# Chandra, HST/STIS, NICER, Swift, and TESS Detail the Flare Evolution of the
Repeating Nuclear Transient ASASSN-14ko

Anna V. Payne NASA Graduate Fellow  Institute for Astronomy, University of
Hawai`i at Manoa, 2680 Woodlawn Dr., Honolulu, HI 96822  Katie Auchettl School
of Physics, The University of Melbourne, Parkville, VIC 3010, Australia  ARC
Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D)
Department of Astronomy and Astrophysics, University of California, Santa
Cruz, CA 95064, USA  Benjamin J. Shappee Institute for Astronomy, University
of Hawai`i at Manoa, 2680 Woodlawn Dr., Honolulu, HI 96822  Christopher S.
Kochanek Department of Astronomy, The Ohio State University, 140 West 18th
Avenue, Columbus, OH 43210, USA  Center for Cosmology and AstroParticle
Physics, The Ohio State University, 191 W. Woodruff Ave., Columbus, OH 43210,
USA  Patricia T. Boyd  Laboratory for Exoplanets and Stellar Astrophysics,
NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA  Thomas W.-S.
Holoien NHFP Einstein Fellow  The Observatories of the Carnegie Institution
for Science, 813 Santa Barbara St., Pasadena, CA 91101, USA  Michael M.
Fausnaugh Department of Physics, and Kavli Institute for Astrophysics and
Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139,
USA  Chris Ashall Institute for Astronomy, University of Hawai`i at Manoa,
2680 Woodlawn Dr., Honolulu, HI 96822  Jason T. Hinkle Institute for
Astronomy, University of Hawai`i at Manoa, 2680 Woodlawn Dr., Honolulu, HI
96822  Patrick J. Vallely Department of Astronomy, The Ohio State University,
140 West 18th Avenue, Columbus, OH 43210, USA  K. Z. Stanek  Department of
Astronomy, The Ohio State University, 140 West 18th Avenue, Columbus, OH
43210, USA  Center for Cosmology and AstroParticle Physics, The Ohio State
University, 191 W. Woodruff Ave., Columbus, OH 43210, USA  Todd A. Thompson
Department of Astronomy, The Ohio State University, 140 West 18th Avenue,
Columbus, OH 43210, USA  Center for Cosmology and AstroParticle Physics, The
Ohio State University, 191 W. Woodruff Ave., Columbus, OH 43210, USA

###### Abstract

ASASSN-14ko is a nuclear transient at the center of the AGN ESO 253$-$G003
that undergoes periodic flares. Optical flares were first observed in 2014 by
the All-Sky Automated Survey for Supernovae (ASAS-SN) and their peak times are
well-modeled with a period of $115.2^{+1.3}_{-1.2}$ days and period derivative
of $-0.0026\pm 0.0006$. Here we present ASAS-SN, Chandra, HST/STIS, NICER,
Swift, and TESS data for the flares that occurred in December 2020, April
2021, July 2021, and November 2021. The HST/STIS UV spectra evolve from blue
shifted broad absorption features to red shifted broad emission features over
$\sim$10 days. The Swift UV/optical light curves peaked as predicted by the
timing model, but the peak UV luminosities varied between flares and the UV
flux in July 2021 was roughly half the brightness of all other peaks. The
X-ray luminosities consistently decreased and the spectra became harder during
the UV/optical rise but apparently without changes in absorption. Finally, two
high-cadence TESS light curves from December 2020 and November 2018 showed
that the slopes during the rising and declining phases changed over time,
which indicates some stochasticity in the flare’s driving mechanism.
ASASSN-14ko remains observationally consistent with a repeating partial tidal
disruption event, but, these rich multi-wavelength data are in need of a
detailed theoretical model.

††journal: ApJ

##  1 Introduction

A tidal disruption event (TDE) occurs when a star crosses the tidal radius of
a central supermassive black hole (SMBH) and is torn apart by the SMBH’s tidal
forces (Hills, 1975; Rees, 1988; Phinney, 1989; Evans & Kochanek, 1989). This
results in roughly half of the stellar mass becoming unbound and the remainder
forming an accretion disk that feeds the SMBH, producing a luminous multi-
wavelength flare. TDEs exhibit diverse behaviors across the electromagnetic
spectrum. This includes a wide range of X-ray brightening timescales and
luminosities (e.g., Auchettl et al. 2017; Holoien et al. 2018; Wevers et al.
2019; Kajava et al. 2020; Hinkle et al. 2021b); optical spectral properties
such as the presence or absence of Hydrogen, Helium, and Bowen fluorescence
lines (e.g., Gezari et al. 2012; Arcavi et al. 2014; Holoien et al. 2014a,
2016a, 2016b; Brown et al. 2016, 2017; Holoien et al. 2018; Leloudas et al.
2019; Nicholl et al. 2019; Holoien et al. 2020; van Velzen et al. 2021; Hinkle
et al. 2022); and differences in their blackbody luminosity, radius, and
temperature evolution (Hinkle et al., 2020, 2021a).

The ultraviolet (UV) spectral properties of TDEs are less well explored. The
small sample of TDEs with UV spectra includes ASASSN-14li (Cenko et al.,
2016), iPTF-16fnl (Brown et al., 2018), iPTF-15af (Blagorodnova et al., 2019),
PS18kh (AT 2018zr: Hung et al. 2019), and ZTF19abzrhgq (AT 2019qiz: Hung et
al. 2020), in addition to the ambiguous nuclear transient (ANT) ASASSN-18jd
which showed properties indicative of both TDEs and active galactic nuclei
(AGNs) (AT 2018bcb: Neustadt et al. 2020). UV spectra of TDEs are generally
characterized by a hot continuum and broad lines including Ly$\alpha$, N V
$\lambda 1240$, Si IV $\lambda 1400$, and C IV $\lambda 1550$ in emission and
absorption. Somewhat surprisingly, they show little temporal evolution when
multiple epochs are available. To date, UV spectra have only been obtained
after maximum light and during the decline, weeks to months after maximum
light.

UV spectra of AGN are characterized by broad and narrow emission lines on top
of a roughly power-law continuum spectrum (e.g., Krolik 1999). Typical
spectral features include Ly$\alpha$ $\lambda 1215$, C IV $\lambda 1549$, Si
IV $\lambda 1400$, N V $\lambda 1240$, O V $\lambda 1035$, C III] $\lambda
1908$, and Mg II $\lambda 2800$, as well as broad absorption lines generally
on the blue wing of the lines. Both the lines and the continuum are time
variable. The vast majority show modest levels of continuum variability that
are reasonably, but not perfectly, modeled by a damped random walk stochastic
process (e.g., Kelly et al. 2008; Kozłowski et al. 2010; MacLeod et al. 2010;
Zu et al. 2013). The UV continuum variability in turn drives changes in the
broad emission lines which are frequently studied using reverberation mapping
(Blandford & McKee 1982; Peterson 1993; Peterson et al. 2004). In particular,
the emission lines are observed to narrow as the AGN becomes brighter, as
expected from photoionization models.

A small fraction of AGNs are “changing look quasars” where the structure of
the emission lines completely changes between narrow and broad line dominated
spectra, usually with a significant change in the continuum brightness (e.g.,
Bianchi et al. 2005; Denney et al. 2014; Shappee et al. 2014; MacLeod et al.
2016; Hon et al. 2022). In the case of “rapid turn-on” events, the blue
continuum and broad lines appear on time scales of a few months (e.g., Gezari
et al. 2017; Frederick et al. 2019; Gromadzki et al. 2019; Trakhtenbrot et al.
2019a; Ricci et al. 2020). There are also ANTs which share characteristics of
both TDEs and more normal AGN variability (Neustadt et al. 2020; Hinkle et al.
2021c; Holoien et al. 2021; Yu et al. 2022).

Here we present observations of the latest four flares of ASASSN-14ko, a
nuclear transient at the center of the AGN ESO 253$-$G003 that undergoes
periodic flares (Payne et al., 2021, 2022). This transient was initially
discovered by the All-Sky Automated Survey for Supernovae (ASAS-SN, Shappee et
al. 2014; Kochanek et al. 2017) in 2014 but classified as a Type IIn supernova
with a blue continuum projected very close to the nucleus of a Type 2 Seyfert,
although AGN activity was not ruled out as a possibility (Holoien et al.,
2014b). However, the subsequent seven years of ASAS-SN data revealed that the
flare was not a one-time event – the flares recur at regular intervals well-
fit by a timing model with a mean period of roughly $115$ days and a negative
period derivative. Each flare is consistently characterized by a single
UV/optical brightening event that rapidly rises and smoothly declines over
$\sim 40$ days. The X-ray flux dims rapidly (days) during the UV rise and then
recovers. The host galaxy ESO 253$-$G003 is a complex merger remnant with two
AGNs and a large tidal arm (Tucker et al., 2020). The brighter northeastern
nucleus is the source of the periodic flares (Payne et al., 2022).

