 right panel shows the correlation between these two quantities for
the observations S33-S78 when the soft X-ray flux steadily increased (green
data points). In comparison, we note that there was no correlation between the
soft and hard X-rays during the ‘initial flare’ where the non-parametric
Spearman rank correlation coefficient is 0.60 with a confidence level $\sim
98.0\%$(See Fig. 4 left panel). To understand if there is indeed any relation
between the soft excess and the power law emission (in the current bright-
soft-state), we correlated the respective fluxes obtained from the best fit
models, black-body and power law respectively. We note that we required a
blackbody component to model the soft excess only for the observations S49 to
S78. We do not find any statistically significant correlation as shown in Fig
5 left panel. This indicates that the soft excess and power law variations are
independent of each other, and the apparent correlation between the
$0.3-2\rm\,\,{\rm keV}$ vs $2-10\rm\,\,{\rm keV}$ flux is driven by the power
law component which is very soft ($\Gamma\sim 2.5-3$). Similarly, we find no
correlations between the UV and the power-law flux and UV and blackbody flux
(See Fig. 5 middle and right panel).

We also checked for individual soft X-ray flares between different
observations and found e.g., the soft X-ray has flared by a factor of a few
times in the span of a few days: between S45 and S46, S48 and S49, S50 and
S51, and between S65 and S66 (See Table 2). However, the hard X-ray flux not
always shown an increase corresponding to a soft X-ray increase e.g., between
S50 and S51, the soft X-ray flux increased but the hard X-ray flux decreased.
This is demonstrated in the lack of correlation between the soft excess flux
and the power-law flux.

###  3.4 Evolution of hardness ratio

Interestingly, the hardness ratio (HR) (defined as $F_{2-10.0\rm\,\,{\rm
keV}}/F_{0.3-2.0\rm\,\,{\rm keV}}$) for this source always varied between
$0.002\pm 0.001$ (during the initial flare S01) to $0.500\pm 0.044$ (during
S36). If we compare these values to the pre-flare value of $0.417\pm 0.021$
(X1) we see the HR has always been either lower or within errors compared to
the pre-flare value. During the ‘initial flare’, the HR showed both hard-when-
bright (while rising) and soft-when-bright (while coming back) behaviour. The
HR increased at the observation when the corona was revived in Oct 2018. In
the post-flare state, during observations S15-S33, when the source showed
minimal X-ray variations, the HR still varied considerably within this range.
See Fig. 1 panel 3. We do not detect any fixed pattern for the HR variability
in response to the X-ray flux changes. Nor do we find any significant
correlation between $L_{\rm 0.3-10\rm\,\,{\rm keV}}$ and HR (See Fig. 6).
During the recent bright-soft-state observed from S33, the HR varied
considerably between these limits. Currently (S78), in the bright-soft-state,
we find the HR to be $\sim 0.172$

###  3.5 Correlation between $\Gamma$ vs $L_{2-10\rm\,\,{\rm keV}}$

We plotted the power law slope $\Gamma$ and the hard X-ray luminosity
($L_{2-10\rm\,\,{\rm keV}}$) for the bright-soft-state (Fig. 7). It is evident
from the figure that in the post-flare state, the power-law slope is
insensitive to the variations in the $2-10\rm\,\,{\rm keV}$ luminosity.

