# Swift/UVOT discovery of Swift J221951-484240: a UV luminous ambiguous
nuclear transient

S. R. Oates1, N. P. M. Kuin2, M. Nicholl3,1, F. Marshall4, E. Ridley1, K.
Boutsia5, A. A. Breeveld2, D. A. H. Buckley6,7,8, S. B. Cenko4,9, M. De
Pasquale10, P. G. Edwards11, M. Gromadzki12, R. Gupta13,14, S. Laha4,15,16, N.
Morrell5, M. Orio17,18, S. B. Pandey13, M. J. Page2, K.L. Page19, T.
Parsotan4,15,16, A. Rau20, P. Schady21, J. Stevens22, P. J. Brown23, P.A.
Evans19, C. Gronwall24,25, J.A. Kennea24, N.J. Klingler4,15,16, M. H.
Siegel24, A. Tohuvavohu26, E. Ambrosi27, S.D. Barthelmy4, A.P. Beardmore19,
M.G. Bernardini28, C. Bonnerot1, S. Campana28, R. Caputo4, S. Ciroi30, G.
Cusumano27, A. D’Aì27, P. D’Avanzo28, V. D’Elia29,31, P. Giommi31, D.H.
Hartmann32, H.A. Krimm33, D. B. Malesani34,35,36, A. Melandri29, J. A.
Nousek24, P.T. O’Brien19, J.P. Osborne19, C. Pagani19, D.M. Palmer37, M.
Perri31,29, J. L. Racusin4, T. Sakamoto38, B. Sbarufatti28,24, J. E.
Schlieder4, G. Tagliaferri28, E. Troja39,40 & D. Xu40,41 E-mail:
<EMAIL_ADDRESS>
(Accepted…Received…)

###### Abstract

We report the discovery of Swift J221951-484240 (hereafter: J221951), a
luminous slow-evolving blue transient that was detected by the Neil Gehrels
Swift Observatory Ultra-violet/Optical Telescope (Swift/UVOT) during the
follow-up of Gravitational Wave alert S190930t, to which it is unrelated.
Swift/UVOT photometry shows the UV spectral energy distribution of the
transient to be well modelled by a slowly shrinking black body with an
approximately constant temperature of $T\sim 2.5\times 10^{4}$ K. At a
redshift $z=0.5205$, J221951 had a peak absolute magnitude of $M_{u,AB}=-23$
mag, peak bolometric luminosity $L_{max}=1.1\times 10^{45}~{}{\rm
erg\,s}^{-1}$ and a total radiated energy of $E>2.6\times 10^{52}$ erg. The
archival WISE IR photometry shows a slow rise prior to a peak near the
discovery date. Spectroscopic UV observations display broad absorption lines
in N V and O VI, pointing toward an outflow at coronal temperatures. The lack
of emission in the higher H Lyman lines, N I and other neutral lines is
consistent with a viewing angle close to the plane of the accretion or debris
disc. The origin of J221951 can not be determined with certainty but has
properties consistent with a tidal disruption event and the turn-on of an
active galactic nucleus.

###### keywords:

gravitational waves, black hole physics, galaxies: nuclei, transients: tidal
disruption events, ultraviolet: general

##  1 Introduction

The Advanced Laser Interferometer Gravitational Wave Observatory (LIGO; LIGO
Scientific Collaboration; Aasi et al., 2015) and the Advanced Virgo detector
(Virgo; the Virgo Scientific Collaboration; Acernese et al., 2015) began the
third observing run (“O3”) in search of Gravitational Wave (GW) events on 2019
April 1 (The LIGO Scientific Collaboration and the Virgo Collaboration.,
2019). The Neil Gehrels Swift Observatory (henceforth Swift; Gehrels et al.,
2004) participated in the search for the electromagnetic (EM) counterpart of
GW sources. In total Swift observed, with varying degrees of coverage, 18 of
the GW candidate alerts released by the LIGO-Virgo Collaboration (LVC). One
effect of scanning very large areas of the sky for the EM counterpart is the
discovery of a multitude of transient phenomena that are not necessarily
related to the GW itself. During the O3 run the Swift Ultra-violet/Optical
Telescope (UVOT; 1600–8000 Å; Roming et al., 2005) serendipitously found 27
optical transients that changed in magnitude at 3$\sigma$ level compared with
archival $u$ or $g$-band catalogued values (Oates et al., 2021). Determining
the nature of all the optical/UV transients that reside in GW error regions is
important to confirm or rule out their possible association with the GW
trigger and these serendipitous UV sources detected may also be of interest in
their own right. Indeed this is the case for Swift J221951-484240 (henceforth
J221951; Oates et al., 2019a, b), which we investigate further in this paper.
This source was observed by the Swift/UVOT telescope as part of the follow-up
campaign to identify the EM counterpart to the GW trigger S190930t, which was
initially classified as a neutron star - black hole (NS-BH) merger (LIGO
Scientific Collaboration & Virgo Collaboration, 2019).

Since detection, Swift has continued to monitor this source and we have
obtained additional photometric and spectral observations with HST ACS+COS,
SALT, Magellan/IMACS, VLT/X-shooter, ATCA, AstroSat and GROND. With the HST
COS spectrum, we identify J221951 at a redshift of $z=0.5205\pm 0.0003$ (see
§3.6), which is outside the distance range of the GW source reported on
GraceDB, ruling out its association with S190930t. At this redshift it had a
peak absolute magnitude of $M_{u,AB}=-23$ mag and total energy release in the
optical/UV over the 2.5 years of observations of $>2.6\times 10^{52}$ erg,
making it one of the most luminous transients ever recorded. In the following,
we report on these observations and investigate the nature of this extremely
luminous UV transient, ultimately comparing it to tidal disruption events
(TDEs111A TDE is a bright flare that arises as a consequence of a star being
torn apart as it passes too close to the centre of a supermassive black hole
(Hills, 1975; Rees, 1988; Loeb & Ulmer, 1997).) and active galactic nuclei
(AGN).

This paper is organized as follows. We provide the data analysis in § 2,
results in § 3 and discussion and conclusions follow in § 4 and § 5. All
uncertainties throughout this paper are quoted at 1$\sigma$ unless otherwise
stated. Throughout, we assume the Hubble parameter $H_{0}=70\;{\rm
km\,s}^{-1}\;{\rm Mpc}^{-1}$ and density parameters $\Omega_{\Lambda}=0.7$ and
$\Omega_{m}=0.3$. All magnitudes are given in the AB system, except for WISE
photometry which is provided in the Vega system.

##  2 Observations

S190930t triggered LIGO/Virgo at 14:34:08 UT on the 2019 September 30,
$T_{0,GW}$ (LIGO Scientific Collaboration & Virgo Collaboration, 2019). It was
reported to be at a distance of $108\pm 38$ Mpc, on GraceDB222
https://gracedb.ligo.org/ using the BAYESTAR skymap. S190930t had a high false
alarm probability of 2.05 $\rm{yr}^{-1}$. At the time of the announcement,
this trigger met the Swift follow-up criteria and Swift/UVOT observed 50.1
deg2, equating to 2 per cent of the total localization probability; determined
from a convolution of the LIGO-Virgo probability map and the 2MASS Photometric
Redshift catalogue (Bilicki et al., 2014; Evans et al., 2016). As this GW
event was only detected by a single detector, this event did not meet the
criteria to be included in the Gravitational-Wave Transient Catalog (GWTC-2)
of compact binary coalescences observed by Advanced LIGO and Advanced Virgo
(Abbott et al., 2020).

J221951 was identified in the UVOT GW pipeline as a Q0 source333For a source
to be given a Q0 identification it implies that in the detection image it must
be brighter than 19.9 mag and is either a new source or a known source that is
two magnitudes brighter than a catalogued value (see Oates et al., 2021, for
further details on the Swift/UVOT GW pipeline and quality flags). and given
the initial identification Q0_src93 (Oates et al., 2021). J221951 was detected
at a $u$-band magnitude of $19.48\pm 0.20$ mag at $T_{0,GW}+14.37\,{\rm ks}$
(Oates et al., 2019a). The UVOT position is RA, Dec(J2000)= 334.96599,
-48.71116 deg with an estimated uncertainty of 0.7 arcsec (radius, 90 per cent
confidence). Examining archival images, no source was detected at the location
in GALEX NUV or FUV images (Bianchi, Conti & Shiao, 2014; Bianchi, Shiao &
Thilker, 2017). However, a faint source consistent with this position was
identified in the DSS archival image and in the catalogues of the Dark Energy
Survey (DES; Abbott et al., 2018, 2021), VISTA (McMahon et al., 2013) and WISE
(Cutri & et al., 2012; Cutri et al., 2021). The source appears nuclear when
comparing our ACS imagery (see below) to DES images going back to 2014.

Initially, it was suggested that this source was a flare of a red dwarf star
(Oates et al., 2019a). However, follow-up observations, performed by UVOT at
$T_{0,GW}+2.1$ days (Oates et al., 2019b), showed the source was blue and at a
magnitude consistent with the initial detection ($u=19.67\pm 0.22$). At the
time of detection, follow-up observations were also performed by J-GEM (Kamei
et al., 2019), Chilescope observatory (Belkin et al., 2019) and spectrally
with SALT (Buckley et al., 2019). However, no spectral features could be
identified and therefore the redshift of this source could not be constrained.

Below we summarize the archival data and the photometry and spectroscopy
obtained for J221951. The photometry is provided in Fig. 1 and Supplementary
Table S.1. A log of the spectroscopic observations is given in Table 1.

Figure 1: Panel (a) displays the 4 UVOT UV filter light curves of J221951
together with data from AstroSat, GROND, GALEX, WISE, J-GEM and Chilescope
observatory; data from the latter two telescopes are from GCNs (Kamei et al.,
2019; Belkin et al., 2019). The different filter light curves have been
scaled, with the scaling given in the legend. No correction for Galactic
extinction, corresponding to a reddening of $E(B-V)=0.012$ (Schlafly &
Finkbeiner, 2011) has been applied. Upper limits for the GALEX FUV and NUV
observations are given by black down-pointing arrows. Included as left
pointing arrows are the archival detections for the different filters,
obtained from DES, VISTA and WISE. No correction has been made to the
photometry of J221951 for this archival source. The dotted vertical lines
indicate the times at which spectra were taken. The light curve shows a
gradual decrease in brightness over the course of observations. The light
curve brightens three times ($\sim$58766, 58843 and 59172 MJD), which appear
to reset the brightness level. At peak J221951 is brighter than archival
values at all UV/optical/IR wavelengths, by more than 1-3 mag. Superimposed on
the decay, most apparent in the $uvw2$ light curve, are three rebrightenings
which show changes in magnitude of $\sim 0.5$ mag, after which, the light
curve continues to decay but at an elevated brightness compared to that pre-
brightening. Panel (b) displays the change over time of 5 different colours,
which are given in the legend. The colour curves have been corrected for
Galactic extinction and host subtracted. The left-pointing arrows indicate
archival colours. Compared to historic values J221951 is much bluer, changing
in $g-r$ by $-0.68$ mag by the first observation. Within errors, $uvw2-uvw1$
is constant in colour, while $g-r$ and $r-i$ become bluer and $uvw2-g$ becomes
redder with time.

###  2.1 Swift BAT Observations

We analyzed all of the publicly available (on
HEASARC444https://heasarc.gsfc.nasa.gov/docs/archive.html) BAT “survey” mode
data, from 29th Oct 2019 to 21st April 2022, which are also known as detector
plane histograms or DPHs. BAT survey data are accumulated in histograms
onboard the spacecraft, with typical integration times of between 300 seconds
and around $2000$ seconds. An 80-channel binned spectrum is recorded for each
of the active detectors, which are saved in the DPH files. For a detailed
explanation of the reprocessing and analysis of the BAT survey mode data see
Laha et al. (2022b). We do not detect a signal at the $3\sigma$ level above
the background in any of the BAT exposures. Integrating from the time of
detection until the last observation we find the average $5\sigma$ upper limit
on the $14-195\,\rm keV$ flux is $6\times 10^{-9}\rm erg\,cm^{-2}\,s^{-1}$.

###  2.2 Swift/XRT Observations

We processed the XRT data using the online analysis tools provided by the UK
Swift Science Data Centre (Evans et al., 2007, 2009). J221951 is not detected
in single visits or in a stacked image created from the $279\,{\rm ks}$ of
observations taken over 2.5 years, from 2019 Sep 30 to 2022 Apr 21. Individual
visits are typically of a few ks duration, with limiting count rates of
$(2-3)\times 10^{-3}\,{\rm counts\,s^{-1}}$ (0.3-10 keV). This is equivalent
to an unabsorbed flux density of $<1\times 10^{-13}\,{\rm erg}\,{\rm
cm}^{-2}\,{\rm s}^{-1}$ and luminosity $<10^{44}\,{\rm erg\,s^{-1}}$ in the
0.3-10 keV energy range, assuming a photon index of $\Gamma=$ 1.7, a Galactic
absorbing column $N_{H}=9.8\times 10^{19}\,{\rm cm}^{-2}$ and an intrinsic
absorbing column of $N_{H}=3\times 10^{20}\,{\rm cm}^{-2}$. Stacking all the
XRT images, we find a deeper limiting count rate of $1.4\times 10^{-4}\,{\rm
counts\,s^{-1}}$ (0.3-10 keV), in $279\,{\rm ks}$, equivalent to an unabsorbed
flux density of $<5.5\times 10^{-15}\,{\rm erg}\,{\rm cm}^{-2}\,{\rm s}^{-1}$
and a luminosity of $L_{X}<6\times 10^{42}\,{\rm ergs^{-1}}$. Assuming this
limit over the duration of Swift observations this places an upper limit on
the total energy released in X-rays (0.3-10 keV) as $E_{X}<5\times
10^{50}\,{\rm erg}$.