In this paper, we present the data and analysis of the four flares which
peaked in December 2020, April 2021, July 2021, and November 2021. In Section
2, we discuss the data used in this paper. In Section 3, we show the
photometric light curves, updated timing model, and how the latest flares
compare to previous flares. In Section 4 we analyze the UV spectra, and in
Section 5 we investigate the X-ray emission. In Section 6 we discuss how the
latest flares fit into interpretations of ASASSN-14ko’s origins and we give
our conclusions in Section 7. For a flat $\Omega_{m}=0.3$ universe, the
luminosity distance is $\approx 188$ Mpc and the projected scale is $\approx
0.85$ kpc/arcsec. The Galactic extinction is $\mathrm{A}_{\mathrm{V}}=0.116$
mag (Schlafly & Finkbeiner, 2011).

##  2 Observations

The following sections describe the data used in this analysis. All
photometric data discussed here are presented in Table 1.

###  2.1 ASAS-SN

ASAS-SN is an ongoing all-sky survey that uses four 14-cm telescopes on a
common mount to monitor the sky to find bright, nearby transients (Shappee et
al., 2014; Kochanek et al., 2017). There are currently five units located in
Hawai`i, Chile, Texas, and South Africa that are hosted by the Las Cumbres
Observatory global telescope network (Brown et al., 2013). ASAS-SN images are
processed with a fully automatic pipeline that utilizes the ISIS image
subtraction package (Alard & Lupton, 1998; Alard, 2000). Reference images are
used to subtract the background and host emission from all science images, and
aperture photometry is then performed using the IRAF apphot package (Tody,
1986, 1993). The data were calibrated using stars from the AAVSO Photometric
All-Sky Survey (APASS; Henden et al. 2015). All low-quality images were
inspected by eye, and images affected by clouds or other systematic problems
are removed.

###  2.2 HST/STIS

We observed ASASSN-14ko using STIS and the FUV/NUV MAMA detectors (HST IDs:
16451, 16498; PI: Shappee). We used the $52.\\!\\!^{\prime\prime}0\times
0.\\!\\!^{\prime\prime}2$ slit and the G140L (1425 Å, FUV-MAMA) and G230L
(2376 Å, NUV-MAMA) gratings. FUV and NUV spectra were obtained on 2020-12-18,
2020-12-22, 2020-12-29, 2021-03-06, and FUV-only spectra were obtained on
2021-03-29 and 2021-04-01. Each visit consisted of 2 or 3 exposures per
grating with exposure times ranging from 336 to 482 seconds for the G230L
grating and 200 to 1732 seconds for the G140L grating. We used the one-
dimensional spectra produced by the HST pipeline since the trace of
ASASSN-14ko was clearly present in the two-dimensional frames. For each epoch,
we combined the individual exposures with an inverse-variance-weighted average
of the one-dimensional spectra and then merged the FUV and NUV channels.

###  2.3 Swift XRT & UVOT

We requested Swift ToO observations (ToO ID: 14775, 14971, 15393, 15636,
16037, 16546; PI: Payne) using the UltraViolet/Optical Telescope (UVOT) and
the X-Ray Telescope (XRT) to coincide with the predicted December 2020, April
2021, July 2021, and November 2021 flares. The UVOT data were obtained in six
filters (Poole et al., 2008): $V$ (5425.3 Å), $B$ (4349.6 Å), $U$ (3467.1 Å),
$UVW1$ (2580.8 Å), $UVM2$ (2246.4 Å), and $UVW2$ (2054.6 Å). The wavelengths
used here are the pivot wavelengths calculated by the SVO Filter Profile
Service (Rodrigo et al., 2012). The Swift team announced in November 2020 that
a loss of sensitivity over time requires an updated photometric correction for
the three UVOT UV filters, and we used the updated correction for this
analysis (see Hinkle et al. 2021a). We used the HEAsoft (HEASARC 2014)
software task uvotsource to extract the source counts using a
$16.\\!\\!^{\prime\prime}0$ radius aperture and used a sky region of $\sim$
$40.\\!\\!^{\prime\prime}0$ radius to estimate and subtract the sky
background.

The XRT data were reduced using the HEAsoft task xrtpipeline. We obtained a
background subtracted count rate from each observation using a source region
with a radius of 50” centered on the position of ASASSN-14ko and a 150” radius
source free background region centered at
($\alpha$,$\delta$)=($05^{h}25^{m}09.87^{s},-45^{\circ}56^{\prime}47.94^{\prime\prime}$).
All count rates were corrected for encircled energy fraction.

###  2.4 TESS

The Transiting Exoplanet Survey Satellite (TESS, Ricker et al. 2014) observed
ASASSN-14ko during Sectors 31-33, which occurred between 2020-10-21 and
2021-01-13, with the flare occurring during Sectors 32-33. Following a similar
procedure to that used for the ASAS-SN data, we used the ISIS package to
perform image subtraction on the 10-minute cadence full frame images (FFIs) to
obtain high fidelity light curves as described in Vallely et al. (2019, 2021).
This is the second flare captured by TESS, following an earlier observation
during Sectors 3-5 of the flare that peaked in November 2018 (Payne et al.,
2022).

Table 1: Photometry of ASASSN-14ko used in this analysis. The luminosities are in units of $10^{40}$ $\mathrm{ergs}$ $\mathrm{s}^{-1}$ and the X-ray luminosities are between $0.3-10.0$ keV. Only the first observation in each band is shown here to demonstrate its form and content. Table to be published in its entirety in machine-readable form in the online journal.  JD | Band |  25mmLλ ($\mathrm{ergs}$ $\mathrm{s}^{-1}$ ) |  35mmLλ Error ($\mathrm{ergs}$ $\mathrm{s}^{-1}$)
---|---|---|---
58983.65 | X-ray | 186 | 50.3
58983.65 | $W2$ | 4.89 | 0.29
58983.68 | $M2$ | 3.99 | 0.43
58983.65 | $W1$ | 2.29 | 0.16
58983.65 | $U$ | 1.15 | 0.09
58983.65 | $B$ | 0.82 | 0.08
58983.66 | $V$ | 0.62 | 0.08
58849.34 | $g$ | 0.63 | 0.08
58425.46 | $I_{\mathrm{TESS}}$ | 0.019 | 0.005

###  2.5 NICER

ASASSN-14ko was also observed using the Neutron star Interior Composition
ExploreR (NICER: Gendreau et al. 2012) and its X-ray timing instrument (XTI).
ASASSN-14ko was observed a total of 115 times between 2020 Dec 10 and 2021 Dec
01 (ObsIDs: 3201740101$-$3201740118,4662010101$-$4662022801, PI: Payne), for a
total cumulative exposure of 277.7ks.

The data were reprocessed using the nicerdas version 8c and the task nicerl2.
Here standard filtering criteria were used111See Bogdanov et al. (2019) or
Section 2.7 of Hinkle et al. (2021b), as well as the latest gain and
calibrations files. Time averaged spectra and count rates were extracted using
xselect. We used the ARF (nixtiaveonaxis20170601v005.arf) and RMF
(nixtiref20170601v003.rmf) files that are available with the NICER CALDB. All
spectra were grouped using a minimum of 20 counts per energy bin. As NICER is
a non-imaging instrument, background spectra were generated using the
background modeling tool
nibackgen3C50222https://heasarc.gsfc.nasa.gov/docs/nicer/tools/nicer_bkg_est_tools.html.

###  2.6 Chandra

Finally, we acquired two Chandra ACIS-S DDT observations (observation IDs:
24875, 24876; PI: Payne). The first observation occurred on 2020-12-26 with an
exposure time of 27.69 ks, and the second observation occurred on 2021-01-21
with an exposure time of 28.68 ks. The data were analyzed using CIAO 4.13 and
CALDB 4.9.4. The data were reduced using the CIAO command chandra_repro, while
spectra were extracted from each reprocessed observation using the CIAO
command specextract. All spectra were grouped using a minimum of 20 counts per
bin. To analyze the spectral data from Swift, NICER and Chandra data, we used
the X-ray spectral fitting package (XSPEC) version 12.12.0 (Arnaud, 1996) and
$\chi^{2}$ statistics.