###  3.6 The evolution of $\alpha_{\rm OX}$

The ratio between the X-ray and UV, which we refer to as $\alpha_{\rm OX}$, is
calculated from the ratio of the monochromatic fluxes in the UV ($2500$Å) and
X-rays ($2\rm\,\,{\rm keV}$), i.e., $\alpha_{\rm OX}=-0.385\log(F_{\rm
2\rm\,\,{\rm keV}}/F_{\rm 2500\AA})$ (Tananbaum et al., 1979; Lusso et al.,
2010). This is an important diagnostic parameter to understand if the
accretion disk and the X-ray-emitting corona are physically connected. The
$\alpha_{\rm OX}$ had attained a very high value of $\sim 2$ during the
‘initial flare’; however, in the post-flare state, the values are mostly
consistent with that of the pre-flare state. In the recent bright-soft-state,
the $\alpha_{\rm OX}$ has dropped. This is because there is no change in the
UV flux, while the X-ray flux has increased. Figure 8 shows the relation
between $\alpha_{\rm OX}$ and $L_{\rm 2500\AA}$ and we find that the recent
bright-soft-state data points denoted by blue dots deviate considerably from
the standard AGN behaviour. We plot the ‘highest flaring state’ data point on
this plot to give an idea of where the source was in this phase space during
the ‘initial flare’.

###  3.7 The 5-GHz Radio observations

The point source radio flux density at 5 GHz was at a minimum (a factor of
four times lower than the pre-flare value) when the X-rays were just starting
to increase. Over a period of the next one thousand days, the radio flux
density gradually increased (Yang et al., 2022), which coincided with the time
period when the UV and X-rays returned to their pre-flare values (see Fig 1).
However, the latest VLBA observation in August 2022 showed a decrease in the
core radio flux density compared to March 2021. This decrease also coincides
with the latest rise in the soft X-ray flux (bright-soft-state). The radio
flux density could be related to either the hard or the soft X-ray or both in
a more complex way.

The ratio between the radio and the X-ray (known as the Güdel Benz relation)
has been plotted in Fig.9. We find that the ratio constantly decreased during
the initial flare, it picked up a little in 2021 and then started decreasing
again. The values of the ratio are well within the spread of the values found
typically in radio-quiet AGN in the Palomar-Green sample studied by Laor &
Behar (2008). We see only a minor change in the morphology (size became
smaller) of the extended emission but not in intensity.

###  3.8 The optical spectra

Both the optical spectra from LDT and HCT show mostly narrow emission lines,
as reported in our earlier work. We also detect broad H$\alpha$ with a typical
FWHM of $1172\pm 327\rm\,\,{\rm km\,s^{-1}}$. We do not see any change in the
narrow or broad emission line intensities and the line ratios from our earlier
work (Laha et al., 2022) except for OIII where the line intensity has nearly
doubled. The reduced spectra and the best-fit model from HCT are shown in
Figure 10 and Figure 11 respectively showing line intensities consistent with
Laha et al. (2022).

Figure 6: Hardness–intensity diagram for 1ES 1927+654. The ‘initial flare’
S01-S14A observations are plotted as red squares. The post-flare states
S15-S31 (when the source got back to the pre-flare state) are plotted as blue
circles. The bright-soft-state S33-S78 are plotted as green triangles. (see
Table 2 for details). Figure 7: The photon index $\Gamma$ and the
$2-10\rm\,\,{\rm keV}$ X-ray luminosity plot during the bright-soft-state that
is S33-S78. The $\Gamma$ is relatively insensitive to the hard X-ray
luminosity.

Figure 8:  Left: The $\alpha_{\rm OX}$ vs $L_{2500\mbox{\AA}}$ for the
different Swift observations of the source 1ES 1927+654 as reported in Table 2
and Laha et al. (2022). The red line is the best-fit correlation from Lusso &
Risaliti (2016) representing the standard AGN disk-corona relation. The pink
diamond represents the 2011 pre-CL state of 1ES 1927+654. Right: The evolution
of $\log(\lambda_{\rm\,Edd})$ and the $\alpha_{OX}$ during the initial flare
(red squares) and the post-flare state (blue circles). See Table
LABEL:Table:lambdaedd for details). The solid red line is the generic AGN
behaviour, as obtained by (Lusso & Risaliti, 2016) for a sample of AGNs. We do
not see any correlation between these parameters either during the initial
flare or the post-flare states.  Figure 9:  The light curve of the ratio of
the 5GHz mono-chromatic central radio luminosity with the $2-10\rm\,\,{\rm
keV}$ luminosity, popularly known as the Güdel-Benz relation. See Table 4 for
details. The $0.3-2.0\rm\,\,{\rm keV}$ X-ray flux vs the 5GHz monochromatic
radio flux exhibits no correlation. See Table 2 and Table 4 for the X-ray and
Radio flux values. The shaded region represents the standard range of the
Güdel-Benz relation for radio-quiet AGNs.  Figure 10: Optical spectrum of 1ES
1927+654 obtained with HCT. The data is shown in grey, the decomposed stellar
template is shown in red, and the host subtracted spectrum is shown in blue.
The spectrum was smoothened by a 5-pixel box car for visualization purposes
only.