###  2.3 Swift/UVOT Observations

After the detection of J221951 in the $u$ band on 2019 September 30, UVOT
continued to observe J221951 in all six optical/UV filters until 2021 Aug 11,
after which observations were performed in the $u$ and UV filters.
Observations were taken in image mode only. To begin with, observations were
performed every few days to a week cadence, which decreased to approximately a
monthly cadence as the source faded. On the 3rd of December 2020, we observed
with a higher cadence to investigate variability. All images were downloaded
from the Swift data archive555https://www.swift.ac.uk/archive/index.php. The
source counts were extracted from single or summed exposures using a source
region of 5 arcsec radius. Background counts were extracted using an annular
region with an inner radius of 15 arcsec and an outer radius of 35 arcsec. The
count rates were obtained from the images using the Swift tool uvotsource.
Finally, the count rates were converted to AB magnitudes using the UVOT
photometric zero-points (Poole et al., 2008; Breeveld et al., 2011). The
analysis pipeline used UVOT calibration 20201215. The UVOT detector is less
sensitive in a few small
patches666https://heasarc.gsfc.nasa.gov/docs/heasarc/caldb/swift/
docs/uvot/uvotcaldb_sss_01b.pdf for which a correction has not yet been
determined. Therefore, we have checked to see if any of the sources of
interest fall on any of these patches in any of our images and exclude 15
individual UV exposures for this reason.

###  2.4 Dark Energy Survey

DES (Abbott et al., 2018) observed the field of J221951 over several occasions
in the $g$, $r$, $i$, $z$ and $Y$ filters, starting November 2013. Images from
2013 until 2018 are available for J221951 in NOIRLab Astro Data
Lab777https://datalab.noirlab.edu/.

We used a custom wrapper for photutils to perform both aperture and point
spread function (PSF) fitting photometry at the location of J221951 in these
DECam images, finding consistent results between the two approaches. We
calibrated the zeropoint of each image using local stars in the Pan-STARRS DR2
catalogue (Flewelling et al., 2020). At the location of J221951, a red point-
like source is well detected in all epochs taken prior to the detection of
J221951 by UVOT. The source shows no significant temporal variability.

###  2.5 HST COS & ACS

We observed J221951 with the Cosmic Origins Spectrograph (Green, Wilkinson &
Morse, 2003; Dixon & Niemi, 2010) on the Hubble Space Telescope (Bahcall,
1986) on the 8th May 2020. In two orbits we obtained medium resolution spectra
with the G130M and G140L gratings (program ID 16076, P.I. S. Oates). The
spectra were processed with the standard pipeline (OPUS_VER=HSTDP 2020_5;
calibration software system version caldp_20201012; and CALCOS code version
3.3.10) on MJD 59139.38. The spectra were spliced together using the IRAF
‘splice’ task. We also obtained an image, split in 4 exposures, with the
Advanced Camera for Surveys (Clampin et al., 2000) (total exposure time = 2256
s) during one orbit using the F475W filter (4746 Å; width 420 Å) which led to
an improved position for J221951 of RA=22:19:51.80, Dec=-48:42:40.90 (J2000),
Fig. 2. The source magnitude was $M(F475W)=20.255\pm 0.002\,{\rm mag}$.
Standard observation and processing were used. We investigated if the source
was nuclear by comparing to DES DECam “Resampled” images in $g,r,z$ and $Y$
bands from 2014 onward by overlaying the ACS. We find that J221951 is centred
on the nucleus in DECam to within 0.045 arcsec (3-sigma) which is equivalent
to an angular separation of $<0.3$kpc.

Figure 2: HST ACS image of J221951. J221951, indicated by a red circle, is
consistent with being a point source.

###  2.6 GROND

On 2019 October 31, $g^{\prime}r^{\prime}i^{\prime}z^{\prime}JHK$ observations
of J221951 were taken with the Gamma-Ray Optical/Near-infrared Detector
(GROND; Greiner et al., 2008) mounted on the MPG 2.2 m telescope at the ESO La
Silla observatory, Chile. The source was re-observed with GROND on a further
seven occasions between August and December of 2021, and again in May 2022.
The data were reduced and analysed with the standard tools and methods
described in Krühler et al. (2008). The optical and NIR magnitudes were
obtained using aperture photometry and absolute calibration was performed
using field stars within the GROND field of view covered by the Sky Mapper
Southern Sky Survey (Keller et al., 2007) and the Two Micron Sky Survey
(2MASS; Skrutskie et al., 2006) for the
$g^{\prime}r^{\prime}i^{\prime}z^{\prime}$ and the $JHK$ bands respectively.

###  2.7 AstroSat

The Ultra-violet Imaging Telescope (UVIT) onboard AstroSat (Pati et al., 2003)
observed J221951 thrice on the 16th November 2020, 23rd November 2020, and a
few days later on the 9th December 2020 (proposal ID A10_024, P.I. S. B.
Pandey). Observations were taken with the F169M Sapphire (central wavelength
of $\lambda=1608{\rm\AA}$, and width $\Delta\lambda=290{\rm\AA}$) and F172M
Silica (central wavelength of $\lambda=1717{\rm\AA}$, and width
$\Delta\lambda=125{\rm\AA}$) filters, respectively. We aligned the images by
comparing field stars against those also found in GALEX FUV images. We
performed the aperture photometry on Level 2 UVIT images to extract the source
brightness. We used the standard zero points provided in Tandon et al. (2017).
J221951 was detected at all three epochs (see Fig. 1).

###  2.8 SALT

The South African Large Telescope (SALT; Buckley, Swart & Meiring 2006) using
the Robert Stobie Spectrograph (RSS, Burgh et al. 2003) obtained several
spectra with the PG0300 grating starting 2019 October 4 (1800 s), then 2019
October 22 ($2\times 1200$ s), and on 2020 May 11 (1500 s); the latter on the
same day of the HST COS observation though with full moon. The mean resolving
power of $R\sim 420$ (14.8 Å resolution). To reduce the spectra we used the
PyRAF-based PySALT package (Crawford et al.
2010)888https://astronomers.salt.ac.za/software/, which includes corrections
for gain and cross-talk, and performs bias subtraction. We extracted the
science spectrum using standard IRAF999https://iraf-community.github.io/
tasks, including wavelength calibration (Argon calibration lamp exposures were
taken, one immediately before and one immediately after the science spectra),
background subtraction, and 1D spectra extraction. Due to the SALT design,
absolute flux calibration is not possible101010The pupil (i.e. the view of the
mirror from the tracker) moves during all SALT observations, causing the
effective area of the telescope to change during exposures. Therefore,
absolute flux calibration cannot be done. See Buckley et al. (2006) and
Crawford et al. (2010) for details.. However, by observing spectrophotometric
standards during twilight, we were able to obtain relative flux calibration,
i.e. allowing recovery of the correct spectral shape and relative line
strengths.

###  2.9 Las Campanas Observatory

On 2019 November 11 a low-resolution spectrum was taken, consisting of
$2\times 1200$ s exposures in the range 3700-9250 Å, using the IMACS
instrument on the Baade Telescope of Las Campanas Observatory. The S/N at
1.16Å/pix resolution is low, 5-6, and there is a gap between 6430 Å and 6524 Å
due to the location of the spectrum on the detector, which lies across two of
the eight CCDs. The spectrum is blue and fairly featureless. Calibration was
done using a standard star spectrum.

###  2.10 ATCA

On 2020 Jan 11, we observed the source position with the Australia Telescope
Compact Array (ATCA), which consists of six 22 m diameter dishes (Wilson et
al., 2011). Observations, made under project code C1730, were made when the
telescope was in its 6A array configuration, with a maximum baseline of 5.9
km. The observing bands were centred at 5.5 and 9.0 GHz, with a 2 GHz
bandwidth in both bands. Four 10-minute scans of J221951 were made, with each
scan bookended by 2.5 minute scans on the phase calibrator PKS 2204-540. PKS
1934-638 was used as the primary flux density calibrator. Data reduction was
carried out following the standard procedures in miriad (Sault, Teuben &
Wright, 2011, 1995). No source was detected at the position of J221951 with 3
$\sigma$ upper limits of 117 $\mu$Jy at 5.5 GHz and 90 $\mu$Jy at 9 GHz.
Assuming a flat spectrum, the 5.5 GHz flux is equivalent to a luminosity of
$\sim 2\times 10^{39}\,{\rm erg\,s^{-1}}$, across a bandwidth of 2 GHz.

###  2.11 GALEX

GALEX observed the field of J221951 6 times in the NUV and 4 times in the FUV
between 2004 and 2008 for a total of 2.5 ks and 2.6 ks. No source is detected
in either band at the location of J221951. Using the Galex Merged catalog of
sources (MCAT; Morrissey et al., 2007) we derive $3\sigma$ magnitude upper
limits in the NUV and FUV of $23.8$ and $24.1$, respectively. For comparison,
the GALEX NUV filter spans 1771-2831 Å, which is broader than the individual
UVOT UV filters. The NUV filter covers a similar wavelength range as the
UVOT’s $uvw1$ and $uvm2$ filters. The FUV filter spans 1344-1786 Å, which is
bluer than UVOT $uvw2$.

###  2.12 WISE

In the ALLWISE catalogue (Cutri et al., 2021) a source is detected within 1.6
arcsec in the W1 and W2 filters only, with non-detections in the W3 and W4
filters (W1 $=16.90$, W2 $=16.78$, W3 $>12.47$, W4 $>9.12$). WISE/NEOWISE
(Wright et al., 2010; Mainzer et al., 2011) has been observing the field of
J221951 biannually since May 2014. At the time of writing, the most recent
data release provides observations until October 2021. We obtained photometry
for the individual exposures from the IRSA/IPAC infrared data science archive.
The individual images during a single visit are taken within a 1-2 day period.
A weak source is detected at the location of J221951 in most of the W1 images.
In W2, a weak source is detected in approximately half of the exposures. In
the single exposure photometry, there appears to be a slight increase in flux
in observations taken between 2019 October 18 and 2019 October 21 close to the
time J221951 was detected by Swift/UVOT, however, the data are noisy. We,
therefore, used the coadder
tool111111https://irsa.ipac.caltech.edu/applications/ICORE/ to produce stacked
images. We used sextractor (Bertin & Arnouts, 1996) to obtain the photometry.
We display all the WISE visits in Fig. 3. We created a stack of the W1 and W2
images taken from May 2010 until May 2014 and measure a magnitude of $16.81\pm
0.10$ and $16.61\pm 0.30$, in W1 and W2, respectively, consistent with that
reported in the ALLWISE catalogue (Cutri et al., 2021). In a stack of the
October 2019 observations, the first WISE visit after J221951 was detected by
UVOT, the source is 1.1, and 1.4 magnitudes brighter in the W1 and W2 filters,
respectively, compared to the stacks of the data taken prior to May 2014.
J221951 is not detected in later W2 per visit stacks. In W1 it fades in the
first 6 months by $\sim 0.3$ magnitudes and by October 2021 is consistent with
pre-2014 level.

Figure 3: Long term light curve of J221951 containing data from DES, WISE,
GROND, UVOT and J-GEM. The grey dotted line represents $T_{0,GW}$, the trigger
time of the gravitational wave event S190930t. The GALEX observations took
place between 53248 MJD and 54679 MJD (August 2004 and July 2008).

###  2.13 X-shooter

We observed J221951 with the X-shooter echelle spectrograph (Vernet et al.,
2011), mounted on the European Southern Observatory (ESO) Very Large
Telescope, on 16th July 2021 (PI Oates, program ID 107.22RT). X-shooter
provides continuous coverage from $\approx 3000-25000$ Å in the observer
frame. Data were obtained in on-slit nodding mode and reduced using the ESO
reflex pipeline. The pipeline applies de-biasing, flat-fielding, geometric
transformations of the echelle orders, wavelength calibration, cosmic ray
removal, and extraction to a one-dimensional spectrum. Flux calibration is
achieved using standard star observations in the same setup.