##  3 UV/Optical Light Curve Analysis

The December 2020, April 2021, July 2021, and November 2021 flares are the
third, fourth, fifth, and sixth events, respectively, in which the UV
properties were observed in conjunction with the optical evolution. These data
enable us to search for similarities and differences in the UV properties of
six flares.

###  3.1 UV/Optical Evolution

We showed previously in Payne et al. (2021) and Payne et al. (2022) that
ASASSN-14ko’s flares are essentially periodic, so each new flare is another
opportunity to update the timing model of the optical peaks. In Payne et al.
(2021) we found significant residuals between the observed and calculated peak
times (Observed - Calculated, O-C), which is indicative of a period
derivative. To analyze how this trend continued, we measured the optical peaks
of the most recent flares that occurred in December 2020, April 2021, July
2021, and November 2021 (as shown on Figure 1: flares 18-21) by fitting the
ASAS-SN $g$-band light curves with a fifth-order polynomial and measured the
errors on the peak times by bootstrap resampling the data. These peaks
occurred on $59205.7\pm 0.19$, $59315.2^{+0.6}_{-0.5}$, $59433.3^{+6}_{-3}$,
and $59532.2\pm 0.9$, respectively. The comparatively larger error on July
2021’s peak timing is due to a gap in the ASAS-SN light curve due to bad
weather, so we also measured this event’s peak timing using the Swift B-band
light curve whose peak time was MJD $59423.9^{+0.7}_{-0.8}$. In total, there
are now 21 flares observed by ASAS-SN since 2014 after combining the new
flares reported here with those in Payne et al. (2021) and Payne et al.
(2022).

For the flares that occurred during November 2018 (flare 12) and December 2020
(flare 18), TESS also observed these flares so the peak times measured from
the TESS light curves were also included alongside the peak times measured
from ASAS-SN. We fit the model

$t=t_{0}+nP_{0}+\frac{1}{2}n^{2}P_{0}\dot{P}+\frac{1}{6}n^{3}P_{0}\dot{P}^{2},$
(1)

and obtain starting time $t_{0}=2456852^{+10}_{-11}$ JD, mean period
$P_{0}=115.2^{+1.3}_{-1.2}$ days, and a period derivative $\dot{P}=-0.0026\pm
0.0006$. The errors on the peak times were expanded in quadrature by 0.8 days
to obtain a reduced $\chi^{2}$ of unity for 10 degrees of freedom. The O-C
diagram for all observed peaks with the updated timing model is shown in
Figure 1. The parabolic trend in the O-C timings continues. This timing model
predicts that the next flares will peak in the optical on MJD $59639.7$ (UT
2022-03-01.2), $59747.5$ (UT 2022-06-17.0), $59855.9$ (UT 2022-10-03.4),
$59962.1$ (UT 2023-01-17.6), and $60069.0$ (UT 2023-05-04.5).

In Payne et al. (2022), we had concluded that $\dot{P}$ had changed and used
only the flare timings obtained after flare 9 for the timing model to predict
when subsequent flares would occur. With these four new flares it now seems
that the timing of flare 10 and flare 11 were anomalous and the older (flares
1-9) and recent flares (flares 12-21) are consistent with a single $\dot{P}$.
Certainly for a repeating TDE we should expect some stochasticity in $\dot{P}$
— it should be problematic for this interpretation if $\dot{P}$ did not vary.

Figure 1: O-C plot for all of ASASSN-14ko’s observed outbursts comparing the
time of the observed peak with the estimated peak if we assume a constant
period. The black circles correspond to the the flares reported in Payne et
al. (2021, 2022) and the blue squares represent the flares reported here. The
solid line is the model described in Section 3.1.

After updating the timing model following each new flare, we used the latest
predictions to schedule the observations of the next flare. Figure 2 shows the
UV/optical host-subtracted light curves for the six most recent flares from
November 2021, July 2021, April 2021, December 2020, September 2020, and May
2020. In order to directly compare the flare evolution across all flares, we
stacked the host-subtracted light curves using the optical peak times
predicted by the timing model as shown in Figure 3. The flares are
consistently characterized by a single, asymmetric UV and optical rise and
fall without obvious substructure. The six flares are not identical. The peak
UV luminosity changes and the differences are largest in the shortest
wavelength $W2$ and $M2$ UV bands. This contrasts with the optical bands,
which have not shown discernible changes in peak luminosity between flares.
The July 2021 flare was significantly less luminous than all the others across
all bands except for the $V$-band, which was noisier. The July 2021 flare was
the first indication that the flares can have significantly different peak
luminosities.

Figure 2: The Swift and NICER X-ray and UV luminosities and hardness ratios
for the six flares in 2020-2021. The vertical dashed lines indicate the
$g$-band peaks predicted from the updated timing model described in Section 3.
The vertical solid bars are color-coded to the HST/STIS spectra epochs shown
in Figure 6 and the Chandra spectra epochs shown in Figure 10. Upper limits
are indicated by downward facing triangles. The X-ray luminosities cover
$0.3-10.0$ keV and the hard and soft X-rays are defined between $0.3-2.0$ keV
and $2.0-10.0$ keV respectively.  Figure 3: Swift XRT and NICER X-ray and
host-subtracted Swift UVOT, and ASAS-SN $g$-band light curves aligned using
the updated timing model described in Section 3 including May 2020 (circles),
September 2020 (triangles), December 2020 (squares), April 2021 (pentagons),
July 2021 (stars), and November 2021 (Xs) flares. The vertical bars are color-
coded to the HST/STIS spectra epochs shown in Figure 6 and the Chandra spectra
epochs shown in Figure 10.

We fit a blackbody model to the UV+U SED as we have done with previous nuclear
transients (e.g., Hinkle et al. 2020). We used Markov Chain Monte Carlo
methods to find the best-fit blackblody parameters for the SED, using flat
priors of
$1000~{}\mathrm{K}~{}\leq~{}\mathrm{T}~{}\leq~{}55000\mathrm{~{}}{K}$ and
$10^{11}~{}\mathrm{cm}~{}\leq~{}\mathrm{R}~{}\leq~{}10^{17}~{}\mathrm{cm}$ in
order to not overly influence the fits. The evolution in luminosity, effective
radius, and temperature from these blackbody fits are shown in Figure 4. The
blackbody models were poor fits if we included the $B$ and $V$ data. This
could be due to deviations from a blackbody SED as a consequence of assuming
the inter-flare flux is just the host galaxy. Since the UV dominates the
energetics, using UV+U only should still give a reasonable characterization of
the flares. For all flares observed in the UV, the peak blackbody luminosity
and temperature occurred several days before the corresponding ASAS-SN
$g$-band peaks. The blackbody radius begins to decline around the times of the
$g$-band peaks. The peak blackbody luminosities of the flares were all
consistent except for the July 2021 flare which was fainter but had a similar
peak blackbody radius to the others. This is consistent with the luminosity
evolution shown in Figure 3.

Figure 4: Evolution of ASASSN-14ko’s luminosity (top), effective radius
(middle), and temperature (bottom) from blackbody fits to the host-subtracted
UV/optical Swift data. The colors and symbols match those used in Figure 3 and
the data are aligned using the timing model described in Section 3.

###  3.2 TESS Light Curve Analysis

TESS observed ASASSN-14ko’s flares during Sectors 3-5, which captured the
November 2018 flare (flare 12) (Payne et al., 2021), and again during Sectors
31-33 for the December 2020 flare (flare 18). TESS’s second view of
ASASSN-14ko’s flaring behavior enables us to compare the properties of two
individual flares using high-cadence data to investigate how they changed over
that two year time frame. We aligned the data using the updated timing model
and plot this in Figure 5. Due to the timing of the TESS sectors, the full
decline was not captured for the December 2020 flare, but the full rise to
peak was observed.

Previous TDEs and some supernovae have been fit by the model

$f=f_{0}+f_{1}(t-t_{0})^{a}$ (2)

finding $\alpha\simeq 1.9-2.8$ (ASASSN-19bt with $\alpha=2.10\pm 0.12$,
Holoien et al. 2019; ASASSN-19dj with $\alpha=1.9\pm 0.4$, Hinkle et al.
2021b; ZTF19abzrhg with $\alpha=1.99\pm 0.01$, Nicholl et al. 2020). In Payne
et al. (2021), we fit the first 25% of the November 2018 rise with a single
power-law to find $\alpha=1.01\pm 0.07$. There is a certain arbitrariness to
these models in selecting the time period over which to do the fit because the
actual light curve has to deviate from the power law as it approaches the
peak. The sector shift during the rise in December 2020 also makes it
difficult to use this model.