Figure 11: Emission line fitting of the optical spectrum obtained by HCT. The
plot shows observed data (solid line with flux error in gray), best-fit total
model (red), stellar contribution (green), and host-subtracted spectrum (gray)
with individual narrow lines (Gaussian). Without any broad component in
H$\beta$ and H$\alpha$ in the left and middle panels respectively and with one
broad (green -Gaussian) component in H$\alpha$ in the right panel. The
spectrum was smoothened by a 5-pixel box car for visualization purposes only.

##  4 Discussion

We have followed up the CL-AGN 1ES 1927+654 with X-ray and UV observations
with Swift, radio observation from VLBA, and optical observation from LDT and
HCT. Our earlier paper Laha et al. (2022) reported the observations till 31st
December 2021, encompassing the entire phase of the ‘initial flare’. Here we
report the multi-wavelength observations from 1st January 2022 till 5th May
2023, particularly highlighting the recent bright-soft-state of the source. In
light of our multi-wavelength observations, we address the following
scientific questions.

###  4.1 The recent bright-soft-state and the origin of soft X-ray excess

The soft X-ray excess (SE) is the excess emission (over the power law) in the
soft band $0.3-2\rm\,\,{\rm keV}$ in an AGN spectrum which typically takes the
shape of a black body with temperatures ranging $0.1-0.3\rm\,\,{\rm keV}$. The
origin of the SE is still highly debated, (Noda & Done, 2018; García et al.,
2019a; Ghosh & Laha, 2020, 2021) as different sources show different spectral
and timing behaviour of the SE with respect to the other bands of the AGN
continuum such as the UV (accretion disk) and the $2-10\rm\,\,{\rm keV}$
(power law). The two most popular models describing the SE are (a) Thermal
Comptonization from warm ($kT_{e}\sim 0.1-0.5\rm\,\,{\rm keV}$) optically-
thick ($\tau\sim 10-20$) corona (Mehdipour et al., 2011; Done et al., 2012;
Ghosh et al., 2018; Petrucci et al., 2018, 2020) and (b) reflection of the
primary continuum from ionized disk (García et al., 2014; Ghosh et al., 2016;
García et al., 2019b).

The thermal Comptonization assumes the existence of a warm ($kT\sim
0.1\rm\,\,{\rm keV}$) optically thick medium above the accretion disk. A
fraction of the accretion disk photons interact with this plasma and gets up-
scattered to create the black body shaped soft excess. Therefore this model
points towards a direct link between the UV and SE flux. This model predicts
an increase in the UV should be related to an increase in the soft X-rays and
vice-versa.

The relativistic reflection, on the other hand, assumes that the SE arises out
of the reflection of the hard X-ray photons from the primary continuum
emission from the ionized accretion disk. The individual emission lines
arising out of the reflection get smoothed out (broadened beyond detection) by
the gravitational effects. This model, therefore, assumes a direct link
between the $2-10\rm\,\,{\rm keV}$ flux and the SE flux.