Table 1: Log of the spectroscopic observations UT Start | Time (MJD) | Telescope | Instrument | Grating | Exposure Time (s) |
---|---|---|---|---|---|---
2019-10-04 | 58760 | SALT | RSS | PG0300 | 1800 |
2019-10-22 | 58778 | SALT | RSS | PG0300 | $2\times 1200$ |
2019-11-11 | 58798 | Magellan-Baade | IMACS | Spectroscopic2/Gri-300-17.5 | $2\times 1200$ |
2019-11-22 | 58809 | SALT | RSS | PG0300 | 1600 |
2020-05-08 | 58977 | SALT | RSS | PG0300 | 1500 |
2020-05-08 | 58977 | HST | COS/FUV | G140L | 300 |
2020-05-08 | 58977 | HST | COS/FUV | G130M | 2275 |
2021-07-15 | 59411 | VLT | X-shooter | UVB,VIS,NIR | 2500,2400,2600 |

##  3 Results

Figure 4: Colour-colour diagram of the photometry of J221951, together with
ambiguous nuclear transients (ANTs) ASASSN-15lh (Leloudas et al., 2016) and
ASASSN-18jd (Neustadt et al., 2020), a classic tidal disruption event
AT2019qiz (Nicholl et al., 2020b) and the UV transient sources followed-up by
Swift/UVOT during the O3 follow-up and identified as candidate AGN, see Oates
et al. (2021). Left: $u-g$ versus $g-r$. Right: $g-r$ versus $r-i$. In both
panels, the size of the markers for the black-edged data points indicates the
time since peak or time since discovery in days, the key is given in the
legend. For J221951, we use the time since UVOT discovery, MJD 58756. For the
AGN arrows connect points of the same source in chronological order. In the
left panel, the dotted lines divide the figure into regions identifying
objects as red, blue and ultra-blue, adapted from Lawrence et al. (2016). In
both panels, the grey region indicates the colour location of 90 per cent of
SDSS spectroscopic quasars (see Lawrence et al., 2016, for details), and we
display a 2-D histogram, given in grey, of the SDSS colours of 10,000 stars
from the $Gaia$ DR2 catalogue, selected from a region at high Galactic
latitude. In the right panel, the blue region represents the location of the
blue cloud galaxies, and the red region represents the red sequence galaxies,
both out to $z=0.22$ (adapted from Lawrence et al., 2016). The green star,
without a black edge, in the right-hand panel (on the right-hand side of the
figure) is the pre-outburst colour of J221951 and represents the colour of the
host galaxy. All the other $gri$ values have been host corrected. All values
have been corrected for Galactic extinction. We have not corrected for host
extinction. Correction for host extinction would move points down and to the
left in both panels.

###  3.1 Temporal Evolution

In the top panel of Fig. 1, we display the IR/optical/UV photometry of J221951
obtained by UVOT (only the 3 UV and $u$ filters are displayed), GROND,
AstroSat, WISE, J-GEM and the Chilescope observatory (the latter two from
GCNs; Kamei et al., 2019; Belkin et al., 2019) along with archival values
obtained with GALEX, DES and WISE. Overall, the light curve shows a gradual
decrease in brightness with time. In addition to the decaying nature of the
light curve, there are three rebrightenings ($\sim$58766, 58843 and 59172 MJD)
which show changes in magnitude of $\sim 0.5$ mag and which appear to reset
the brightness level. The light curve continues to decay from the peak of the
flare, rather than returning to the value expected from the extrapolation of
the power-law observed pre-flare. These flaring episodes are most clearly
observed in the $uvw2$ filter. The long-term light curve of J221951, including
photometry taken pre-and post-UVOT detection, is displayed in Fig. 3. At peak
J221951 is brighter than archival values at all UV/optical/IR wavelengths, by
more than 1-3 mags, with the largest change observed in the blue filters.
There is some marginal evidence to suggest the rise of the optical/UV emission
lags the IR emission, though this cannot be claimed with certainty due to the
lack of optical/UV emission between 58429 and 58759 MJD. The most recent
observations, around 1000 days after the initial detection, indicate J221951
continues to be brighter than archival values in the UV through to the
$r$-band, while in redder filters, J221951 is comparable in brightness with
historic values.

The start time of J221951 is uncertain, but constraints can be placed using
individual DES, J-GEM $i$ band measurements. The last visit by DES taken in
November 2018 is at a brightness consistent with historic values, suggesting
J221951 began no earlier than 10 months prior to September 2019 (see Fig. 3).
This implies the start time $T_{0}$ is within a $\sim 330$ day window between
58429 and 58759 MJD. For comparison with other types of objects, we will take
the mid-time of this range, 58594 MJD, as $T_{0}$.

Using $T_{0}$ as the start time, we fit the $uvw2$ count rate (CR) light
curve, with a series of functions of increasing complexity. We initially fit a
power-law ($CR=Nt^{\alpha}$, where $N$ is the normalisation, and $\alpha$ is
the temporal decay index), which gives $\alpha=-1.32\pm 0.06$, although the
fit is poor with $\chi^{2}/d.o.f=286/62$. With a broken power-law (two power-
laws connected by a break at $t_{break})$, the fit traces the general
underlying behaviour. The fit has improved, but the $\chi^{2}/d.o.f$ is still
poor with $\chi^{2}/d.o.f=229/60$. An F-test suggests the break is required
with a confidence of $3\sigma$. The broken power-law fit has parameters
$\alpha_{1}=-0.84^{+0.17}_{-0.09}$, the break time
$t_{break}=58935^{+30}_{-18}$ MJD and $\alpha_{2}=-1.82\pm 0.15$. The poor
$\chi^{2}/d.o.f$ is likely due to the rebrightenings, which appear to reset
the brightness level. The uncertainty on the $T_{0}$ will also affect our
estimate of the decay indices of the fits. For instance setting $T_{0}$ as
$T_{0,GW}$ and fitting a power-law results in an $\alpha=-0.39\pm 0.01$, but
with a much worse $\chi^{2}/d.o.f=847/62$.

In the bottom panel of Fig. 1 we display the colour evolution. The $uvw2-uvw1$
and $uvw2-u$ colours do not show strong evolution with time although there is
some variation, which appears to correspond to the peaks and troughs of the
light curve behaviour in the panel above. However, the error bars are large
compared to the colour curves built with the ground-based data. For the
$uvw2-g$, $g-r$, $r-i$ we notice there is a strong change in colour between
the first two data points, becoming redder for $uvw2-g$ and $r-i$, but bluer
for $g-r$. There is a gap in these colour curves, until 59420 MJD, after which
the colour curves remain approximately at the same level until the end of
observations, however, this level is slightly redder for $uvw2-g$ and $r-i$
and slightly bluer for $g-r$, compared to the last data point before the gap.
Also shown in Fig. 1 is the archival $g-r$ colour (indicated by an arrow).
Compared to historic values J221951 has become bluer, changing in $g-r$ by
-0.68 mag by the first observation post-detection.

The WISE $W1-W2$ colour changes from $0.20\pm 0.32$ pre-outburst to $0.56\pm
0.28$ near the peak. The $W1-W2$ has been used to provide a rough assessment
of the AGN or stellar dominance of a galaxy ($W1–W2>0.8$ for AGN-like or
$W1–W2<0.5$ for galaxy-like; Stern et al., 2012; Yan et al., 2013). A value of
0.20 mag is consistent with being galaxy-like and not dominated by an AGN. The
change to 0.56 at peak, suggests that the surrounding dust is being heated,
but $W1-W2$ is still less than that observed in AGN.

In Fig. 4, we compare the $u-g$, $g-r$ and $r-i$ colours determined from
archival photometry and the photometry taken during the evolution of J221951.
The $u-g$ vs $g-r$ colour evolves downwards with time, while the $g-r$ vs
$r-i$ colour moves towards the top left corner with time. In both instances,
the strongest colour evolution is observed in the first few days after
detection. Initially, the colour of the transient is similar to that of the
candidate AGN from Oates et al. (2021) and quasars (QSOs) and it changes in
colour away from these objects as it evolves with time. Note there is a
substantial gap in GROND observations and so we only have colour information
in these panels from observations at very early and very late times.

###  3.2 Spectral Energy Distributions

We also constructed spectral energy distributions (SEDs) for 8 epochs where we
have quasi-simultaneous data from facilities in addition to Swift. To each
data point of each SED, which includes UVOT and ground observations we add a
$5\%$ systematic error. We fit the SEDs with XSPEC version 12.12.0 (Arnaud,
1996). In these SEDs, we use host subtracted photometry, with the host values
taken as the archival DES, VISTA and WISE values. The UVOT data are not host
subtracted; this is reasonable given that J221951 is $>1$ magnitude brighter
than the underlying host galaxy in the redder g-band at all epochs and
similarly J221951 is brighter than the NUV GALEX limit of 23.7 by $>1$ mag at
all epochs. Two of the SEDs were built using values from GCNs, $gri$ for 58758
MJD (Kamei et al., 2019) and $r$ for 58760 MJD (Belkin et al., 2019). For the
$gri$ filters in the SED at 58758 MJD, we assume an error of 0.1 mag since
photometric errors are not provided in Kamei et al. (2019). The remaining 6
SEDs were built using the optical/UV UVOT filters and GROND filters. For the
MJD 58787 SED, we also include host subtracted WISE W1 and W2 photometry. We
fit each SED with a power-law and then with a single blackbody. In both cases,
we include a dust component with Galactic reddening of $E(B-V)=0.012$
(Schlafly & Finkbeiner, 2011). We tested whether a host extinction component
improved the fits. The host $E(B-V)$ was consistent with zero, similar to that
derived from the X-shooter spectrum. The X-shooter spectrum does not display a
dip at 2175 Å in the rest-frame, implying that either the object has a Small
Magellanic Cloud type extinction law without extra absorption at 2175 Å or a
very low $E(B-V)$ of the host. We, therefore, do not use a second dust
component in our SED modelling.

For the first 3 SEDs, a power-law is preferred over a single blackbody, while
for the latter 5 epochs, a single blackbody is preferred. However, the
$\chi^{2}/d.o.f$ for both the power-law and single blackbody fits for the SEDs
from MJD 58787 (the third SED) onwards were poor. We therefore also tried
fitting a model consisting of two blackbodies. For the MJD 58787 SED, this
model is marginally preferred over the single power-law at $2.5\sigma$. For
the subsequent SEDs, the two blackbody model does not provide a better fit. We
note that by these late epochs, the brightness of J221951 in the reddest
filters is comparable to the host making it difficult to constrain the second
blackbody component. In Fig. 5, we display the SEDs of MJD 58787 and MJD
59437, together with the three models. The necessity of two blackbody
components for the MJD 58787 SED is apparent. For the MJD 59437 SED, the
H-band data point is well above the extrapolation of the single component
blackbody model, suggesting that the second thermal component is required. The
temperature of the second component decreases with time. The spectral fits are
provided in Table 2, with the convention for flux density,
$F\propto\nu^{-\beta}$.

We also constructed SEDs using UVOT data only for each observation performed
by Swift/UVOT for which at least three filters were obtained. We fit two
simple models to each UVOT SED: a power-law and a blackbody, again including
in both instances a dust component with Galactic reddening. From 64 SEDs, we
find a weighted average index of $\beta=0.49\pm 0.04$, which is $4\sigma$
shallower than the $\beta=2/3$ predicted for a standard thin accretion disc at
UV/optical wavelengths (Shakura & Sunyaev, 1973). For the blackbody fits the
average temperature is $23000\pm 410\,{\rm K}$. The mean and standard
deviation of the $\chi^{2}$/d.o.f of the power-law fits and the blackbody fits
is $0.80\pm 0.65$ and $0.89\pm 0.62$, respectively.

Table 2: The best-fit parameters to eight SEDs with ground-based photometry in addition to 6 filter UVOT photometry. The parameters given are: spectral index $\beta$, intrinsic blackbody temperature $T_{BB}$ and the $\chi^{2}/d.o.f$.  Time (MJD) | Model | $\beta$ |  $T_{BB,1}$ (K) |  $T_{BB,2}$ (K) | $\chi^{2}/d.o.f$ | Null Hypothesis
---|---|---|---|---|---|---
58758 | Pow | $0.19\pm 0.10$ | - | - | 5/7 | $6.1\times 10^{-1}$
58758 | Bbody | - | $21300\pm 1000$ | - | 7/7 | $4.3\times 10^{-1}$
58760 | Pow | $0.25\pm 0.10$ | - | - | 2/5 | $8.1\times 10^{-1}$
58760 | Bbody | - | $21100\pm 1000$ | - | 7/5 | $1.9\times 10^{-1}$
58787 | Pow | $0.03\pm 0.06$ | - | - | 35/11 | $2.2\times 10^{-4}$
58787 | Bbody | - | $20200\pm 700$ | - | 38/11 | $6.8\times 10^{-5}$
58787 | Bbody Bbody | - | $20800\pm 700$ | $2800\pm 400$ | 13/9 | $1.6\times 10^{-1}$
59437 | Pow | $0.12\pm 0.12$ | - | - | 29/11 | $2.2\times 10^{-3}$
59437 | Bbody | - | $20500\pm 1100$ | - | 18/11 | $8.7\times 10^{-2}$
59437 | Bbody Bbody | - | $20700\pm 1200$ | $2300\pm 500$ | 15/9 | $1.0\times 10^{-1}$
59470 | Pow | $0.15\pm 0.14$ | - | - | 51/9 | $6.4\times 10^{-8}$
59470 | Bbody | - | $19400\pm 1300$ | - | 36/9 | $4.4\times 10^{-5}$
59470 | Bbody Bbody | - | $19500\pm 1300$ | $1200\pm 100$ | 28/7 | $2.2\times 10^{-4}$
59497 | Pow | $0.33\pm 0.11$ | - | - | 46/8 | $1.9\times 10^{-7}$
59497 | Bbody | - | $18300\pm 1000$ | - | 22/8 | $5.2\times 10^{-3}$
59497 | Bbody Bbody | - | $18300\pm 1000$ | $580\pm 50$ | 20/6 | $2.8\times 10^{-3}$
59528 | Pow | $0.12\pm 0.13$ | - | - | 46/9 | $6.9\times 10^{-7}$
59528 | Bbody | - | $18900\pm 1200$ | - | 21/9 | $1.1\times 10^{-2}$
59558 | Pow | $0.12\pm 0.13$ | - | - | 46/9 | $6.9\times 10^{-7}$
59558 | Bbody | - | $18900\pm 1200$ | - | 21/9 | $1.1\times 10^{-2}$
Figure 5: Spectral energy distributions of J221951 at MJD 58787 (blue), MJD
59172 (green), MJD 59200 (cyan) and MJD 59437 (red). For the MJD 58787 and MJD
59437 we overlay three models: power-law (dotted), single blackbody (dot-
dashed) and two blackbodies (dashed and solid lines). For the two blackbody
model we display the two single blackbodies (dashed) and the combination of
these components (solid). A two blackbody model is the best fit for the MJD
58787 SED. Visually a two blackbody appears to be the best fit for the MJD
59437 SED, although is not statistically required. The MJD 59172, MJD 59200
SEDs include AstroSat data and show the turnover of the higher temperature
black body. We only overlay the best-fit blackbody for these two SEDs. The
typical AGN Type 1 spectrum (Richards et al., 2006), divided by a factor of
10, is given in purple. Overall, the SEDs of J221951 do not resemble the
typical AGN Type 1 SED.