Here we instead use the model used by Vallely et al. (2021) to model the rise
time of core-collapse supernovae with TESS data,

$f(t)=\frac{h}{(1+z)^{2}}\left(\frac{t-t_{1}}{1+z}\right)^{a_{1}\left(1+a_{2}\dot{(}t-t_{1})/(1+z)\right)}+f_{0}$
(3)

up to near the light curve peak for $t>t_{1}$ and as $f(t)=f_{0}$ for
$t<t_{1}$. Here $f_{0}$ is any residual background flux, $t_{1}$ is the
beginning of the model rise, and the factors of $1+z$ are introduced to
account for redshift time-dilation, although the low redshift of the host
galaxy makes the correction small. Parameter $a_{2}$ is mathematically related
to the rise time, $t_{rise}=t_{peak}-t_{1}$, between the start of the rise at
time $t_{1}$ and the time of the peak $t_{peak}$. This model is able to adjust
to the changing curvature as the transient approaches its peak and so can be
fit over a broad enough time range for the slope of the initial rise to be
unaffected by this choice (although the parameters for the curvature
approaching peak will be). Using this model we find $a_{1}=1.10\pm 0.04$ and
$a_{1}=1.50\pm 0.10$ for November 2018 and December 2020, respectively. The
fits for both flare events using Equation 3 are shown in Figure 5. The fit
parameters are summarized in Table 2. For our estimates of $t_{rise}$, we
estimate $t_{peak}$ directly from the light curve using a polynomial fit to
the data around peak. As shown by these model fits in Figure 5, the December
2020 flare started to rise earlier and took a day longer to rise to peak than
the November 2018 flare. In addition, the December 2020 flare took longer to
rise to 50% of the peak flux. We refit the previously-studied TDEs with
publicly available photometric data with observed early-time rises with
Equation 3 in order to compare to ASASSN-14ko and found $\alpha_{1}=5.12\pm
0.12$ and $\alpha_{2}=-0.02\pm 0.001$ for ASASSN-19bt, and $\alpha_{1}=1.70\pm
0.44$ and $\alpha_{2}=-0.01\pm 0.001$ for ASASSN-19dj. The early-time rise of
ASASSN-14ko is similar to ASASSN-19dj but different than ASASSN-19bt.

Like the November 2018 flare (Payne et al., 2021), the December 2020 flare
also smoothly declined in brightness, as shown in Figure 5. As in Payne et al.
(2021), we fit the decline of both flares using a power-law and exponential
model. Since only part of the decline was captured for the December 2020
flare, we fit both flares over this restricted time range (3 to 21 days after
peak). The power-law model is

$f(t)=z-h\left(\frac{t-t_{\mathrm{0}}}{\mathrm{days}}\right)^{\alpha}$ (4)

with $t_{\mathrm{0}}$ being the time of disruption, constrained to be before
the start of the rise, $t_{1}$, determined above. However, in Payne et al.
(2021) we found that an exponential decline

$f(t)=ae^{-(t-t_{\mathrm{peak}})/\tau}+c$ (5)

was a better fit for the November 2018 flare. We find that the December 2020
flare is also best fit by the exponential model with a reduced $\chi^{2}$ of
0.80 with 2419 degrees of freedom versus a reduced $\chi^{2}$ of 1.02 with
5851 degrees of freedom for the power-law model fit after inflating the errors
to give the power-law model a reduced $\chi^{2}\simeq 1$. There is a
difference in the degrees of freedom because the power-law model also includes
the pre-rise quiescence data as part of the fit whereas the exponential model
does not. The best-fit parameters for both models are also shown in Table 2.

Compared to the November 2018 flare, the December 2020 flare began to rise
earlier but took longer to rise to a fainter peak, and then declined more
slowly. Although the full decline was not observed by TESS, it is likely that
the December 2020 flare also took longer to return to quiescence than the
November 2018 flare.

(a) (b) (c)

Figure 5: (a) TESS image subtraction light curves of the December 2020 (red) and November 2018 (gray) flares. The two light curves are aligned using the updated timing model described in Section 3. Squares show the data binned at 8 hour intervals. The unbinned December 2020 flare data has more scatter because of the increased cadence of the observations (10 minutes instead of 30 minutes). (b) The rising phase of the two flares with the corresponding best-fit curved power-law models (Equation 3). The bottom panel shows the flux residuals. (c) The declining phase of the two flares with the best-fit exponential decline models shown in black and red for November 2018 and December 2020, respectively. For both (b) and (c) the TESS data binned in 4 hour intervals are shown by squares and Xs in the color corresponding to the model fits.  Table 2:  Best-fit parameters for the peak, rise, and decline models of the TESS light curves for the November 2018 flare and December 2020 flare.  | November 2018 flare | December 2020 flare
---|---|---
Peak Fit |  |
$t_{peak}$ (JD) | 2458435.50${}^{+0.13}_{-0.10}$ | 2459206.13$\pm 0.10$
$F_{\lambda peak}$ (10-15 ergs s-1 cm-2 $\AA^{-1}$) | $\hskip 27.0301pt0.40\pm 0.001$ | $\hskip 27.0301pt0.34\pm 0.001$
Curved Power Law Rise Fit |  |
$t_{1}$ (JD) | $2458429.71\pm 0.04$ | $2459199.25\pm 0.09$
$h$ (ergs s-1 cm-2 $\AA^{-1}$) | $\hskip 27.0301pt0.14\pm 0.01$ | $\hskip 27.0301pt0.06\pm 0.01$
$a_{1}$ | $\hskip 27.0301pt1.10\pm 0.04$ | $\hskip 27.0301pt1.50\pm 0.10$
$a_{2}$ (Days-1) | $\hskip 19.91692pt-0.07\pm 0.04$ | $\hskip 19.91692pt-0.06\pm 0.10$
$f_{0}$ (ergs s-1 cm-2 $\AA^{-1}$) | $\hskip 19.91692pt-0.01\pm 0.001$ | $\hskip 19.91692pt-0.01\pm 0.001$
$t_{50}$ (days) | $\hskip 27.0301pt1.61\pm 0.12$ | $\hskip 27.0301pt2.49\pm 0.13$
$t_{rise}$ (days) | $\hskip 27.0301pt5.80\pm 0.12$ | $\hskip 27.0301pt6.88\pm 0.13$
Power Law Decline Fit |  |
$t_{0}$ (JD) | $2458429.8\pm 0.8$ | $2459199.1\pm 1.0$
$z$ (ergs s-1 cm-2 $\AA^{-1}$) | $\hskip 19.91692pt-0.01\pm 0.001$ | $\hskip 19.91692pt-0.01\pm 0.001$
$h$ (ergs s-1 cm-2 $\AA^{-1}$) | $\hskip 19.91692pt-3.5\pm 0.7$ | $\hskip 19.91692pt-2.3\pm 0.5$
$\alpha$ | $\hskip 19.91692pt-1.01\pm 0.06$ | $\hskip 19.91692pt-0.81\pm 0.05$
Exponential Decline Fit |  |
$a$ (ergs s-1 cm-2 $\AA^{-1}$) | $\hskip 27.0301pt0.47\pm 0.02$ | $\hskip 27.0301pt0.45\pm 0.03$
$\tau$ (days) | $\hskip 22.76219pt22.3\pm 1.2$ | $\hskip 22.76219pt33\pm 3$
$c$ (days) | $\hskip 19.91692pt-0.12\pm 0.02$ | $\hskip 19.91692pt-0.13\pm 0.03$
|  |

##  4 UV Spectroscopic Analysis

We present the six epochs of HST/STIS UV Galactic extinction corrected spectra
in Figure 6. They were obtained during both the UV rise and decline for the
December 2020 flare, as denoted by the color-coded vertical bars in Figure 3.
We also obtained two UV spectra a few days prior to the start of the UV rise
of the April 2021 flare when the X-ray flux was anticipated to be in a low
state based on the Swift XRT light curves of the prior flares (see Section 5).

There were dramatic changes in both the UV continuum and the spectral lines as
the December 2020 flare evolved. The FUV continuum was at its bluest state in
the first observation $-4$ days prior to the optical peak and when the UV flux
was still increasing. Several days later at epochs of +0 and +7 days, when the
UV flux was declining, the UV continuum flattened. The continuum was faintest
at $-36$, $-12$, and $-9$ days.