The SE behaviour in source 1ES1927+654 is very unique. During the ‘initial
flare’ (during 2017-2019, reported in Ricci et al. 2021; Laha et al. 2022;
Masterson et al. 2022), the source started off very soft when the UV was
gradually decreasing. When the $2-10\rm\,\,{\rm keV}$ band X-rays completely
vanished, there was still some traces of the SE as detected by XMM-Newton
(Ricci et al., 2021). This phase of the SE was characterized by an unusually
varying lower temperature (kT$\sim 0.08-0.1\rm\,\,{\rm keV}$) black body. The
temperature was lower than that usually measured for this source in the pre-
flare state (kT$\sim 0.2\rm\,\,{\rm keV}$). The presence of SE in the absence
of a power law component is inconsistent with the reflection scenario because,
without the primary power law, there should not be any reflection in the first
place. This is also unusual in the warm Comptonization scenario because the UV
flux was very high at the time when the SE was the lowest unless the warm
Comptonizing corona also vanished in some way. When the X-rays revived with a
flare in Oct 2018 (see Fig. 1) the UV was still monotonically decreasing with
a power law decline $y\propto t^{-0.91}$. There was no correlation between the
UV and X-ray fluxes, or even between the soft and the hard X-ray fluxes (see
Fig. 4 left panel). These results make it hard to describe the SE of this
source with either of the two models mentioned above.

During the recent bright-soft-state since May 2022, which became more
prominent in Dec 2022, we find that the UVW2 flux density changes are confined
within a fluctuation range of $\leq 30\%$. The mean value of the UV flux in
the bright-soft-state is $1.96\times 10^{-15}$ and a standard deviation of
$0.19\times 10^{-15}$ (in units of
$\rm\,erg\,cm^{-2}\,s^{-1}\mbox{\AA}^{-1}$). While the UV flux shows minimal
change, the soft X-ray flux has increased by a factor of five compared to the
pre-flare state. The hard X-ray flux has also increased by a factor of $\sim
2$ but has shown substantial fluctuations in the latest observations (see Fig.
2 for details). We do not see any correlations between the UV and soft X-rays
and UV and hard X-rays (see Fig. 12). This is in agreement with the lack of
correlation observed between the soft excess flux (modelled by the blackbody)
and the coronal flux (modelled by the power-law) in the recent bright-soft-
state (Fig.5). These results are inconsistent with both the models describing
the soft excess as the reflection from the ionized disk and/or thermal
Comptonization scenario.

Previous studies using broadband spectroscopy with XMM-Newton concluded that
the $1\rm\,\,{\rm keV}$ emission feature (detected in XMM-Newton and NICER
observations) could arise out of the reflection of the hard X-ray photons of
the accretion disk (Masterson et al., 2022). They suggested relativistic
reflection happening during the TDE, resulting in the $1\rm\,\,{\rm keV}$
emission line. The spectroscopic fits could not distinguish between the
thermal Comptonization and the reflection models for the origin of the SE.

Other CL-AGN sources have also shown the absence of correlation between the
X-rays and the UV and between the soft and the hard X-rays. For example, Mrk
590 (Ghosh et al., 2022) where we found that the SE completely vanished when
the power law and the UV were still dominant. We also note that in 1ES1927+654
since the start of the bright-soft-state, the $\Gamma$ has not varied
significantly, staying between $2.01^{+0.50}_{-0.90}$ and
$2.96^{+0.36}_{-0.37}$, consistent within the large errors.

Now the question is: what is pumping the energy to the X-rays if the UV flux,
and hence the standard accretion rate, is so silent? To understand this, we
need to have some estimate of the energy. First, we calculate the energetics
of this system in one of the shortest variability time scales, which in this
case is 4 days (cadence of our Swift observation). We note that the highest
soft and hard X-ray flux change happened between S48 and S49 (gap of 4 days).
Assuming a constant increase profile, we estimated the soft band energy pumped
within four days is equal to $9.7\times 10^{47}\rm erg$. During that time, the
total hard X-ray energy pumped is equal to $3.7\times 10^{47}\rm erg$. While
the energy pumped in UV amounts to $1.9\times 10^{47}\rm erg$, which roughly
equals $14\%$ of the total X-ray energy pumped during this period.