###  3.3 Optical to X-ray flux ratio, $\alpha_{OX}$

For both TDEs and AGN, the UV to X-ray spectral slope, $\alpha_{OX}$
(Tananbaum et al., 1979; Wevers, 2020), can be measured as:

$\alpha_{OX}=-\frac{\log(f_{\nu,X}/f_{\nu,O})}{\log(\nu_{X}/\nu_{O})}$ (1)

where $f_{\nu,X}$ and $f_{\nu,O}$ are the X-ray and optical flux densities at
rest-frame $2\,{\rm keV}$ and $2500$ Å, respectively, and $\nu_{X}$ and
$\nu_{O}$ are the X-ray and optical rest-frame frequencies at $2\,{\rm keV}$
and $2500$ Å, respectively. Using the extinction-corrected observed $u$-band
flux ($\lambda_{central}=3501$ Å), at peak brightness, as a proxy for
$f_{\nu,O}$ at rest-frame $2500$ Å and the X-ray unabsorbed flux limit from an
individual XRT visit, scaled to rest-frame 2 keV, assuming a photon index of
$\Gamma=$ 1.7, we obtain a value of $\alpha_{OX}\gtrsim 1.6$ at 58788 MJD.

###  3.4 Bolometric Light curve

We construct the bolometric light curve of J221951 from the Swift/UVOT data
using superbol (Nicholl et al., 2018). The method allows us to integrate under
the SED inferred from the multi-colour data at each epoch, and fit a blackbody
function to estimate the temperature, radius, and missing energy outside of
the observed wavelength range. For TDEs, a blackbody is an excellent
approximation of the near-UV and optical emission (e.g. van Velzen et al.,
2021). However, we note that the radius is computed under the assumption of
spherical symmetry, which may not reflect the potentially complex geometry in
TDEs. We include Galactic extinction but do not correct for the host
extinction, which for this source is likely to be negligible (see §3.2). The
bolometric light curve is plotted in Fig. 6. In the middle and bottom panels
of Fig. 6, we display the effective temperature and radius of J221951. The
maximum luminosity is $L_{max}=(1.1\pm 0.7)\times 10^{45}\,{\rm erg}\,{\rm
s}^{-1}=(2.9\pm 1.1)\times 10^{11}L_{\odot}$. The bumps seen in the photometry
are also seen in the luminosity evolution, although the increase in the size
of the errors makes these features less apparent. The effective temperature of
J221951 has remained roughly constant throughout with an average temperature
of $T=2.8\times 10^{4}\,{\rm K}$, with a typical error for each inferred
temperature of $\sim 7000\,{\rm K}$. This is consistent with the values
determined in §3.2. The blackbody radius evolves in a similar fashion as the
luminosity of J221951. Trapezoidal integration of the blackbody luminosity
over the span of Swift observations in rest frame days gives a total emitted
energy of $E=2.6\times 10^{52}\,{\rm erg}$, a lower limit since we miss the
peak of the light curve. This corresponds to a lower limit on the accreted
mass of $M_{acc}=0.14M_{\odot}$, for an accretion efficiency of $\eta=0.1$.

In Fig. 7, we compare the luminosity of J221951 with a sample of TDEs. J221951
is more luminous than the bulk population of TDEs and is on par with the
luminosity of ASASSN-15lh, ASASSN-17jz and ASASSN-18jd (Dong et al., 2016;
Holoien et al., 2021; Neustadt et al., 2020), which are luminous members of
the population of ambiguous nuclear transients (ANTs; Holoien et al., 2021;
Margutti et al., 2017a). ANTs are transients for which it is not clear if they
are TDEs or related to AGN activity, with properties consistent with both
classes. The temporal evolution of J221951 most closely resembles ASASSN-15lh
after its initial peak. Other objects showing similar slowly declining light
curve behaviour are the ANTs ASASSN-18el (Trakhtenbrot et al., 2019; Ricci et
al., 2020; Laha et al., 2022a; Hinkle et al., 2022b) and ASASSN-20hx (Hinkle
et al., 2022a), though they are overall an order of magnitude fainter. J221951
continues to be detected and, whether it is a TDE or an ANT, it is one of the
longest observed to date (see also van Velzen et al., 2019b).

Figure 6: Top: the bolometric light curve of J221951 derived from the UVOT
photometry. Middle: temperature evolution. Bottom: evolution of the blackbody
radius. The luminosity and blackbody radius evolve similarly, while the
temperature is approximately constant. Time is given in MJD. Figure 7: The
bolometric light curve of J221951, with peak time taken as the UVOT discovery
date, MJD 58756, together with a sample of tidal disruption events: PTF09ge
(Arcavi et al., 2014); PS1-10jh (Gezari et al., 2012); PS1-11af (Chornock et
al., 2014); ASASSN-14ae (Holoien et al., 2014); ASASSN-14li (Holoien et al.,
2016b); ASASSN-15oi (Holoien et al., 2016a); iPTF16fnl (Blagorodnova et al.,
2017); AT2017eqx (Nicholl et al., 2019); AT2018dyb (Leloudas et al., 2019);
AT2018hyz (Short et al., 2020; Gomez et al., 2020); AT2018zr (van Velzen et
al., 2019a); AT2019ahk (Holoien et al., 2019b); AT2019qiz (Nicholl et al.,
2020b) and ambiguous nuclear transients: ASASSN-15lh (Dong et al., 2016;
Leloudas et al., 2016; Godoy-Rivera et al., 2017; Brown et al., 2014),
ASASSN-18jd (Neustadt et al., 2020), ASASSN-17jz (Holoien et al., 2021),
ASASSN-18el (Trakhtenbrot et al., 2019), PS16dtm (Blanchard et al., 2017),
ASASSN-20hx (Hinkle et al., 2022a). J221951 is comparable in luminosity to
ASASSN-15lh and ASASSN-18jd.

###  3.5 TDE model fit

If we assume J221951 is a TDE, we can derive physical parameters from our
multiband light curves using the Modular Open Source Fitter for Transients
(MOSFIT; Guillochon et al., 2018) with the TDE model from Mockler, Guillochon
& Ramirez-Ruiz (2019). This model assumes a mass fallback rate derived from
simulated disruptions of polytropic stars by a super massive black hole (SMBH)
of $10^{6}M_{\odot}$ (Guillochon, Manukian & Ramirez-Ruiz, 2014), and uses
scaling relations and interpolations for a range of black hole masses, star
masses, and impact parameters. The free parameters of the model, as defined by
Mockler, Guillochon & Ramirez-Ruiz (2019), are the masses of the black hole,
$M_{BH}$, and star, $M_{\ast}$; the scaled impact parameter $b$; the
efficiency $\eta$ of converting accreted mass to energy; the normalisation and
power-law index, $R_{ph,0}$ and $l_{ph}$, connecting the radius to the
instantaneous luminosity; the viscous delay time $T_{\nu}$ (the time taken for
matter to circularise and/or move through the accretion disc) which acts
approximately as a low pass filter on the light curve; the time of first
fallback, $t_{0}$, which is equivalent to our $T_{0}$, assuming the TDE model
is correct; the extinction, proportional to the hydrogen column density
$N_{H}$ in the host galaxy; and a white noise parameter, $\sigma$. The priors
follow those used by Nicholl et al. (2022), and reflect the range of SMBH
masses where optically-bright TDEs are expected (e.g. van Velzen, 2018), the
range of impact parameters covering both full and partial disruptions,
accretion efficiencies for non-rotating to maximally-rotating black holes, and
a broad range of possible photospheres and viscous timescales (see Mockler,
Guillochon & Ramirez-Ruiz, 2019, or details). In addition, we use the time of
the last DES i-band archival observation as a lower limit on the $T_{0}$
prior.

The fits are applied using the python package, Dynasty (Speagle, 2020), which
implements dynamic nested sampling methods to evaluate the posterior
distributions of the model parameters. We plot the median and 16th-84th
percentiles of the light curve posterior distribution from 100 realisations of
the Markov Chain in Fig. 8. The model provides a good fit to the optical/UV
bands but is unable to reproduce the undulations that are most clearly
observed in the UV bands. Mockler, Guillochon & Ramirez-Ruiz (2019) also found
for ASSASN-14li and ASASSN-15oi, that the observed photometry deviates from
the decline of the model light curve. They suggest that for these TDEs
additional late-time components may contribute, which are not modelled by
MOSFIT. From the fit to J221951 we derive the posterior probability
distributions of the parameters, listed in Table 3, with two-dimensional
posteriors plotted in Supplementary Figure S.1. With the light curve observed
to decay from the start of observations, the start time, $T_{0}$, inferred
with MOSFIT is MJD $58579.85^{+33.70}_{-31.41}$. The physical parameters point
to the disruption of a $\sim 0.6{\rm M_{\odot}}$ star by a black hole of mass
$\log(M_{BH}/M_{\odot})=\,7.12$. The scaled impact parameter,
$b=0.63^{+0.08}_{-0.05}$, corresponds to a median physical impact parameter
$\beta=R_{t}/R_{p}=1.04$, where $R_{t}$ is the tidal radius and $R_{p}$ is the
orbital pericentre. For the inferred SMBH mass, $R_{t}=3.4R_{S}$, where
$R_{S}$ is the Schwarzschild radius, which is equivalent to $4.3\times
10^{-6}\,{\rm pc}$. Using the remnant mass versus $\beta$ curve from Ryu et
al. (2020, their Fig. 4) for a 0.5-0.7 $M_{\odot}$ star, up to $\sim 25$ per
cent of the star could have survived this encounter.

Comparing these parameters to those derived in the same fashion for a sample
of 32 TDEs (Nicholl et al., 2022), we find the black hole mass at the high
end, star mass in the normal range and that the impact parameter is more
consistent with the TDE-H spectroscopic class than the TDE-H+He121212TDEs may
be divided into sub-classes spectroscopically (van Velzen et al., 2021). These
classes are: TDE-H: TDEs with H I lines, TDE-He: TDEs with He II lines only,
and TDE-H+He: TDEs with a mixture of H I, He II and N III lines.. Compared to
other TDEs from Mockler, Guillochon & Ramirez-Ruiz (2019) and Nicholl et al.
(2022), the $T_{\nu}$ posterior is flat up to relatively high values, $\sim
10\,{\rm days}$, though there is similar posterior support for lower values
$\lesssim$ days. This broad distribution is likely due to the lack of
constraints on the rise time of this source, together with the slow decay rate
of the light curve. Viscous delays would broaden the light curve (relative to
the fallback rate) around the peak, and the wide range of possible rise times
allows for a wider range of viscous reprocessing than in faster-evolving TDEs
with early data.

Figure 8: Fits to the multicolour light curve using the TDE model in MOSFIT (Guillochon et al., 2018; Mockler, Guillochon & Ramirez-Ruiz, 2019). We plot the median and 16th-84th percentiles of the light curve posterior distribution from 100 realisations of the Markov Chain. Table 3: Priors and marginalized posteriors for the MOSFIT TDE model. Priors are flat within the stated ranges, except for $M_{\ast}$, which uses a Kroupa initial mass function. The quoted results are the median of each distribution, and the error bars are the 16th and 84th percentiles. These errors are purely statistical; Mockler, Guillochon & Ramirez-Ruiz (2019) provide estimates of the systematic uncertainty. $t_{0}$ is observer frame days before the first detection.  Parameter | Prior | Posterior | Units
---|---|---|---
$\log(M_{BH}/M_{\odot})$ | [5,8] | $7.12^{+0.12}_{-0.09}$ |
$M\ast$ | [0.001,100] | $0.62^{+0.41}_{-0.16}$ | $M_{\odot}$
$b$ | [0,2] | $0.63^{+0.08}_{-0.05}$ |
log $\epsilon$ | [-2.3, -0.4] | $-0.60^{+0.13}_{-0.22}$ |
log $R_{ph,0}$ | [-4,4] | $0.43\pm 0.07$ |
$l_{ph}$ | [0,4] | $0.63^{+0.09}_{-0.08}$ |
log $T_{\nu}$ | [-3,3] | $-0.20^{+1.01}_{-1.30}$ | d
$t_{0}$ | [-500,0] | $-177^{+34}_{-31}$ | d
log $N_{H,host}$ | [19,23] | $18.79^{+1.01}_{-1.42}$ | cm-2
log $\sigma$ | [-4,2] | $-0.76^{+0.03}_{-0.04}$ |

###  3.6 Spectroscopic Analysis

From the HST UV spectrum, given in Fig. 9, we are able to determine the
redshift of J221951. The UV spectrum shows the cutoff caused by the Lyman
limit absorption and, exceptionally, also the higher level Lyman lines, up to
Ly$-11$, giving a redshift of $z=0.5205\pm 0.0003$. We use the higher level
Lyman lines for the redshift determination since they are cleaner (not
saturated or blended with other lines) than Ly-$\alpha$ and Ly-$\beta$. A
P-Cygni profile is observed in the Ly-$\alpha$ line core, indicative of an
outflow. Ly-$\beta$ is blended with the absorption edge of the nearby O VI
resonance doublet. A fit with Voigt profiles of the Lyman lines from
Ly-$\alpha$ up to Ly$-11$ shows that the Lyman lines are broadened by
velocities of $\approx 90~{}{\rm km\,s^{-1}}$ with natural line broadening
from interstellar hydrogen in the host galaxy $N_{HI}$$\approx 1\times
10^{18}\,{\rm cm^{-2}}$. For the fit an additional emission at a level of
${\rm 2.510^{-17}erg\,cm^{-2}\,s^{-1}\,\AA^{-1}}$ has been adopted and
velocity broadening for the Lyman continuum of $1800\,{\rm km\,s^{-1}}$.