The spectral lines during the flare are very different from the quiescent
spectrum, as shown in Figure 6, where we also offset each epoch to show the
change in spectral features over time. The prominent features are the standard
UV lines of AGNs, including Ly$\alpha$, N V $\lambda 1240$, Si IV
$\lambda\lambda 1394,1403$, C IV $\lambda 1550$, C III] $\lambda 1909$ and Mg
II $\lambda 2800$. In addition to these features, we detect narrow, high- and
low-ionization absorption features at $z=0$ that we interpret as originating
from the Milky Way ISM, as discussed in the Appendix.

The evolution of the spectral lines over the duration of the flare is most
dramatic for Ly$\alpha$, N V $\lambda 1240$, Si IV $\lambda\lambda 1394,1403$,
and C IV $\lambda 1550$. We fit either single or multiple-component Gaussian
models to Si IV, C IV, C III], and Mg II, as shown in Figure 7 with the
resulting fit parameters summarized in Table 3. We estimated the local
continuum by fitting a polynomial to the adjacent featureless regions
surrounding the lines. Although it appears that Ly$\alpha$+N V shows a similar
trend in its profile evolution, we do not fit these profiles due to the
complexity created by the blended lines and the telluric features. Velocities
of each component were then obtained using these fits. Most components have
velocities on the order of several thousand km/s. During the UV rise at $-4$
days, a strong blue absorption feature is seen for the S IV and C IV lines
which weakens and then vanishes during the declining phase of the UV light
curve. The absorption feature is also absent in the quiescent spectrum. The
presence of the absorption feature is clearest for the C IV $\lambda 1550$ and
Si IV $\lambda\lambda 1394,1403$ lines because there are no other nearby
lines. The velocity of this feature is $\sim 4000-5000$ km/s. Based on the
evolution of the C IV $\lambda 1550$ and Si IV $\lambda\lambda 1394,1403$
lines, the strong absorption feature centered at $1235~{}\mathrm{\AA}$ is
possibly associated with the blue wing of N V $\lambda 1240$. As the
absorption features vanish, broad emission lines of C IV and Si IV appear with
similar velocity widths but $\sim$4,000 km/s redwards of the absorption
features. C III] and Mg II are only seen in emission and show only modest
changes. The presence of Mg II is a significant difference from all previous
UV spectra of TDEs.

We obtained two UV spectra on 2021-03-29 and 2021-04-01, $-12$ and $-9$ days
prior to the optical peak to determine the spectral properties at the time of
the minimum X-ray emission. There were no large differences between these two
spectra and the quiescent spectrum from 2021-03-06, aside from the appearance
of a weak absorption feature on the blue side of C IV in the $-9$ day spectrum
and at the X-ray minimum. This feature may be the beginning of the prominent
blue absorption feature present at $-4$ days relative to peak. There was a
significant difference in the minimum X-ray luminosity between the two flares,
with the December 2020 minimum being significantly fainter than the April 2021
minimum for both the total and soft X-ray luminosity, as discussed below.

Table 3:  ASASSN-14ko absorption and emission features and their evolution over time. $\lambda_{0}$ is the expected rest-frame wavelength of each ion whereas $\lambda_{c}$ is measured from the Gaussian fits of each component. Equivalent widths are negative for emission lines and positive for absorption lines.  Ion | Phase & Date Observed |  $\lambda_{0}$ (rest $\mathrm{\AA}$) | Central Velocity (rest km s-1) | Equivalent Width (rest $\mathrm{\AA}$) | FWHM (km s-1)
---|---|---|---|---|---
Si IV |  $-4$d, 2020-12-18 | $\lambda\lambda 1394,1403$ | $-2630\pm 107$ | $\hskip 11.38109pt2.9^{+0.9}_{-0.6}$ | $3690\pm 260$
Si IV | +0d, 2020-12-22 | $\lambda\lambda 1394,1403$ | $\hskip 7.11317pt1310\pm 1390$ | $\hskip 4.2679pt-3.7^{+1.9}_{-2.4}$ | $6370\pm 3260$
Si IV | +7d, 2020-12-29 | $\lambda\lambda 1394,1403$ | $\hskip 7.11317pt1130\pm 197$ | $\hskip 4.2679pt-3.9^{+0.5}_{-0.5}$ | $4820\pm 460$
Si IV |  $-36$d, 2021-03-06 | $\lambda\lambda 1394,1403$ | $\hskip 7.11317pt1360\pm 116$ | $-13.1^{+4}_{-8}$ | $2900\pm 270$
Si IV |  $-12$d, 2021-03-29 | $\lambda\lambda 1394,1403$ | $\hskip 7.11317pt1500\pm 116$ | $\hskip 4.2679pt-3.3^{+0.2}_{-0.3}$ | $3040\pm 270$
Si IV |  $-9$d, 2021-04-01 | $\lambda\lambda 1394,1403$ | $\hskip 7.11317pt1620\pm 122$ | $\hskip 4.2679pt-2.5^{+0.3}_{-0.3}$ | $2380\pm 290$
C IV |  $-4$d, 2020-12-18 | $\lambda 1550$ | $-2990\pm 367$ | $\hskip 11.38109pt8.3_{-1.4}^{+1.3}$ | $4500\pm 510$
|  |  | $-2180\pm 40.5$ | $\hskip 11.38109pt2.2_{-0.5}^{+0.8}$ | $\hskip 4.2679pt980\pm 140$
|  |  | $\hskip 11.38109pt212\pm 154$ | $\hskip 4.2679pt-1.7_{-1}^{+0.8}$ | $1410\pm 500$
|  |  | $\hskip 11.38109pt922\pm 61.7$ | $\hskip 4.2679pt-0.4_{-0.5}^{+0.2}$ | $\hskip 4.2679pt340\pm 190$
C IV | +0d, 2020-12-22 | $\lambda 1550$ | $\hskip 11.38109pt289\pm 965$ | $\hskip 4.2679pt-7.7_{-2.1}^{+2.0}$ | $6810\pm 2270$
C IV | +7d, 2020-12-29 | $\lambda 1550$ | $-2280\pm 118$ | $\hskip 11.38109pt0.7^{+0.2}_{-0.2}$ | $\hskip 4.2679pt840\pm 280$
|  |  | $\hskip 9.95845pt15.4\pm 46.3$ | $\hskip 4.2679pt-4.0_{-0.4}^{+0.5}$ | $1270\pm 110$
|  |  | $\hskip 7.11317pt1490\pm 69.5$ | $\hskip 4.2679pt-0.9_{-0.8}^{+0.3}$ | $\hskip 4.2679pt590\pm 160$
C IV |  $-36$d, 2021-03-06 | $\lambda 1550$ | $\hskip 4.2679pt-164\pm 84.9$ | $\hskip 4.2679pt-2.0_{-1.2}^{+0.7}$ | $\hskip 4.2679pt850\pm 240$
|  |  | $\hskip 11.38109pt390\pm 106$ | $-13.2_{-1.1}^{+1.5}$ | $3470\pm 250$
C IV |  $-12$d, 2021-03-29 | $\lambda 1550$ | $\hskip 4.2679pt-675\pm 77.2$ | $\hskip 4.2679pt-0.5_{-0.3}^{+0.4}$ | $\hskip 4.2679pt330\pm 210$
|  |  | $\hskip 11.38109pt243\pm 50.2$ | $-11.6_{-0.7}^{+0.9}$ | $2000\pm 100$
C IV |  $-9$d, 2021-04-01 | $\lambda 1550$ | $-2440\pm 48.2$ | $\hskip 11.38109pt0.3_{-0.10}^{+0.13}$ | $\hskip 4.2679pt240\pm 120$
|  |  | $\hskip 4.2679pt-135\pm 174$ | $\hskip 4.2679pt-8.3_{-1.6}^{+2.1}$ | $1440\pm 210$
|  |  | $\hskip 7.11317pt1270\pm 598$ | $\hskip 4.2679pt-4.3_{-2}^{+1.7}$ | $1860\pm 770$
C III] |  $-4$d, 2020-12-18 | $\lambda 1909$ | $\hskip 1.42262pt-78.3\pm 219$ | $\hskip 4.2679pt-5.9_{-0.5}^{+0.4}$ | $5390\pm 520$
C III] | +0d, 2020-12-22 | $\lambda 1909$ | $\hskip 4.2679pt-490\pm 86.2$ | $\hskip 4.2679pt-7.2_{-0.4}^{+0.5}$ | $2870\pm 200$
C III] | +7d, 2020-12-29 | $\lambda 1909$ | $\hskip 4.2679pt-670\pm 95.6$ | $-13.9_{-0.7}^{+0.6}$ | $4840\pm 220$
C III] |  $-36$d, 2021-03-06 | $\lambda 1909$ | $\hskip 4.2679pt-620\pm 97.1$ | $-19.8_{-1.1}^{+1.0}$ | $4510\pm 230$
Mg II |  $-4$d, 2020-12-18 | $\lambda 2800$ | $\hskip 4.2679pt-406\pm 278$ | $\hskip 4.2679pt-1.4^{+0.3}_{-0.3}$ | $2490\pm 660$
Mg II | +0d, 2020-12-22 | $\lambda 2800$ | $\hskip 4.2679pt-256\pm 192$ | $\hskip 4.2679pt-2.1^{+0.3}_{-0.3}$ | $3000\pm 450$
Mg II | +7d, 2020-12-29 | $\lambda 2800$ | $\hskip 4.2679pt-363\pm 267$ | $\hskip 4.2679pt-2.8_{-0.4}^{+0.3}$ | $4740\pm 630$
Mg II | -36d, 2021-03-06 | $\lambda 2800$ | $\hskip 4.2679pt-395\pm 128$ | $\hskip 4.2679pt-7.0_{-0.7}^{+0.6}$ | $3950\pm 300$
|  |  |  |  |
Figure 6: HST/STIS UV spectra of ASASSN-14ko during the December 2020 and
April 2021 flares. The $-4$ (blue), $+0$ (orange), and $+7$ (red) day spectra
are relative to the December 2020 optical peak timing of MJD 59205.65, and the
$-36$ (black), $-12$ (pink), and $-9$ day (purple) spectra are relative to the
April 2021 optical peak timing of MJD 59315.22. The spectra have been
corrected for Galactic extinction and the major geocoronal airglow lines are
marked with the green $\oplus$ symbol. The middle panel shows the full spectra
offset to better illustrate the change in line profiles over time. The bottom
panels are zoomed-in to the labeled lines and not offset, showing the change
in flux at different epochs of the flares. The major BALs are shaded in
purple, blue, and green regions.  Figure 7: Continuum subtracted Si IV, C IV,
C III], and Mg II profiles from the HST/STIS UV spectra. The black lines show
the spectra, the red lines show the best fitting Gaussian profiles, and the
blue, purple, green and cyan dotted lines indicate the best fitting components
of those profiles for C IV.