Now let’s consider the energy pumped in a longer time scale since the rise of
the bright-soft-state phase, which is between S35(May 2022) and S76(May 2023).
During these eleven months (344 days), the energy pumped in the soft and the
hard bands are $3.57\times 10^{50}\rm erg$ and $0.59\times 10^{50}\rm erg$,
respectively. During this one year long time period the UV band did not show
any significant change at all (see Fig. 2), with fluctuations consistent
within one standard deviation around the mean value. Fig 13 shows the soft and
the hard X-ray band rise and the best fit linear regression line. The
integrated energy for soft and hard bands quoted above (for 11 months) has
been estimated by the area under the best fit linear regression curve.

In addition, we can estimate the mass accretion rate required to create the
luminosity in the soft X-rays from the standard accretion theory,
$L=\eta\dot{M}c^{2}$, where $L$ is the accretion luminosity, $\eta$ is the
accretion efficiency, $\dot{M}$ is the mass accretion rate, and $c$ is the
speed of light. This is assuming that whatever is creating the soft X-rays is
accreting and deriving its energy from the gravitational binding energy. If we
assume a value of $\eta=0.1$ for standard AGNs, we get an accretion rate of
$\sim 0.003M_{\odot}\,\rm yr^{-1}$ needed to create this bright-soft-sate.

From the energetics, it is evident that whatever is producing the SE is
pumping out more energy than either the UV or hard X-ray source. Since the
energy source presumably is ultimately accretion of matter onto the SMBH, the
SE emitting region must be receiving the majority of this energy. This could
mean that the SE region is either the dominant accreting flow or there is a
conduit pumping the accretion energy from the disk to the SE region rather
than radiating it inside the disk.

If warm Comptonization produced the soft excess, then one would expect a
direct relation between the UV luminosity and the soft X-ray luminosity e.g.,
as observed in NGC 3516 (Mehdipour et al., 2022), but we do not find in 1ES
1927+654. Moreover, one would expect to observe absorption in the soft X-rays
due to the atomic opacity in the warm Comptonizing corona (García et al.,
2019a), which we do not see in the XMM-Newton EPIC-pn data of the source in
the bright-soft-state (Ghosh et al. in preparation). The reflection scenario,
on the other hand, cannot explain a weaker ($\sim 2$ times) hard X-ray
variation (which is the primary flux) compared to the stronger ($\sim 5$
times) consistent rise of soft X-ray, with the soft X-rays gaining $\sim 10$
times more energy than the primary power law in a matter of 11 months. This is
also true if we see the initial flare (Laha et al., 2022) where no correlation
between the soft and hard X-ray was found.

Therefore, in this source, we need a different mechanism to produce the soft
X-ray excess. We conjecture that the soft excess in this source may not be the
canonical soft excess we find in typical AGN. This is supported by several
observational results from both the initial flare (Dec 2017- Dec 2019) and the
bright soft state (May 2022- May 2023), such as (1) no correlation between the
SE and other X-ray and UV components (2) SE modeled by an unusually varying
lower temperature black body (kT$\sim 0.08-0.1\rm\,\,{\rm keV}$) in initial-
flare when the coronal emission vanished, (3) The current consistent rise in
the SE having a black body temperature mostly constant but on the higher side
(kT$\sim 0.15-0.2\rm\,\,{\rm keV}$).

Figure 12:  Left: The soft X-ray $0.3-2\rm\,\,{\rm keV}$ flux vs the UVW2 flux
density during the bright soft state (i.e., observations S33-S78) exhibits no
significant correlation. The bright-soft-state plotted in green, the pre-flare
XMM data plotted in magenta and the highest flux state in magenta.See Table 2
for details. Right: The hard X-ray $2-10\rm\,\,{\rm keV}$ flux vs UVW2 flux
density during the same period as in the left panel similarly shows no
statistically significant correlation.