Figure 9: The HST COS spectrum between 900-1360Å. The blue line is a Voigt
profile fit to all the Lyman lines for $N_{HI}$=$1\times 10^{18}$ cm-2 and a
line-broadening velocity of 90 km s-1 The bottom figures show details. An
underlying free-free emission component has been adopted. Left: the Lyman
series can be seen up to quite high numbers. Right: The Ly-$\alpha$ absorption
is all confined to the blue wing, the red wing shows weak emission. A blue
shifted absorption from Ly-$\alpha$ at higher velocity of 650 km s-1 seems to
be present blue-ward of the [O V] 1213.8 Å emission line. There are similar
high-velocity features in Ly-$\beta$ and Ly-$\gamma$ Figure 10: SALT and LCO-
Magellan-Baade optical spectra of J221951. The spectra have been smoothed
using a 1D Box filter kernel, with a kernel width of 5 for the SALT spectra,
and a kernel width of 10 for the LCO-Magellan-Baade spectrum. The LCO-
Magellan-Baade spectrum is markedly different in shape compared to the SALT
spectra. The $B$ band flux from the acquisition image matches well, suggesting
the flux calibration at the $B$ band is correct, however, we cannot rule out
the shape difference as being due to calibration issues because we lack
photometric measurements at multiple wavelengths taken at the same time. The
grey bands show the telluric bands. The darker grey bands absorb more strongly
than the light grey band. The SALT spectra have a telluric correction applied.

Figure 11: Time evolution, in MJD, of the Mg II (left) and H$\beta$ (right)
line profiles.

We also see absorption lines of S VI at 933.4, 944.5 Å, N V at 1238.8, 1242.8
Å and O VI at 1031.9, 1037.6 Å. The red wings of the 1242.8 Å and 1037.6 Å
lines match and we see extended absorption in the blue, consistent with a
velocity of $-1800~{}{\rm km\,s^{-1}}$ in the absorption of O VI and
$-1750~{}{\rm km\,s^{-1}}$ in the N V lines, with an accuracy of about
$50~{}{\rm km\,s^{-1}}$. The absorption in these high ionisation lines extends
to much larger velocities than we see in Ly-$\alpha$. Notably, the emission on
the red part of the lines is small, suggesting no simple spherical geometry
for the emitting region.

In addition to the Ly-$\alpha$ core P-Cygni profile which extends over only a
$400~{}{\rm km\,s^{-1}}$/1.6 Å range, a broader emission feature, which is
likely also Ly-$\alpha$, surrounds that profile, best seen in Fig. 13. The
emission extends from $-2000$ to $2000~{}{\rm km\,s^{-1}}$ in the form of many
peaks, or a broad emission that is cut through by many absorption lines, see
Fig. 9.

Apart from the previously mentioned lines, we find absorption lines of N I
964.1 Å, C III 977.0 Å. An emission feature at 1073 Å might be a blend of Mg I
1073.5 Å with Si IV 1072.96 Å; increasing noise makes further identifications
in the COS spectrum unreliable. Together these lines indicate various stages
of ionisation. Emission features are seen, but cannot be identified as being
either from host or Galaxy in origin. A prominent absorption line at a rest
wavelength of 940.6 Å cannot be identified.

The optical spectra, except for the X-shooter spectrum, are presented in Fig.
10. The continuum changes colour with time, in general becoming bluer.
However, the Magellan spectrum observed on 2019 November 11 is considerably
bluer than all other spectra but coincides with a minimum in the UV light
curves. We checked if there were any issues with the acquisition or reduction
of this spectrum by overlaying the blue band flux measurement derived from the
$B$ band magnitude ($B=20.0\pm 0.1$ mag) of the IMACS acquisition images,
taken immediately beforehand. The $B$ band flux from the acquisition image
matches well, suggesting the flux calibration at the $B$ band is correct,
however, we cannot rule out the shape difference as being due to calibration
issues because we lack photometric measurements at multiple wavelengths taken
at the same time.

The evolution of the Mg II 2800 Å and H$\beta$ line profiles is shown in Fig.
11. Mg II is observed to be initially double peaked, with the peaks narrowing
over time and moving closer together, while the H$\beta$ shows a reduction in
the emission of its red wing with time. A possible explanation for the double
peaked Mg II is emission from a bipolar source or an accretion or debris disc.
However, this does not explain why the H$\beta$ profile is different to that
of Mg II. An alternative possibility is that it is a broad Mg II emission line
with central Mg II absorption from the interstellar medium. If we examine Mg
II and H$\beta$ at the time of the HST observation, we see the width is
consistent with the width of the broad Ly$\alpha$ profile observed in the HST
spectrum. This suggests that all three are broad emission lines with narrow
absorption from the interstellar medium superimposed.

At the location of [O III] 5007 Å line, the spectra are affected by telluric
absorption. Since this [O III] line is important in identifying the presence
of an AGN, we used a SALT spectrum of Feige 110, observed at a similar airmass
as the SALT spectra of J221951, to derive a telluric correction. In the
telluric corrected spectra of J221951, we do not see evidence for strong [O
III] 5007 Å emission. Using the 58760 MJD spectrum, we measure a $3\sigma$
upper limit of $4.3\times 10^{-17}{\rm erg}\,{\rm cm^{-2}}\,{\rm s^{-1}}$ on
the flux of the [O III] 5007 Å emission line. Dividing this by the continuum
flux $3.1\times 10^{-17}{\rm erg}\,{\rm cm^{-2}}\,{\rm s^{-1}}{\rm\AA^{-1}}$
at 4861 Å, following the method of Boroson & Green (1992), we find an
equivalent width for the [O III] 5007 Å emission line of $<1.4$ Å.

The X-shooter spectrum, taken at a much later time, is given in Fig. 12. The
spectrum shows a broad H$\alpha$ emission line, though the S/N is low because
it falls at the edge of the VIS arm of the spectrograph. We also detect
emission from Mg II 2800 Å as shown also in Fig. 11. A red excess is observed
in the X-shooter spectrum. This is consistent with a second blackbody
component observed in the SEDs, see §3.2.

We compare the SALT spectrum of J221951 taken at 58760 MJD and the X-shooter
spectrum taken at 59411 MJD with a sample of spectra from other objects,
including QSOs, known TDEs and ANTs in Fig. 13 and Fig. 14. Examining the UV
spectra of J221951, the broad emission features resemble those found in the
spectra of three TDEs, which were also shown to have broad absorption
features: ASASSN-14li, iPTF15af, iPTF16fnl, and the low ionization broad
absorption line QSO (BALQSO). The most significant absorptions in the J221951
spectrum, N V and O VI, are observed in the optical spectrum of the ANTs
ASASSN-15lh and ASASSN-17jz, however, the absorptions in the spectrum of
J221951 are much broader. When examining the optical spectra of J221951, it
most closely resembles the optical spectrum of the ANT, ASASSN-18jd.

Figure 12: X-shooter spectrum taken on 59411 MJD. The spectrum has been binned
using a $\rm 10\,\AA$ median filter (light blue) and then smoothed with a
Savitsky-Golay filter (dark blue). ${\rm H}\alpha$ 6565 $\rm\AA$ and Mg II
2800 $\rm\AA$ emission lines have been labeled. The spectrum turns up at the
IR end, consistent with a second thermal component observed in the SEDs, e.g
Fig. 5.

###  3.7 Host Galaxy Properties

We built a host SED using archival observations from GALEX, DES, VISTA and
WISE. We apply the stellar population synthesis models in Prospector (Leja et
al., 2017, 2018) to the archival photometry, demonstrating that it is that of
an underlying galaxy. We derive key physical parameters of the galaxy, which
include the stellar mass, metallicity, current star-formation rate and the
widths of five equal-mass bins for the star-formation history, and three
parameters controlling the dust fraction and reprocessing (see Leja et al.,
2017, for details). Leja et al. (2017) identify important degeneracies between
age–metallicity–dust, and the dust mass–dust attenuation curve. Prospector is
specifically designed to account for such degeneracies in parameter estimation
using Markov Chain Monte Carlo analysis to fully explore the posterior
probability density. The best-fitting model is shown compared to the archival
photometry in Fig. 15 and the two-dimensional posteriors are plotted in
Supplementary Figure S.2. We find a stellar mass $\log({\rm M_{\ast}}/{\rm
M_{\odot}})=10.8\pm 0.1$ and a metallicity below or marginally consistent with
solar, $\log\,Z/Z_{\odot}=-1.24^{+0.56}_{-0.47}$. Prospector fits to TDE hosts
have also favoured low metallicities (Ramsden et al., 2022; Hammerstein et
al., 2023). This may be expected since these galaxies tend to be below the
mass of the Milky Way. We also find a low specific star-formation rate, log
$sSFR=-12.0\pm 1\,{\rm yr^{-1}}$ in the last 50 Myr, where the reported values
and uncertainties are the median and 16th/84th percentiles of the marginalized
posterior distributions. The host galaxy SED is consistent with no AGN
contribution, with the fraction of bolometric luminosity from an AGN,
$f_{AGN}<0.06$. This is consistent with the WISE W1-W2 colour of 0.20 mag,
which suggests the archival IR emission is galaxy-like and not dominated by an
AGN (Stern et al., 2012; Yan et al., 2013). Overall, the Prospector fit
suggests that the host galaxy is consistent with a recently quenched galaxy
with high star formation rate between 200-700 Myr ago and no strong AGN
activity.

We compute the BH mass using the BH mass - bulge mass relation,
$M_{BH}-M_{bulge}$ (Kormendy & Ho, 2013; McConnell & Ma, 2013; Ramsden et al.,
2022). We are unable to decompose the host galaxy light into bulge and disc
components since the host is too faint. We estimate the bulge mass from the
total mass of the galaxy using the average bulge to total light (B/T) ratio,
for a $\log\,(M_{\ast}/M_{\odot})\sim 10.8$ the ratio is $\sim 0.67$ (Stone et
al., 2018), indicating that a large fraction of the mass of the galaxy is
within the bulge. This gives $\log\,(M_{bulge}/M_{\odot})\sim 10.6$. We first
use the relationship derived from TDE host galaxies in Ramsden et al. (2022)
and then compare this to the value produced by Kormendy & Ho (2013), which has
been calibrated mainly at BH masses greater than those able to produce a TDE.
Using the relationship derived using TDE host galaxies (Ramsden et al., 2022),
we obtain an expected value of the SMBH mass of
$\log\,(M_{BH}/M_{\odot})=6.9$. This value is consistent with the mass derived
from the TDE model in §3.5. This mass is at the top end of the SMBH masses
associated with TDEs (Wevers et al., 2017, 2019b; Nicholl et al., 2022) and
within the theoretically expected mass range for TDEs (Kochanek, 2016). If we
use the Kormendy & Ho (2013) ${\rm M_{BH}}-{\rm M_{bulge}}$ relation the value
of the SMBH mass we obtain a higher expected value of
$\log\,(M_{BH}/M_{\odot})=8.2$, such a value is larger than the Hills mass for
a $1\,{\rm M_{\odot}}$ star (Hills, 1975)131313The Hills mass is the largest
BH mass for a given stellar mass that will result in a TDE and not swallow the
star whole without disruption (Hills, 1975). For a star of $0.1-1\,{\rm
M_{\odot}}$, the Hills mass of a Schwarzschild BH is $10^{7}-10^{8}\,{\rm
M_{\odot}}$., suggesting that if this is the correct BH mass, either the mass
of the disrupted star is larger than 1 M⊙, contrary to the modelling in §3.5,
or that J221951 is not a TDE as the star should have been swallowed whole
without disruption.