##  5 Analysis of the X-ray Emission

As previously shown in Payne et al. (2021, 2022), ASASSN-14ko’s X-ray
evolution during the flares differs from its UV/optical evolution. The X-ray
luminosity rapidly drops during the UV/optical rise, recovers near peak and
then declines again before recovering. Here we discuss the new X-ray data from
Swift and NICER during December 2020, April 2021, July 2021, and November
2021.

###  5.1 X-ray Light Curve from Swift XRT and NICER

The X-ray light curves and their hardness ratios are compared to the
UV/optical light curves in Figure 2 and the aligned light curves are shown in
Figure 3. The X-ray luminosity follows the previously observed trends. The
X-ray luminosity consistently has a minimum ${\sim}5$ days before the optical
peak but the depth of the minimum is variable. The largest decreases were for
the September 2020 and December 2020 flares, and the smallest decrease was for
the April 2021 flare.

Near the minimum, the hardness ratio (HR) also changes. During quiescence
before the flares, the HR is ${\sim}-0.5$ and it then hardens to
HR${\sim}0.1-0.5$ at the minimum. There was a secondary X-ray minimum at $+10$
days only in the September 2020 flare. This secondary minimum also had a
harder spectrum with HR $\sim 0.25$. This “softer when brighter, harder when
fainter” behavior is illustrated in Figure 8. Across all flares, both the hard
and soft X-ray luminosities dip together, as shown in Figure 9, but the soft
X-rays consistently nearly disappear completely during the minimums. The April
2021 flare was unusual in that both the total X-ray and soft X-ray luminosity
were unusually bright compared to the other flares. This indicates that the
X-rays were uncharacteristically bright when the UV spectra were observed at
$-12$ and $-9$ days relative to the April 2021 flare optical peak. Overall,
Figure 9 shows that ASASSN-14ko’s evolution is strongest in the softest energy
band.

###  5.2 X-ray Spectra from Chandra

Due to the spatial resolution of Swift XRT and NICER, it was not possible to
separately analyze the X-ray properties of the two AGN nuclei present in ESO
253$-$G003. As shown by the contours in Figure 10, Chandra detects the two
nuclei as separate sources coincident with the two nuclei identified by Tucker
et al. (2020) in the optical.

We extracted X-ray spectra for both sources for the two epochs observed at
$+4$ and $+26$ days relative to the December 2020 $g$-band peak. For both
epochs, the absorbed power-law xspec model
$\textrm{tbabs}\times\textrm{zashift}\times\textrm{powerlaw}$ with a photon
index of $1.34\pm 0.11$ at $+4$ days and $1.32\pm 0.09$ at $+26$ days provided
the best fit for ASASSN-14ko. These two spectra and the models are shown in
Figure 10. Comparing these two spectra indicates that the northern nucleus was
brighter at $+4$ days during the UV/optical light curve decline versus at
$+26$ days. We also extracted spectra for the southern nucleus at each epoch
but found that the fit was not well constrained. The merged spectrum combining
both epochs was best fit as an absorbed power law with a photon index of
$1.61^{+0.65}_{-0.58}$, as shown in Figure 10. The spectral properties are
summarized in Table 4. The Fe K$\alpha$ line seen in the archival XMM-Newton
and NuSTAR observations (Payne et al., 2021) originates from the southern
nucleus and not from ASASSN-14ko.

Table 4:  Best-fit parameters for absorbed power-law models of the Chandra X-ray spectra of the northern and southern nucleus. The first observation had 55 degrees of freedom and the second observation had 31 degrees of freedom for the northern nucleus. The southern spectrum was produced by merging the two epochs, and this spectrum was fit with an additional Gaussian with energy $6.57^{+0.26}_{-0.21}$ keV and width $1.25_{-0.21}^{+0.27}$ keV.  MJD | MJD-MJD${}_{\mathrm{g~{}peak}}$ |  25mmNH |  |  |  |
---|---|---|---|---|---|---
(1022 g-1cm-1) |  43mmAbsorbed Flux (0.3-10.0 keV) |  |  |  |  |
(erg/cm2/s) |  7mm log10(Lx) |  |  |  |  |
(erg/s) | Photon Index $\Gamma$ |  $\chi^{2}$ per dof |  |  |  |
Northern: |  |  |  |  |  |
59210.0 | $+4.4$ | $0.002\pm{0.07}$ | (1.4${}_{-0.2}^{+0.3}$) $\times 10^{-12}$ | 42.77${}_{-0.07}^{+0.08}$ | $1.34\pm 0.11$ | 0.68
59236.3 | $+30.7$ | $0.002\pm 0.10$ | (7.6${}_{-1.8}^{+1.7}$) $\times 10^{-13}$ | 42.50${}_{-0.11}^{+0.10}$ | $1.32\pm 0.09$ | 0.84
Southern: |  |  |  |  |  |
… | … | 0.035 | (5.8${}_{-0.5}^{+0.5}$) $\times 10^{-13}$ | 42.38${}_{-0.04}^{+0.04}$ | $1.61^{+0.65}_{-0.58}$ | 1.75
Figure 8: The connection between the hardness ratio evolution and the X-ray
luminosity for all ASASSN-14ko flares observed with Swift XRT and NICER.
Figure 9: Swift XRT X-ray luminosities separated into total luminosity between
$0.3-10.0$ keV, hard X-rays between $2-10.0$ keV, and soft X-rays between
$0.3-2.0$ keV. The bottom panels in each column show the data stacked using
the timing model.