Figure 13:  Left: The linear regression fit to the soft X-ray energy increase
during the recent bright-soft-state. Right: Same as left, but for the hard
X-rays. We have estimated the total integrated energy input in the soft and
hard X-rays during this time period by integrating under the best-fit line, as
shown above. See Section 4.1 for details. The dotted horizontal line
represents the pre-flare flux values obtained from the 2011 XMM-Newton
observation.

###  4.2 The relation between $\alpha_{\rm OX}$ vs $L_{\rm 2500\AA}$ and
$\alpha_{\rm OX}$ vs $\lambda_{\rm\,Edd}$

The relation between $\alpha_{\rm OX}$ and $L_{\rm 2500\AA}$ has been found to
be tightly related for AGN across cosmic timescales (Lusso et al., 2010; Lusso
& Risaliti, 2016), which implies a close relation between the accretion disk
(UV) and the corona (X-ray). Figure 8 left panel shows the data points for the
source 1ES 1927+654 during the initial flare (in red) and the bright-soft-
state (in blue), and in neither of the cases, the source behaviour follows the
commonly detected correlation (red line). We note that the pre-flare state (in
pink diamond) was close to that of the expected slope, and possibly the AGN
disk corona emission was related at that time. We also note that when the
source went back to the pre-flare state after the initial flare, the red dots
and blue circles were near the expected slope. However, in neither the
‘initial flare’ nor the bright-soft-state, the disk corona relation holds.

Similarly, from Fig. 8 right panel, we note that the $\alpha_{\rm OX}$ vs
$\lambda_{\rm\,Edd}$ show no fixed pattern. Ruan et al. (2019) compared the
observed correlations between $\alpha_{\rm OX}$ and $\lambda_{\rm\,Edd}$ in
AGN (including low accretion states $\lambda_{\rm\,Edd}\sim 10^{-2}$) to those
predicted from the observations of X-ray binary outbursts. They found that the
observed correlations in AGN are very similar to the accretion state
transitions in typical X-ray binary outbursts, including the inversion of this
correlation at $\lambda_{\rm\,Edd}\sim 10^{-2}$. The right panel of Fig. 8
shows that 1ES 1927+654 never went below $\log\lambda_{\rm\,Edd}<-1.2$, and
hence we are confined to only the high accretion phase space of the study by
Ruan et al. (2019). The bright-soft state is visible (red squares at the top),
but as the UV faded, the X-rays grew in strength, leading to the ‘high-hard’
state (lowest pink square at the bottom right). The red line represents the
general AGN behaviour as obtained by Lusso & Risaliti (2016).

###  4.3 $\Gamma$ vs $\lambda_{\rm\,Edd}$ and $\Gamma$ vs $L_{\rm
2-10\rm\,\,{\rm keV}}$ evolution

In a large sample of $\sim 7500$ AGN Sobolewska & Papadakis (2009) found that
the average spectral slope $\Gamma$ does not correlate with source luminosity
or BH mass while it correlates positively with average $\lambda_{\rm\,Edd}$.
From Fig. 3, we find that in the ‘initial flare’, there was no correlation
between $\Gamma$ and $\lambda_{\rm\,Edd}$ (red data points). However, the
bright-soft-state is different (in blue points). We find that although there
has been an increase in the $\lambda_{\rm\,Edd}$ in this phase, the power law
$\Gamma$ is very narrowly distributed around $\sim 2.5$. Similarly the
$\Gamma$ vs $L_{2-10\rm\,\,{\rm keV}}$ plot (Fig. 7) shows that the power-law
slope is insensitive to the variation in hard X-ray luminosity. The conclusion
is that the power-law slope is insensitive to the luminosity and
$\lambda_{\rm\,Edd}$ in the bright-soft-state.