Figure 13: Comparison of the HST COS FUV spectrum of J221951, taken on 58977
MJD, with FUV spectra of other transient objects: ASASSN-14li (Holoien et al.,
2016b), ASASSN-15lh (Brown et al., 2016), iPTF15af (Blagorodnova et al.,
2019), iPTF16fnl (Brown et al., 2018), ASASSN-18jd (Neustadt et al., 2020),
BALQSO low and high ionizaions (LI and HI; Brotherton et al., 2001). J221951
most closely resembles the UV spectra of the TDEs: ASASSN-14li, iPTF15af,
iPTF16fnl and the low ionization BALQSO, which all have broad absorption
features. Figure 14: Comparison of the optical spectra of J221951 (SALT
spectrum taken on 58760 MJD and X-shooter taken on 59411 MJD) with optical
spectra of other transient objects: QSO (Vanden Berk et al., 2001),
ASASSN-14li (Holoien et al., 2016b), iPTF15af (Blagorodnova et al., 2019),
ASASSN-15lh (Brown et al., 2016), iPTF16fnl (Blagorodnova et al., 2017),
ASASSN-17jz (Holoien et al., 2021), ASASSN-18jd (Neustadt et al., 2020). In
terms of continuum shape and spectral lines, J221951 most closely resembles
the optical spectra of the ANT ASASSN-18jd.

##  4 Discussion

With an absolute magnitude of $M_{u,AB}=-23$ mag, peak bolometric luminosity
$L_{max}=(1.1\pm 0.7)\times 10^{45}\,{\rm erg}\,{\rm s}^{-1}$ and total
radiated energy of $E>2.6\times 10^{52}\,{\rm erg}$, J221951 is one of the
brightest, most energetic and long-lived UV transients observed to date. Based
on some of the basic properties of this event, we are immediately able to draw
conclusions on the nature of J221951 and exclude some classes of transient.
The long-duration of the light curve and lack of an X-ray counterpart tends to
rule out fast-evolving transients, including on-axis Gamma-ray bursts and most
supernovae (SNe), except the long duration SN type IIn, which interact
strongly with the circumstellar medium (e.g. Schlegel, 1990; Smith, 2017).

The broad absorption seen in N V and O VI, and the lack of undulations due to
singly or doubly ionised metals are not expected in SNe (Baron et al., 2000;
Foley & Kirshner, 2013; Yan et al., 2017). The total radiated energy of
J221951 of $>2.6\times 10^{52}$ erg is also in tension with SN IIn theory
(Sukhbold & Woosley, 2016) and even the brightest confirmed interacting
superluminous supernovae (SLSNe; Nicholl et al., 2020a), although there is a
population of nuclear transients with similar total radiated energies that
have some properties consistent with supernovae (e.g. PS1-10adi; Kankare et
al., 2017, see §4.2 for further discussion). J221951 has a constant
temperature at $\sim 2.3\times 10^{4}\,{\rm K}$. Supernovae have been shown to
have similar initial temperatures, however SNe cool below $10^{4}\,{\rm K}$
within a few weeks (Holoien et al., 2019a). The WISE W2 peak luminosity ($\sim
10^{44}\,{\rm ergs^{-1}}$) is also of order a factor 100 brighter than the
brightest known SN in the IR (see Fig. 8 of Jiang et al., 2019). The large
bolometric luminosity suggests an association with a black hole, belonging to
one of two main classes of events that also display broad absorption line
features: TDEs or AGN. In the following, we investigate these two main classes
of event and then discuss the origin of the second, lower-temperature
blackbody component.

Figure 15: Archival photometry of the host galaxy of J221951, and SED fit
using Prospector. The best-fitting model, as well as the 1$\sigma$ dispersion
in model realizations, is shown. The inset shows the derived star-formation
history, which peaks at 10 $M_{\odot}\,{\rm yr^{-1}}$ between a lookback time
of 0.1 and 1 Gyr, and has a steep drop in the last $\sim 0.5$ Gyr. The shaded
areas give the $1\sigma$ uncertainty on their respective parameter.

###  4.1 Tidal disruption events

J221951 is positionally coincident with the nucleus of its host galaxy and is
therefore consistent with a TDE origin. In TDEs, approximately half of the
disrupted material falls back onto the SMBH, likely forming an accretion disc,
while the other half is unbound (Rees, 1988; Lacy, Townes & Hollenbach, 1982).
The spectra of TDEs are typically blue and thermal in nature (e.g. van Velzen
et al., 2020). J221951 is consistent with this picture with the SEDs well fit
by a blackbody with a typical temperature of $T\sim 23000$ K. With H$\alpha$
and weak H$\beta$ being observed in the optical spectra, J221951 would be
classified as a H-only TDE in terms of the van Velzen et al. (2021) spectral
classification scheme for optical TDEs. In addition, the overall light curve
evolution can be fit reasonably well with a TDE model. Comparing the model
parameters of J221951 to a sample of TDEs (Nicholl et al., 2022) suggests that
for J221951 the BH mass is at the high end, the star mass is typical, and the
low impact parameter is most similar to that of a H-only TDE rather than a He
or Bowen TDE, consistent with the spectral classification obtained from the
spectra. The radio luminosity of J221951 is consistent with radio-quiet TDEs
(Alexander et al., 2020). If J221951 does contain a jetted synchrotron source,
we are unlikely to be viewing it on-axis.

We can use the SMBH mass estimates and the peak luminosity to determine the
Eddington ratio. Using the lower mass estimate for the SMBH, based on the
assumption that J221951 is a TDE ($\log\,(M/M_{\odot})\sim 7.1$; see §3.5 &
3.7), we derive an Eddington luminosity of $L_{Edd}=1.6\times 10^{45}\,{\rm
erg}\,{\rm s}^{-1}$, and an Eddington ratio of $L_{max}/L_{Edd}=0.70$. This is
consistent with the typical Eddington ratios measured for a sample of TDEs
with well-constrained SMBH masses, for which the peak luminosities are $\sim
L_{Edd}$ (Wevers et al., 2019b). The $L_{max}/L_{Edd}$ value computed for
J221951 is likely to be an underestimate of the true ratio since we may not
have observed the event at peak brightness and therefore the peak luminosity
was probably larger and closer to $L_{Edd}$. However, if we use the larger
black hole estimate of $\log\,(M_{BH}/M_{\odot})\sim 8.2$ determined from the
Kormendy & Ho (2013) BH mass – bulge mass relation (see §3.7), which
challenges the formation of a TDE in the first place, then this increases
$L_{Edd}=2\times 10^{46}\,{\rm erg}\,{\rm s}^{-1}$ and reduces
$L_{max}/L_{Edd}=0.06$.

For a (minimum) total energy of $E\sim 3\times 10^{52}$ erg, the accreted mass
needed to radiate this energy is $\sim 0.14{\rm M_{\odot}}$ for a typical
efficiency $\epsilon=0.1$. This is much larger than the accreted mass
estimates of most TDEs e.g. $<0.01{\rm M_{\odot}}$ (Holoien et al., 2016a,
2019a; Hung et al., 2021) and is consistent with radiatively efficient
accretion of the bound stellar debris (van Velzen et al., 2019b).

The peak luminosity of J221951 of $L_{max}=1.1\times 10^{45}{\rm erg\,s}^{-1}$
is larger than the bulk population of TDEs and is on par with the luminosity
and energy of ASASSN-15lh, ASASSN-17jz and ASASSN-18jd (Dong et al., 2016;
Holoien et al., 2021; Neustadt et al., 2020). The light curve shows bumps,
which are not typical of TDE light curves (Neustadt et al., 2020), but are
also seen in ASASSN-15lh (Brown et al., 2016) and ASASSN-18jd. TDEs tend to
show smooth monotonic declining behaviour, though some do show variability and
moderate rebrightening episodes (e.g. AT 2018fyk; Wevers et al., 2019a).
ASASSN-15lh, ASASSN-17jz and ASASSN-18jd are ANTs (Holoien et al., 2021) and
so their nature is also uncertain and under debate. Neustadt et al. (2020)
discuss ASASSN-18jd as being either due to a TDE or as a rapid turn-on AGN.
Holoien et al. (2021) suggest ASASSN-17jz was a SN IIn occurring in or near
the disc of an existing AGN, and that the late-time emission is caused by the
AGN transitioning to a more active state. For ASASSN-15lh, the literature is
more extensive. Initially, ASASSN-15lh was deemed to be a hydrogen poor
superluminous supernova (SLSN-I; Dong et al., 2016), but with an absolute peak
magnitude more than 1 magnitude brighter than typical SLSNe-I. While some
studies agreed with the SLSNe interpretation (Godoy-Rivera et al., 2017; Brown
et al., 2016), there is a larger consensus that the properties of the UV light
curve, spectra and the host galaxy together implies ASASSN-15lh is more
consistent with a TDE origin (e.g. Brown et al., 2016; Leloudas et al., 2016;
Margutti et al., 2017b; Krühler et al., 2018). Recently, an even more luminous
ANT has been discovered, AT2021lwx (Subrayan et al., 2023; Wiseman et al.,
2023). It is located at a redshift of 0.995, even higher than J221951. As with
the other events discussed in this paragraph, there is debate over whether
AT2021lwx is a TDE or some other accretion event around a SMBH, particularly
so in this case, because the light curve fit with MOSFIT requires the unlikely
disruption of a $\sim 14\,{\rm M_{\odot}}$ star by a $10^{8}\,{\rm M_{\odot}}$
SMBH (Subrayan et al., 2023; Wiseman et al., 2023). This stellar mass is
unusually large, whereas the MOSFIT parameters derived for J221951 are more
consistent with likely TDE configurations.

The host galaxies of J221951, ASASSN-15lh ASASSN-17jz and ASASSN-18jd are also
similar. For J221951 we determine the host mass to be
$\log\,(M/M_{\odot})=10.8\pm 0.1$ and the specific star formation rate
$\log\,sSFR=-12\pm 1\,{\rm yr^{-1}}$. For ASASSN-15lj, Leloudas et al. (2016)
showed the host to be a massive red galaxy with a small rate of ongoing star
formation with host mass $\log\,(M/M_{\odot})=10.95^{+0.15}_{-0.11}$, a star
formation rate $SFR<0.02\,{\rm M_{\odot}\,yr^{-1}}$ and a specific star
formation rate of $\log\,sSFR<\,-12.5\,{\rm yr^{-1}}$. For ASASSN-18jd the
host mass is $\log\,(M/M_{\odot})=11.23^{+0.03}_{-0.33}$ and a star formation
rate of $SFR=0.6^{+0.1}_{-0.3}\,{\rm M_{\odot}\,yr^{-1}}$ (Neustadt et al.,
2020). For ASASSN-17jz the host mass is
$\log\,(M/M_{\odot})=10.74^{+0.11}_{-0.14}$, age $=2.2^{+1.2}_{-1.0}\,{\rm
Gyr}$, and a star formation rate of $SFR=2.9^{+0.4}_{-0.5}\,{\rm
M_{\odot}\,yr^{-1}}$ (Holoien et al., 2021). The black hole mass at the centre
of the host galaxy for ASASSN-15lh has been estimated from galactic scaling
relationships to be $\sim 10^{9}\,{\rm M_{\odot}}$ (Leloudas et al., 2016;
Krühler et al., 2018) and is consistent with the value derived from TDE light
curve model fits (Mummery & Balbus, 2020). For ASASSN-18jd, the black hole
mass is $\log\,(M_{BH}/M_{\odot})=7.6\pm 0.4$ (Neustadt et al., 2020) and for
ASASSN-17jd the black hole mass is $\log\,(M_{BH}/M_{\odot})\sim 7.5$ (Holoien
et al., 2021). For J221951 we estimate a lower value of
$\log\,(M_{BH}/M_{\odot})\sim 7.1$ and an upper value of
$\log\,(M_{BH}/M_{\odot})\sim 8.2$ depending on the scaling relation. For
ASASSN-15lh, the black hole mass is larger than the Hills mass and the same is
true for the upper value of the black hole mass for J221951. For a black hole,
bigger than this Hills mass, the star should have been swallowed whole without
disruption. For ASASSN-15lh, to overcome this in order to allow the tidal
disruption to occur, it has been suggested that the black hole is a rapidly
spinning Kerr SMBH (Leloudas et al., 2016; Krühler et al., 2018; Mummery &
Balbus, 2020). In this case, the Hills mass increases by approximately an
order of magnitude for extreme Kerr spins (Kesden, 2012). There is some
evidence to suggest that TDEs fade more slowly as the SMBH mass increases
(Blagorodnova et al., 2017; Wevers et al., 2017; van Velzen et al., 2019b).
Considering J221951 as a TDE, it would be consistent with this picture since
the black hole mass of J221951 is at the high end of the TDE black hole mass
distribution (Nicholl et al., 2022) and the decay rate is shallower than
typical TDE light curves (see Fig. 7).

X-ray-selected TDEs have harder spectra with $\alpha_{OX}\sim 1.5$ and
optically selected TDEs are softer with $\alpha_{OX}\sim 2.4$. In TDEs, high
(soft) values of $\alpha_{OX}$, associated with high Eddington ratios, are
thought to arise from disc dominated spectra, while low (hard) values of
$\alpha_{OX}$, associated with low Eddington ratios, indicate power-law
spectra (Wevers, 2020). The latter may be more consistent with a jet rather
than a disc. For J221951, $\alpha_{OX}$ is $>1.6$ upon initial detection. This
excludes the hardest spectra, allowing for either a thermal spectrum or a
mixture of power-law and thermal components.