(a) (b) (c)

Figure 10: Chandra X-ray spectra of ESO 253$-$G003’s northern nucleus (a),
which corresponds to ASASSN-14ko, taken $+4$ days and $+31$ days after the
$g$-band peak associated with the December 2020 flare, and ESO 253$-$G003’s
southern nucleus (b). HST WFC3 F275W UV image of ESO 253$-$G003 (PI: J. Lyman,
Proposal ID: 16287) (c) overlaid with contours of the $0.3-10.0$ keV Chandra
image. The blue ellipse denotes the position corresponding to the location of
the flares reported in Payne et al. (2022), the red star shows the position
originally reported for ASASSN-14ko by Holoien et al. (2014b), and the green X
represents the position of the southwestern nucleus from Tucker et al. (2020).
Figure 11: Comparison of the HST/STIS UV spectrum of ASASSN-14ko taken four
days prior to the December 2020 optical peak to other UV spectra. The top
panel shows the TDEs iPTF15af (Blagorodnova et al., 2019), ZTF19abzrhgq (Hung
et al., 2020), PS18kh (Hung et al., 2019), iPTF16fnl Brown et al. (2018),
ASASSN-14li (Cenko et al., 2016). The middle panel shows the composite QSO
spectrum of Vanden Berk et al. (2001), the nitrogen rich QSO SDSS
J164148.19+223225.2 (Batra & Baldwin, 2014), the ambiguous nuclear transient
(ANT) ASASSN-18jd (Neustadt et al., 2020), and the AGN Mrk 817 (Kara et al.,
2021). The bottom panel shows the superluminous supernovae Gaia16apd (Yan et
al., 2017), and Gaia17biu (Yan et al., 2018). All spectra are normalized and
offset for visibility. The ordering of the spectra relative to ASASSN-14ko is
qualitative. The geocoronal airglow emission in each spectra is masked.

##  6 Discussion

We observed four of the most recent flares from ASASSN-14ko from X-ray through
optical wavelengths. At the time of ASASSN-14ko’s discovery as a periodic
transient, ASAS-SN had observed sixteen flares over six years in the optical.
Now, six sequential flares have been observed at X-ray, UV, and optical
wavelengths.

The six flares are UV dominated and have the blackbody SEDs typical of TDEs
but with possible deviations going into the optical. They have different peak
luminosities, indicating that the source is evolving, which was not clear from
the lower quality ground-based light curves. The high-cadence TESS data
further show that the optical flares do differ in terms of the peak luminosity
and rise/decline morphology, but the changes were more subtle than in the UV.
The July 2021 flare’s much fainter UV flux and flatter evolution indicate that
the UV properties can vary widely. It remains to be seen if this was a unique
or particularly rare event.

Detecting these differences is important for understanding the physical nature
of the system. For example, Payne et al. (2021) argued against interpreting
the flares as being due to a star passing through and disrupting the SMBH
accretion disk because the even and odd flares (i.e., up/down through the
disk) were indistinguishable given the quality of the data. Thus far, the six
flares observed in the UV also do not have any obvious differences between
even and odd flares.

For the TDE hypothesis, the flares have to evolve, and the flares have to
eventually ($\sim 100$ years based on $P_{0}=115.2^{+1.3}_{-1.2}$ days and
$\dot{P}=-0.0026\pm 0.0006$) come to an end. In this scenario, the envelope of
a giant star is steadily being stripped at each pericentric passage. This
means that the star has to evolve and the amount of mass stripped should
change with time both secularly and stochastically. The orbit should change
with each pericentric passage, but it should not be completely regular – the
period derivative should vary, and we may see evidence for this in our
inability to fit all the peak times with a single $\dot{P}$ (see Figure 1).
Recent theoretical work on modeling ASASSN-14ko has also supported the partial
TDE hypothesis. Cufari et al. (2022) used analytic arguments and three-body
integrations to show that the Hills mechanism can result in the capture of a
star on an orbit similar to ASASSN-14ko’s observed period. Metzger et al.
(2022) proposed ASASSN-14ko as a system of two stars co-orbiting on an extreme
mass ratio inspiral (EMRI) resulting in a long-lived mass-transfer. King
(2022) propose that ASASSN-14ko is caused by a white dwarf of mass
$0.58M_{\odot}$ on $114$ day orbit but the tidal disruption radius of a white
dwarf is deep inside the event horizon given the black hole mass estimates.

The strong UV spectral evolution is notable not only for the remarkable
evolution of the broad absorption lines, but also for the extremely rapid
timescale over which the line profiles changed. Compared to the UV spectra of
both TDEs and AGNs, ASASSN-14ko’s UV spectra changes in days versus the
timescales of months or years observed for normal TDEs and changing-look AGNs.
The general trend of the UV lines is the formation of a strong blue absorption
feature at $\sim-5000$ km/s relative to the expected rest-frame central
wavelength that appears prior to the optical peaks, and then vanishes and only
a broad emission line around the rest-frame wavelength remains. This trend is
most apparent for the C IV line, which shows the start of the formation of the
blue absorption feature at $-9$ days and evolves into an emission line by $+7$
days. All the absorption features are deep but they are not saturated. For Si
IV at $-4$ days, the line absorbs 17% of the continuum flux at maximum depth
and 68% for C IV at $-4$ days. This implies that the absorbing material must
both have a high optical depth and a high covering fraction. Speculatively, if
the $-12$ day and $-9$ day spectra represent the AGN UV emission which is
unchanging during the flare, then we can subtract these from the $-4$, 0 and
$+7$ day flaring spectra as shown in Figure 12. Then we find that the C IV and
Si IV absorption is optically thick in the core of the line.

Figure 12: The HST/STIS spectra epochs at $-4$, $+0$, $+7$, and $-36$ days
after subtracting off an averaged spectra combining the $-12$ and $-9$ day
epochs. The broad absorption features are optically thick in the core of the
line.  Figure 13: Radii as a function of SMBH mass $M_{BH}$ for the tidal
radii corresponding to a $0.3M_{\odot}$ main-sequence star, the Sun, and a
star with the mass of the Sun and a radius of $10R_{\odot}$
($R_{\mathrm{tidal}}(0.3M_{\odot})$, $R_{\mathrm{tidal}}(M_{\odot})$,
$R_{\mathrm{tidal}}(M_{\odot},10R_{\odot})$ shown as solid lines);
Schwarzschild radius (dotted line); semimajor axis assuming $P=115.2$ days
(dash-dotted line); radii corresponding to a range of circular velocities
(dashed lines); and blackbody radii spanning $10^{14.2}$ cm to $10^{14.9}$ cm
taken from Figure 4 (shaded). The $\log_{10}(M_{BH})$ shown for both plots
matches the range reported in Payne et al. (2021).

We compare the UV spectra during ASASSN-14ko’s UV rise at $-4$ days and
quiescence at $-36$ days to the UV spectra of TDEs, AGNs, and superluminous
supernovae (SLSNe) in Figure 11. The UV continuum of ASASSN-14ko at $-4$ days
most closely resembles the TDEs rather than the AGNs, which have consistently
flat continua from the FUV to the NUV. There are no similarities between
ASASSN-14ko and SLSNe. Overall, ASASSN-14ko at $-4$ days most closely
resembles iPTF15af, since both objects have broad N V, Si IV, and C IV
absorption features at similar velocities along with a blue continuum.
ZTF19abzrhgq also showed broad absorption features around these lines but at
much higher velocities of $\sim 15,000$ km/s (Hung et al., 2020). However, C
IV and Si IV in ASASSN-14ko appear similar to classic P Cygni profiles that
are not apparent in any other TDE spectra. In comparison, the spectrum at
$-36$ days is dominated by emission lines akin to AGNs. The strong broad
absorption features observed at $-4$ days but not at $-9$ days could either
indicate that the feature only forms several days before the optical peak or
that they only form when the X-ray luminosities reach a very low state like in
December 2020.

The Mg II emission line is typically present in AGNs and comes from a
partially ionized region (e.g., Peterson 1993; Richards et al. 2002). TDEs
generally lack Mg II emission, including ASASSN-14li, iPTF15af, iPTF16fnl, and
possibly ASASSN-18jd. Mg II was detected in absorption in ZTF19abzrhgq, but
Hung et al. (2020) attributed this feature to the host ISM and circumnuclear
gas instead of stellar debris associated with the TDE. In ASASSN-14ko, Mg II
is not strongly detected at $-4$, $+0$, and $+7$ days but it is clearly
detected at $-36$ days. This trend is another instance of the UV spectra
looking more TDE-like during the flares and more AGN-like during quiescence.
Since Mg II originates in partially ionized regions, the development of a
luminous hard continuum should suppress Mg II emission. The Mg II line is
always blueshifted from its expected rest frame wavelength, likely as a result
of host galaxy dynamics as was also discussed in Tucker et al. (2020). C III],
which is used to constrain the density of the broad-line region (Ferland et
al., 1992; Peterson, 1997), is detected at all times in ASASSN-14ko, but not
in any other TDE spectra. It was previously observed that the UV spectra of
ASASSN-14li and iPTF16fnl are similar to N-rich QSOs (Brown et al., 2018).