###  4.4 The evolution of core and the extended radio flux density

The physical mechanism of the origin of the unresolved core radio emission
from radio-quiet AGN is still unknown (see Panessa et al., 2019a; Yang et al.,
2020, and references therein). The possible candidates include (a) the AGN
corona (e.g. Yang et al., 2020), (b) low-power jets (e.g. Yao et al., 2021)
(c) shocks from winds (e.g. Zakamska & Greene, 2014) and (d) rapid star
formation (Panessa et al., 2019b, 2022; Kawamuro et al., 2022). 1ES 1927+654
has given us a unique glimpse at the core radio emission during and after the
violent changing look event. From Laha et al. (2022), we note that the core
radio emission was at its lowest when the hard X-ray flux was low. Gradually
with time over the next 3 years (till March 2022, see also Yang et al., 2022),
we found that the radio flux density increased but never reached its pre-flare
value. In a more recent observation in Aug 2022, the core radio flux density
decreased, coincidentally when the soft X-ray flux started to rise (bright-
soft-state). It is, therefore, clear that the core radio flux density is
variable at a time scale of months and hence cannot arise out of star
formation. It is likely that the core radio emission is related to the corona
or the shocks from the winds generated by the violent events, a nascent
evolving jet or a low-power jet decreasing in power. Future coordinated X-ray,
radio and optical-UV monitoring of the source will help us resolve the nature
of the radio emission.

The ratio of unresolved core radio emission at 5 GHz of radio-quiet AGN to the
$L_{\rm 2-10\rm\,\,{\rm keV}}$ luminosity follows a unique relation known as
the Güdel-Benz relation (GB, see, e.g., Laor & Behar, 2008) commonly found in
the coronally active stars. Table 4 lists the GB relation of the source, and
Fig. 9 shows the GB light curve. We find that the GB constantly reduced during
the violent phase, picked up, and then again started to reduce during the
bright-soft-state but still within the expected range for radio-quiet AGN.
Further follow-up radio monitoring is currently being undertaken for a better
understanding of the radio emission of the source.

###  4.5 The optical spectra

We do not see any significant variations in the emission line intensity in the
optical spectra between the 2021 and 2022 observations except for the OIII
emission lines which have nearly doubled their intensity. We think that the
light front from the December 2017 violent event may have just reached the
narrow-line region. We need further optical monitoring to confirm this
scenario.

##  5 Conclusions

We followed up the enigmatic changing look AGN 1ES 1927+654 with multi-
wavelength observations spanning a period over 1 year (Jan 2022- May 2023).
Below we list the most important conclusions.

  * •

We have observed a recent brightening of the soft X-ray flux since May 2022,
although there is no appreciable change in the UV flux. The total energy
pumped into the soft X-rays over a period of $11$ months (20th May 2022- 5th
May 2023) is $3.6\times 10^{50}$ erg, and that of the hard X-rays is
$5.9\times 10^{49}\rm\,\,{\rm erg\,}$, an order of magnitude lower. Both the
warm Comptonization and the disk reflection scenarios may not be adequate to
describe the soft X-rays of this source. The energetics suggest that whatever
is producing the SE is pumping out more energy than either the UV or hard
X-ray source. This implies that the SE region is either the dominant accreting
flow or there is a conduit pumping the accretion energy from the disk to the
SE region rather than radiating it inside the disk.

  * •

In the bright-soft-state we do not detect any correlation between soft X-rays
vs UV and the hard X-rays vs UV. In addition, we do not see any correlation
between the (a) soft excess vs the power-law, (b) soft excess vs UV, and (c)
power-law vs UV. The apparent correlation between the soft and hard X-rays is
driven by the power-law alone due to the soft nature of the coronal emission
($\Gamma\sim 2.5-3$).

  * •

The core radio emission ($<1pc$) at $5$ GHz showed an increase till March 2022
and then a dip in Aug 2022. It is probably linked to the coronal emission or
the shocks from the winds generated by the violent events during the initial
flare, a nascent evolving jet, or a low-power jet decreasing in power.

  * •

The Güdel-Benz relation continuously decreased from the pre-flare state and is
still within the range exhibited by most radio quiet AGN.