Examining the light curve behaviour of J221951, the light curve decays
approximately as a broken power-law, with a change to a steeper decay after
200 days. Around this time, the bolometric light curve behaviour of J221951
appears to have a similar decay rate to that of ASASSN-15lh after its second
light curve peak. Mummery & Balbus (2020) and Leloudas et al. (2016) showed
that this part of the light curve of ASASSN-15lh, at T+100 days, is consistent
with being disc-dominated. Usually, the transition from fallback-dominated to
disc-dominated emission is expected as a flattening of the TDE light curves
and has been observed for a number of optical/UV TDEs ($>$few hundred days;
van Velzen et al., 2019b). However, Mummery & Balbus (2020) showed that disc-
dominated light curves may not be flat and may actually decay. If ASASSN-15lh
and J221951 are similar in origin, this suggests that the late-time behaviour
of J221951 may also be disc dominated. However, Mummery & Balbus (2020) note
that their model is unable to reproduce the very latest UV emission of
ASASSN-15lh, which is above their model prediction. Instead, they suggest that
the late emission in ASASSN-15lh may be due to AGN activity (Krühler et al.,
2018), additional material from the initial disruption returning to the disc,
or even a state transition within the disc at low Eddington ratio (Mummery &
Balbus, 2020).

Comparing the UV spectrum of J221951 with other objects in Fig. 13, we see
that J221951 is different to ASASSN-18jd and ASASSN-15lh. ASASSN-18jd shows
weak N V emission at 1238.8, 1242.8 Å while we see it in absorption for
J221951. ASASSN-15lh does not show any broad absorption features (Brown et
al., 2016), though it does show N V and O VI absorptions, which are broad in
the spectrum of J221951, but narrow for ASASSN-15lh. In terms of the broad
absorption lines, J221951 most closely resembles TDEs ASASSN-14li (Cenko et
al., 2016) and iPTFi6fnl (Brown et al., 2018) and that of the low-ionisation
BALQSO. In general, the UV spectra of the other objects do not typically
extend bluer than 1000-1100 Å, however, the UV spectrum of ASASSN-15lh (Brown
et al., 2016) does cover a similar wavelength range to that of J221951. In
both spectra, absorption from the Lyman series is observed although the
features are more evident for J221951.

Comparing the optical spectra of J221951 with other objects in Fig. 14, we see
that J221951 lacks the narrow line features that are present in the composite
QSO spectrum. In contrast to the UV comparison J221951 looks least like
ASASSN-14li and iPTF16fnl. No absorption features are present in the optical
spectrum of J221951, while they are present in that of iPTF16fnl and the
emission features in ASASSN-14li are much stronger than J221951. Overall,
J221951 most closely resembles ASASSN-18jd in the optical, though has less
prominent hydrogen Balmer lines. The emission lines of J221951 with $FWHM\sim
2200\,{\rm km\,s^{-1}}$ are much narrower than typically observed for TDEs
($\sim 10^{4}\,{\rm km\,s^{-1}}$), but are consistent with that observed for
ASASSN-18jd (Neustadt et al., 2020).

Parkinson et al. (2020) investigated why some TDEs show broad absorption lines
(BALs) while others display broad emission lines (BELs) in their UV spectra,
using synthetic UV spectra for disc and wind-hosting TDEs, produced by a
state-of-the-art Monte Carlo ionization and radiative transfer code. Using a
variety of disc wind geometries and kinematics they naturally reproduce both
BALs and BELs with winds. Sight lines looking into the wind cone, at low
angles relative to the plane of the disc, preferentially produce BALs, while
other orientations preferentially produce BELs. Clumpy winds may also be a
factor as clumping increases both the emission measure and the abundance of
the relevant ionic species. Clumpier winds tend to produce stronger UV
emission and absorption lines. This model suggests that we are viewing this
event at high inclinations, towards the plane of a disc, if J221951 is indeed
a TDE with a wind. In the scheme of Charalampopoulos et al. (2022), which
assumes a reprocessing scenario whereby the optical emission of the TDE is
produced by reprocessed X-rays, at high inclinations TDEs would only be H-only
and lack X-ray emission, which is consistent with that observed for J221951.
This inclination would also suggest we may not observe the relativistic jet,
if present, which is also consistent with the lack of strong X-ray and radio
emission.

Using the properties of the spectrum we can form a picture of this event if it
is indeed a TDE. The Ly-$\alpha$ profile shows two broad emission peaks on
either side in addition to a saturated central absorption (see Fig. 13). The
absorption is likely to be located in our line of sight far from the ionising
source. The emission peaks would be produced through recombination of hydrogen
much closer into the core and may be from a rotating disc-like object
(Sanbuichi, Fukue & Kojima, 1994). The wavelength separation of each of the
broad peaks of H Ly$\alpha$ from the centre suggests a velocity of $\approx
2000~{}{\rm km\,s^{-1}}$. We can determine the location of the emitting
material that is causing the broad emission, specifically in the Hydrogen
lines. Assuming it is orbiting the SMBH in a circular orbit, such that the
kinetic energy equals the gravitational potential energy, then using the SMBH
mass estimates from §3.7 and the velocity of the broad H Ly$\alpha$ emission,
the radius would be $R\sim 5\times 10^{16}-1\times 10^{18}$ cm (0.017 - 0.36
pc).

The Ly-$\beta$ absorption profile blends with the blue absorption trough of
the O VI 1031.9 Å line. The O VI 1037.6 Å line shows a blue-shifted absorption
profile. The extent of the absorption to the blue of Ly-$\beta$ is much
smaller than the velocities derived from the blue wing of the absorption in O
VI of $-1800~{}{\rm km\,s^{-1}}$, consistent with the lower ionisation lines
being from a different component. The N V lines have a velocity edge
consistent with a $1750~{}{\rm km\,s^{-1}}$ outflow, so both are formed in the
same component.

Examination of Fig. 11 shows that over time the red wing of H$\beta$ becomes
fainter and eventually the profile appears as an asymmetric blue-peaking hump.
The He I 5017 Å line is present but weak and no He II lines were found. The
P-Cygni lines of O VI, N V, etc. in the hotter outflows reach projected
outflow velocities of $1800\,{\rm km\,s^{-1}}$, but there is also a cooler
outflow or turbulence of $\approx 90\,{\rm km\,s^{-1}}$ as seen in Hydrogen
absorption lines. The hotter outflow is likely produced in the inner region,
within a fast outflow, while the velocities in the low-ionisation lines are
located further out. Ly-$\alpha$ is optically thick enough to show an outflow.
There, the higher Lyman lines are optically thin, at least from the
Ly-$\gamma$ on, which suggest a H column density of $\sim 10^{18}$ cm-2.
Entrained in the hot outflow is ionised H, which recombines, giving the broad
emission seen in Ly$\alpha$. The slow decay of the emission may suggest on-
going accretion, while the evolution of the width of the Mg II resonance line
emission suggests decreasing densities reduce opacity in the line wings and a
possible expansion of the disc-like structure. A possible model (Fig. 16) can
be envisioned based on the data taking the central source to be a SMBH with a
disc of material that is flared, a wind outflow and with the observer looking
at a high inclination, close to the plane of the disc, consistent with the
scheme of Charalampopoulos et al. (2022). Over time the disc expands outward,
becoming cooler on the outside, whilst maintaining the inner disc radius.

Overall, the peak luminosity of J221951 is higher than the bulk TDE
population, consistent with the luminosity of ANTs: ASASSN-15lh ASASSN-17jz
and ASASSN-18jd, suggesting J221951 is not a standard TDE. However, TDE light
curve modelling suggests that J221951 is a TDE, with a typical star mass but
with a BH mass at the high end. The spectra and modelling suggest it is a
H-only TDE. J221951 has spectral properties consistent with ASASSN-18jd, in
the optical, and BAL TDEs in the UV. The broad emission line features and lack
of X-ray and radio emission may be because we are observing at a high
inclination, close to the plane of the disc.

Figure 16: Artist impression of the geometry of the source. We likely see
closer to the equatorial plane since we do not see the X-rays from the jet.
The centre is a SMBH, the disc surface has wind that causes the blue
absorptions in resonance lines, and near the equatorial plane, the disc is
cooler and provides the narrow absorption lines seen in the low ionisation
lines. Over time the disc expands and accretion decreases.

###  4.2 Active Galactic Nucleus

Given the potential nuclear nature of J221951, we now look at whether an AGN
may be the cause of this transient. The optical spectra of J221951 show a
double-peaked Mg II 2800 Å profile. Mg II 2800 Å is an emission line common to
AGN, but not usually TDEs. ASASSN-18jd, suspected to be either a TDE or a
rapid turn-on AGN, also has Mg II in emission (Neustadt et al., 2020). The
narrow line widths, observed for J221951, of order $2000\,{\rm km\,s^{-1}}$,
are also commonly observed in AGN spectra. In addition, the broad absorption
in the UV N V and O VI resonance lines, suggestive of outflows, also mimic the
behaviour seen in C IV 1550 Å in the rest-frame UV of BALQSOs. However, the
lack of a strong [O III] 5007 Å emission line, with an EW of $<1$ Å, is not
typical of AGN (Shen et al., 2011). Out of 105000 QSOs in the SDSS DR7
catalogue, 532 have a bolometric luminosity less than $L_{bol}=10^{45}\,{\rm
erg\,s^{-1}}$ and of these none has an EW $\lesssim 3$ Å (Shen et al., 2011);
their average EW is 57 Å. The UV to X-ray spectral slope, for J221951 is
$\alpha_{OX}>1.6$, this is not consistent with QSOs but is consistent with
BALQSOs; for QSOs $\alpha_{OX}\sim 1.4$ (see e.g. Marconi et al., 2004); for
X-ray BALQSOs, $\alpha_{OX}\sim 1.90$; and for optical BALQSOs
$\alpha_{OX}\sim 2.20$ (Blustin et al., 2008).

Based on an argument used for ASASSN-18jd, outlined by Neustadt et al. (2020)
using the SDSS survey of quasars (MacLeod et al., 2012), it is unlikely that
the variability of J221951 is due to normal QSO variability. J221951 is 3.4
magnitudes brighter than the archival $g$-band value (which is also likely a
lower limit due to host contamination); the probability of achieving a
$|\Delta m_{g}|>3.5$ mag on a timescale of $<5$ years is $P<2\times 10^{-6}$
(MacLeod et al., 2012). Graham et al. (2017) use CRTS data to provide even
tighter constraints on the likelihood of observing large-magnitude changes.
For $\Delta m=3.0$ mag, they find that the probability of achieving this
change after 3200 days is $10^{-7}$, which would be even larger for shorter
time lags. These probabilities make it unlikely that the variability of
J221951 is due to normal QSO variability.

However, just because such a large flare is unlikely, it is not impossible,
and observations of AGN have discovered new and more extreme forms of
variability, indicating that we are yet to discover the full range in AGN
variability. A class of slow-blue transients, with $\Delta m>1.5\,{\rm mag}$
over $\sim$years, were identified in Lawrence et al. (2016) and a similar
population was discovered by Graham et al. (2017). Lawrence et al. (2016)
state that around 1 AGN in $10^{4}$ displays such behaviour at any given time.
The origin of these large magnitude changes is unknown and may be due to rare
eruptive events from accretion, or microlensing (Lawrence et al., 2016).
Although some instead may be attributed to stellar-related activity, such as
TDEs, SLSNe and mergers of binary black holes (Graham et al., 2017).

While J221951 does share some characteristics of AGN, the lack of narrow line
features, together with no AGN required in the host SED fitting and the WISE
$W1-W2$ colour, we can conclude that the host galaxy of J221951 did not host a
strong AGN prior to the transient. This suggests that there has been no recent
AGN activity prior to it turning on; the narrow line region (NLR) has not been
ionized. We can estimate the distance from the SMBH to the NLR and thus
estimate the minimum time that the AGN must have been inactive for us to
observe no [O III] 5007Å emission. Using the same argument, we can also
estimate when we would expect to see the [O III] 5007Å emission line in the
spectrum if the AGN has newly turned on. Baskin & Laor (2005) state that for a
single zone model, the distance to the $R_{NLR}=40L_{44}^{0.45}\,{\rm pc}$
where $L_{44}$ is $\nu L_{\nu}/10^{44}\,{\rm erg\,s^{-1}}$ at 4861 Å. From the
first optical spectrum of J221951, we derive
$\nu\,L(4861\,{\rm\AA})=1.61\times 10^{44}\,{\rm erg\,s^{-1}}$. This results
in a distance to the NLR, $R_{NLR}$ of 49.5 pc. This implies that the AGN has
not been active for at least 160 years. Therefore, we can conclude that if
J221951 is due to AGN activity, it is ‘turning on’. In this case, we may have
to wait decades before the [O III] 5007 Å emission line is detectable.
Historically, several AGN have been noted to ‘turn-on’ (Frederick et al.,
2019; Yan et al., 2019), whereby galaxies transition from being LINERS to more
active galaxies, such as narrow-line Seyfert 1s or radio-quiet QSOs (Gezari et
al., 2017; Frederick et al., 2019). Some such objects show similar temporal
behaviour to J221951 (e.g. SDSS1115+0544A; Yan et al., 2019).

Overall, J221951 did not host a strong AGN prior to the transient but has
properties consistent with an AGN turning on. The clearest evidence of an AGN
nature would be for J221951 to deviate from its current steadily decaying
behaviour and to show a sustained period of increased flux - not just a bump
or flare. Continuous UV monitoring of this source will therefore be important
for monitoring the late-time behaviour. Deep X-ray observations of this
source, for instance with Chandra or XMM-Netwon, would provide tighter
constraints on the X-ray brightness of the AGN.