Figure 13 tries to place the various radii and velocities in context given the
rough estimates of the SMBH mass from Payne et al. (2021). As discussed there,
the Roche limits for main sequence stars, here illustrated by an
$0.3M_{\odot}$ dwarf and the Sun, are very close to the Schwarzschild radius.
Giants, illustrated using a star with a radius of $10R_{\odot}$, have Roche
limits well outside the horizon. The apparent orbital period corresponds to a
semi-major axis roughly ten times larger than the Roche limit of such a star,
which would be consistent with a star on an elliptical orbit being
periodically stripped at pericenter. Note that a much larger star ($\sim
100R_{\odot}$) would be inconsistent with this picture because its Roche limit
would be very similar to the semimajor axis. The range of black body radii
shown in Figure 4 corresponds to a scale very similar to the Roche limit of
the $10R_{\odot}$ star.

If we interpret the expansion of the black body radius from $10^{14.2}$ to
$10^{14.9}$ cm over 8 days as a physical motion, it implies a velocity of
$34000$ km/s. As a circular velocity, this corresponds to a radius modestly
larger than the black body radius and the giant Roche limit and modestly
smaller than the semi-major axis. The typical absorption and emission line
velocities of $\sim 5000$ km/s, again interpreted as a circular velocity,
correspond to radii an order of magnitude larger than the semi-major axis. The
orbital time scales at these radii are decades, so the changes in the emission
and absorption lines during a flare cannot be driven by physical motions
associated with the flare – they must be due to ionization changes driven by
changes in the luminosity and spectrum during the flare. The light travel
times to these radii are several days to a week, so it should be possible to
use intensive UV spectral monitoring during a flare to make a reverberation
mapping measurement of the characteristic distance of the emission lines
(absorption lines, by definition, have zero lag).

The six flares with X-ray observations are all characterized by an X-ray
luminosity dimming and spectral hardening a few days before the optical peak.
The column density does not change significantly, which rules out the
possibility that the change in X-ray luminosity and spectrum is driven by a
change in the absorption. ASASSN-14ko’s X-ray/UV/optical behavior is similar
to ASASSN-18el (AT2018zf; Nicholls et al. 2018) which underwent a drop in
X-ray luminosity by several orders of magnitude in conjunction with the
UV/optical brightening (Trakhtenbrot et al., 2019b; Ricci et al., 2020, 2021).
Different scenarios have been advanced to explain ASASSN-18el’s behavior. It
could be driven by an instability within the AGN accretion disk that causes a
change in the accretion rate (Trakhtenbrot et al., 2019b) or an inversion of
magnetic flux in a magnetically arrested disk (Scepi et al., 2021; Laha et
al., 2022). Alternatively, ASASSN-18el was a TDE that destroyed the X-ray
corona and inner accretion disk (Ricci et al., 2020, 2021) leading to the
decreased X-ray luminosity and increased UV/optical emission. The X-ray
luminosity then recovers as the X-ray corona reforms. However, for ASASSN-18el
the X-ray spectrum became softer rather than harder as it faded, and the
fading occurred long after the optical peak and lasted for longer.

##  7 Conclusion

ASASSN-14ko is a predictable nuclear transient whose flares are well-
characterized by a timing model, although the individual flares are not
identical. They do not, however, show an even/odd dichotomy which we might
expect from a star disrupting an accretion disk. Using ASAS-SN, Chandra,
HST/STIS, NICER, Swift X-ray/UVOT, and TESS, we presented the photometric and
spectroscopic X-ray/UV evolution and photometric optical evolution. Our
findings can be summarized as follows:

  * •

We refit the timing model first used in Payne et al. (2021) to include the
recent flare events and we find period $P_{0}=115.2^{+1.3}_{-1.2}$ days and
period derivative $\dot{P}=-0.0026\pm 0.0006$.

  * •

The UV/optical light curves always brighten to a single large-amplitude peak,
but the peak luminosity varies between peaks. The UV luminosity peaks show
larger differences than the optical luminosity peaks. This is also reflected
in the blackbody luminosities.

  * •

The two TESS light curves from Sectors 3-5 and 31-33 show that the optical
flares are not truly identical. The earlier flare began to rise earlier but
more slowly to a fainter peak and then declined slower.

  * •

The X-ray luminosity consistently decreases during the UV/optical rise, but
the depth of the minimum varies. Across all flares, the X-ray emission is
consistently harder when fainter and softer when brighter. There seems to be
no associated change in the absorption.

  * •

The Chandra data showed that the northern nucleus of the host galaxy
brightened during the flare, indicating that this nucleus is the source of
ASASSN-14ko’s X-ray emission. For both epochs, the absorbed power-law xspec
model $\textrm{tbabs}\times\textrm{zashift}\times\textrm{powerlaw}$ with a
photon index of $1.34\pm 0.11$ at $+4$ days and $1.32\pm 0.09$ at $+26$ days
provided the best fit for ASASSN-14ko.

  * •

Both the UV continuum and UV spectral lines changed rapidly during the
December 2020 flare as revealed by HST/STIS. The UV spectral lines that
evolved the most dramatically were Ly$\alpha$, N V, Si IV, and C IV which
showed broad absorption features at $\sim 5000$ km/s four days before the
optical peak which vanished and were replaced by broad emission lines by seven
days after peak. Overall, the UV spectra show some similarities to other UV
TDE spectra during outburst but they are more similar to AGN spectra in
quiescence.

We are continuing to monitor the flares and have proposed to obtain more
complete UV spectra phase coverage. We have an extensive body of optical
spectra and these will be analyzed in Payne et al. (in prep).

Software: astropy (Astropy Collaboration et al., 2018), ftools (Blackburn,
1995), HEAsoft (HEASARC, 2014), IRAF (Tody, 1986, 1993), numpy (Harris et al.,
2020), matplotlib (Hunter, 2007)

##  8 Acknowledgements

We thank Rick Pogge for valuable discussions. We thank Las Cumbres Observatory
and its staff for their continued support of ASAS-SN. ASAS-SN is funded in
part by the Gordon and Betty Moore Foundation through grants GBMF5490 and
GBMF10501 to the Ohio State University, and also funded in part by the Alfred
P. Sloan Foundation grant G-2021-14192. A.V.P. acknowledges support from the
NASA Graduate Fellowship through grant 80NSSC19K1679. B.J.S., C.S.K., and
K.Z.S. are supported by NSF grant AST- 1907570. B.J.S. is also supported by
NSF grants AST-1920392, AST-1911074, and AST-1911074. C.S.K. and K.Z.S. are
supported by NSF grant AST-181440. Support for T.W.-S.H. was provided by NASA
through the NASA Hubble Fellowship grant HST-HF2-51458.001-A awarded by the
Space Telescope Science Institute, which is operated by the Association of
Universities for Research in Astronomy, Inc., for NASA, under contract
NAS5-26555. Parts of this research were supported by the Australian Research
Council Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO
3D), through project number CE170100013. T.A.T. is supported in part by
Scialog Scholar grant 24215 from the Research Corporation. J.T.H. is supported
by NASA award 80NSSC21K0136.

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In addition to emission and absorption features detected at the redshift of
the host, the UV HST/STIS spectra exhibit low- and high-ionization absorption
lines at $z=0$. We attribute these features shown in Figure 14 as originating
from the Milky Way interstellar medium (ISM). These transitions consist of
low-ionization elements (ionization energy $<13.6$ eV) N I, Si II, Ly$\alpha$,
C II, Fe II, Mg II, Al II, and O I, in addition to high-ionization elements
(ionization energy $>13.6$ eV) S III, Si IV and C IV. Numerous lines are
affected by neighboring absorption and emission lines, but all absorption
lines from the Galactic ISM are less than
$500~{}\mathrm{km}~{}\mathrm{s}^{-1}$.

Figure 14: Low- and high-ionization absorption lines attributed to the Milky
Way ISM shown for the quiescence spectrum observed on 2021-03-06. The velocity
scale is at rest-frame wavelength at $z=0$ for each line, or in the case of
blended or doublet lines, an average of the two rest-frame wavelengths. Each
velocity is indicated by the vertical dashed lines. The shaded blue regions
are areas of the spectrum affected by neighboring absorption and emission
lines. The gray shaded regions show the spectrum uncertainty.