  * •

We do not detect any correlations between (a) $\Gamma$ vs $\lambda_{\rm\,Edd}$
and (b) Hardness ratio vs $L_{2-10\rm\,\,{\rm keV}}$, neither during the
‘initial flare’ nor during the bright-soft-state.The power-law slope is
insensitive to the luminosity and $\lambda_{\rm\,Edd}$ in the bright-soft-
state.

  * •

The data do not follow the $\alpha_{\rm OX}$ vs $L_{2500\AA}$ and the
$\alpha_{\rm OX}$ vs $\lambda_{\rm\,Edd}$ relations usually observed in
typical AGN where the disc-corona synergy is in place.

  * •

In the optical band we found that the line intensity of OIII has nearly
doubled since 2021. We think that the light front from the violent event in
Dec 2017 may have just reached the narrow-line-region. We need further
monitoring of the source to confirm this scenario.

##  6 Acknowledgements

We thank the anonymous referee for the insightful comments that helped to
better the manuscript. RG and SL thank Missagh Mehdipour for the helpful
discussion. The material is based upon work supported by NASA under award
number 80GSFC21M0002. We acknowledge the use of public data from the Swift
data archive and the Swift team for approving the Swift DDT request. MN is
supported by the European Research Council (ERC) under the European Union’s
Horizon 2020 research and innovation programme (grant agreement No. 948381)
and by funding from the UK Space Agency. XLY is supported by the National
Science Foundation of China (Grant No. 12103076) and the Shanghai Sailing
Program (Grant No. 21YF1455300). KÉG was supported by the Hungarian National
Research, Development and Innovation Office (NKFIH), grant number OTKA
K134213.

##  7 Data availability:

This research has used new and archival data of Swift observatory through the
High Energy Astrophysics Science Archive Research Center Online Service,
provided by the NASA Goddard Space Flight Center. This work has made use of
data from VLBA (available in the NRAO Data Archive: https://data.nrao.edu/)
and EVN observations (available in the EVN Data Archive at JIVE:
http://archive.jive.nl/scripts/portal.php). The National Radio Astronomy
Observatory is a facility of the National Science Foundation operated under
cooperative agreement by Associated Universities, Inc. The EVN is a joint
facility of independent European, African, Asian, and North American radio
astronomy institutes.

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The evolution of the X-ray spectra and the corresponding best-fit model are
shown below to characterise the evolution of 1ES 1927+654 as observed by
Swift-XRT.

Figure 14:  X-ray spectral evolution of 1ES 1927+654 as observed by Swift-XRT.
The X-ray spectrum in May 2018 is shown by black diamond points when the
spectrum was very soft and the spectrum $>1\rm\,\,{\rm keV}$ was about to
vanish (corona vanished). The lowest flux state (still detectable by XRT) was
captured in June 2018 shown in red triangular points. The highest flux state
(S14A, see Laha et al. 2022) in November 2019 is depicted as pink circular
points. The soft X-ray flux rise in Mar 2023 (during bright-soft-state) is
shown in blue star points. See Table 2 and Laha et al. (2022) for details.
Figure 15: The best-fit model obtained from the spectral fitting of the Swift
X-ray observations showing the evolution of 1ES 1927+654. We just included the
best-fit models to better visualize the spectral evolution. The dotted dashed,
and the solid line represent the black body, power-law, and the total spectrum
respectively for each observation. The model for May 2018 is shown by a black
curve featuring a soft spectrum. The highest flux state (S14A, see Laha et al.
2022) in November 2019 is depicted as a magenta curve. S35 represents the
recent observation in May 2022, where the soft and hard X-ray flux returned to
the pre-flare state (as in 2011) and is shown in the orange curve. Here, we
did not require any blackbody component to model the spectrum. The soft X-ray
flux rise in Mar 2023 (during bright-soft-state, S64) is shown in blue curve.
See Table 2 and Laha et al. (2022) for details.