###  4.3 Origin of the low-temperature blackbody

One interesting feature of J221951, is that there is evidence of two blackbody
components in at least two of the SEDs. Two blackbody components were also
observed in PS1-10adi (van Velzen et al., 2016; Kankare et al., 2017), PS16dtm
(Jiang et al., 2017; Petrushevska et al., 2023) and most recently AT2021lwx
(Wiseman et al., 2023). PS1-10adi is an AGN-associated transient that may be
produced by a TDE, SNe or AGN activity (Kankare et al., 2017).
Spectroscopically, the transient has features similar to a narrow line Seyfert
1 galaxy and to certain types of supernovae. For PS1-10adi, the blackbody
temperatures (11000 K and 8000 K evolving to 2500 K and 1200 K) are lower than
those observed for J221951. Jiang et al. (2019) argue that the IR excess
observed in PS1-10adi is a dust echo of a TDE in an AGN, with the UV emission
from the TDE heating and sublimating the dust in the AGN torus. PS16dtm is a
TDE in a narrow line Seyfert 1 galaxy (Blanchard et al., 2017). It similarly
displayed a MIR flare that was also interpreted as a dust echo of a TDE in an
AGN (Jiang et al., 2017). MIR flares have also been found in other TDEs (van
Velzen et al., 2016; Onori et al., 2022) and a systematic search of WISE
observations of galaxies discovered over 100 with IR outbursts, thought to be
the dust echoes of transient accretion events of SMBHs (Jiang et al., 2021).
Jiang et al. (2017) noted that PS16dtm seemed to be detected a few days
earlier in the MIR compared to the optical/UV. This may also be the case for
J221951, see Fig. 3, however, it is difficult to confirm this given the large
errors on the WISE data and the lack of optical data points in between the
last DES visit and the UVOT detection. For PS16dtm, Jiang et al. (2017) note
that the blackbody temperature in the MIR decreases with time, to a value
below the sublimation temperature.

For J221951, a two blackbody component best fits only one of our SEDs which
includes photometry redder than the $v$ band. While a model with two
blackbodies can be fitted to the three subsequent SEDs, it does not provide a
better fit for any, although the fits do suggest that the temperature of this
second component is decreasing as was observed for PS16dtm (Jiang et al.,
2017). Assuming the IR excess observed in the 58787 MJD SED and the X-shooter
spectrum of J221951 is also due to UV heating of nearby dust, then with a
temperature of $\sim 2800\pm 400$ K, the dust is consistent with the
sublimation temperature. Using the formula for the sublimation radius given in
Namekata & Umemura (2016), their Eqn. 2, we can calculate the distance of the
inner edge of the dusty torus from J221951. Taking the peak bolometric
luminosity of $L_{bol}=8.91\times 10^{44}\,{\rm erg\,s^{-1}}$, assuming a
grain radius of 0.1$\,\mu{\rm m}$ and a sublimation temperature of $\sim 2000$
K, we compute a sublimation radius of $3\times 10^{17}\,{\rm cm}$
corresponding to 0.09pc or 110 light days. This distance is typical of the
distances expected of the inner edge of an AGN torus (Suganuma et al., 2006)
and suggests that any preexisting dust within this radius will have been
evaporated by UV emission from J221951.

Another TDE candidate observed to have low temperature blackbody component is
Arp 299-B AT1, which was discovered in the galaxy merger Arp 299 and is
associated with an AGN (Mattila et al., 2018). For Arp 299-B AT1, the
temperature remains constant at $800\,$K beyond 2000 days after the transient
was first observed to rise. At late times ($T_{0}>+800$), the flux of J221951
is comparable to the host value in the reddest filters making it difficult to
constrain the temperature of the second blackbody component. Observations with
JWST would be important in enabling us to measure the IR flux and determine
how the temperature of this second blackbody component evolves with time.

##  5 Conclusion

J221951 was discovered during the follow-up of a gravitational event:
S190930t. It brightened by $>3$ magnitudes in the UV compared to archival data
and coincides with the centre of an optical/IR archival source, previously
observed by DES and VISTA, which we show to be an underlying galaxy. Our
spectroscopic redshift of 0.5205 rules out its association with the
gravitational wave event. However, J221951 is a very unusual and long-lived
UV-luminous nuclear transient. In this paper, we presented our follow-up of
this transient and investigated its nature, whether it is a supernova, tidal
disruption event or related to AGN activity. Below we summarise our key
findings:

  * •

A HST UV spectrum determines a redshift 0.5205 and reveals broad absorption
lines from ionised species such as NV and OVI, along with narrow, low-
ionisation lines of H and N I.

  * •

J221951 has been observed at regular cadence for $\sim 1000$ days and
continues to be detected in the UV, making it one of the longest observed UV
transients with one of the best-sampled UV light curves.

  * •

In the optical/UV the light curve decays from the start of observations.
Several bumps that are more pronounced in the UV are present and appear to
reset the brightness level, such that the light curve resumes its decay from
close to the peak of the bump.

  * •

A supernova explosion is ruled out by a total radiated energy of $\gtrsim
3\times 10^{52}\,{\rm erg}$, as well as the lack of broad absorption lines in
the optical spectrum

  * •

Coincident Swift/XRT observations, do not detect X-ray emission from J221951,
providing an upper limit to the X-ray luminosity of $L_{X}<6\times
10^{42}\,{\rm erg\,s^{-1}}$ (0.3-10 keV). Radio observations by ACTA also do
not detect any radio emission with 3 $\sigma$ upper limits of 117 $\mu$Jy at
5.5 GHz and 90 $\mu$Jy at 9 GHz, with a 5.5GHz luminosity of $<2\times
10^{39}\,{\rm erg\,s^{-1}}$.

  * •

The optical spectra are blue and relatively featureless, displaying only
H$\beta$ and Mg II in emission.

  * •

Spectral energy distributions, created from UVOT data only (with filters
$uvw2$ through to $v$) for which we have the most epochs, are well fit by a
power-law with a slope of $\beta=0.49\pm 0.04$ or a blackbody with an average
temperature of $23000\pm 410\,{\rm K}$. In SEDs constructed using UVOT and
ground-based photometry, a two blackbody model is preferred in one SED, with
evidence for two blackbody components observed in at least one other SED. The
temperature of the second component is $\sim 2800\pm 400$K, which potentially
cools across later SEDs.

  * •

Examining the archival photometry, we determine the host galaxy to be a
massive red galaxy, with a host galaxy stellar mass
$\log\,(M/M_{\odot})=10.8\pm 0.1$ and a low specific star formation rate
$\log\,sSFR=-12\pm 1\,{\rm yr^{-1}}$ in the last 50 Myr.

  * •

From the host SED fitting and the WISE $W1-W2$ colour, we can conclude that
the host galaxy of J221951 did not host a strong AGN prior to the transient.

  * •

Using the Kormendy & Ho (2013) black hole mass – bulge mass scaling relation
we estimate that the mass of the BH is $\log\,(M_{BH}/M_{\odot})\sim 8.2$,
bigger than this Hills mass, which implies for a $\sim 0.6{\rm M_{\odot}}$
star it should have been swallowed whole without disruption and no emission
should have been observed. One solution to this may be that the black hole is
a rapidly spinning Kerr SMBH (Leloudas et al., 2016; Krühler et al., 2018;
Mummery & Balbus, 2020). However, using the black hole mass – bulge mass
scaling relation derived from TDE host galaxies in Ramsden et al. (2022), we
estimate a BH mass of $\log\,(M_{BH}/M_{\odot})=6.9$, which is consistent with
the value derived from the TDE light curve model fits,
$\log\,(M_{BH}/M_{\odot})\sim 7.1$.

  * •

The probability of seeing such a large flare from normal AGN activity is
$P<2\times 10^{-6}$, characterising this as one of the most extreme nuclear
flares to date.

  * •

If due to AGN activity, the lack of narrow emission lines together with the
host fitting and the WISE colour, implies it is caused by the AGN ‘turning
on’.

The progenitor of J221951 is unclear. The optical and UV spectra show features
resembling both TDEs and AGN. Overall its spectral, temporal and host
properties and its energetics are closest in nature to ASASSN-15lh and
ASASSN-18jd. ASASSN-15lh, ASASSN-18jd and J221951 belong to an increasing
population of luminous blue transients, dubbed ambiguous nuclear transients
for which the progenitors are not well constrained, but may be TDEs or due to
AGN activity. Observing the late time evolution of J221951 will provide
important clues as to its nature. For instance, if this source is associated
with an AGN turning on we may expect it to deviate from its current steadily
decaying behaviour. The clearest evidence of an AGN nature would be for
J221951 to show a sustained period where it increased in flux - not just a
bump or flare. Deep X-ray observations of this source, for instance with
Chandra or XMM-Newton, would provide tight constraints on the X-ray
brightness, a late-time continued detection would be indicative of an AGN and
disfavour a TDE origin. Observations with JWST would be important in enabling
us to understand the nature of the second lower-temperature blackbody
component, potentially due to a dusty torus, and how it evolves with time.

The increase in the number of ambiguous nuclear transients, such as J221951,
is blurring the boundary between what is considered TDE and AGN activity.
Sources such as J221951 are important to pinpoint SMBHs that are otherwise
hidden and provide the means to study SMBHs across various degrees of
activity.

##  6 Acknowledgments

This research has made use of data obtained from the High Energy Astrophysics
Science Archive Research Center (HEASARC) and the Leicester Database and
Archive Service (LEDAS), provided by NASA’s Goddard Space Flight Center and
the School of Physics and Astronomy, University of Leicester, UK,
respectively. This publication makes use of data products from the Wide-field
Infrared Survey Explorer, which is a joint project of the University of
California, Los Angeles, and the Jet Propulsion Laboratory/California
Institute of Technology, funded by the National Aeronautics and Space
Administration. This research has made use of the VizieR catalogue access
tool, CDS, Strasbourg, France (DOI : 10.26093/cds/vizier). The original
description of the VizieR service was published in 2000, A&AS 143, 23. This
research has made use of the NASA/IPAC Infrared Science Archive, which is
funded by the National Aeronautics and Space Administration and operated by
the California Institute of Technology. This paper includes data gathered with
the 6.5 meter Magellan Telescopes located at Las Campanas Observatory, Chile.
Some of the observations reported in this paper were obtained with the
Southern African Large Telescope (SALT). This research is based on
observations made with the NASA/ESA Hubble Space Telescope obtained from the
Space Telescope Science Institute, which is operated by the Association of
Universities for Research in Astronomy, Inc., under NASA contract NAS 5–26555.
These observations are associated with program #16076. The Australia Telescope
Compact Array is part of the Australia Telescope National Facility
(https://ror.org/05qajvd42) which is funded by the Australian Government for
operation as a National Facility managed by CSIRO.We acknowledge the Gomeroi
people as the traditional owners of the ATCA observatory site. This research
uses services or data provided by the Astro Data Lab at NSF’s National
Optical-Infrared Astronomy Research Laboratory. NOIRLab is operated by the
Association of Universities for Research in Astronomy (AURA), Inc. under a
cooperative agreement with the National Science Foundation. This publication
uses the data from the AstroSat mission of the Indian Space Research
Organisation (ISRO), archived at the Indian Space Science Data Centre (ISSDC).
Part of the funding for GROND (both hardware as well as personnel) was
generously granted from the Leibniz-Prize to Prof. G. Hasinger (DFG grant HA
1850/28-1). This research made use of Astropy, a community-developed core
Python package for Astronomy (Astropy Collaboration et al., 2013). AAB, NPMK,
MJP, KLP, PAE, APB and JPO acknowledge funding from the UK Space Agency. MDP
acknowledges support for this work by the Scientific and Technological
Research Council of Turkey (TÜBITAK), Grant No: MFAG-119F073. RG and SBP
acknowledge the financial support of ISRO under AstroSat archival Data
utilization program (DS$\\_$2B-13013(2)/1/2021-Sec.2). RG and SBP are also
thankful to the AstroSat UVIT team for helping with UVIT data analysis. EA,
MGB, SC, GC, AD, PDA, AM and GT acknowledge funding from the Italian Space
Agency, contract ASI/INAF n. I/004/11/4. This work is also partially supported
by a grant from the Italian Ministry of Foreign Affairs and International
Cooperation Nr. MAE0065741. PDA acknowledges support from PRIN-MIUR 2017
(grant 20179ZF5KS). DBM is supported by research grant 19054 from Villum
Fonden. 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 a Fellowship from the Alan Turing Institute. MG
is supported by the EU Horizon 2020 research and innovation programme under
grant agreement No 101004719. ET acknowledges support from the European
Research Council (ERC) under the European Union’s Horizon 2020 research and
innovation programme (grant agreement 101002761). DBM is supported by the
European Research Council (ERC) under the European Union’s Horizon 2020
research and innovation programme (grant agreement No. 725246). The Cosmic
Dawn Center is supported by the Danish National Research Foundation.

##  7 Data Availability

The Swift data underlying this article are available in the Swift archives at
https://www.swift.ac.uk/swift_live/, https://heasarc.gsfc.nasa.gov/cgi-
bin/W3Browse/swift.pl,
https://www.ssdc.asi.it/mmia/index.php?mission=swiftmastr. The photometry of
J221951 is available in the online supplementary material. The ATCA data are
available from the Australia Telescope Online Archive –
https://atoa.atnf.csiro.au/. The HST and GALEX observations are available from
the MAST Portal –
https://mast.stsci.edu/portal/Mashup/Clients/Mast/Portal.html. The X-shooter
spectrum and GROND data are available from the ESO main archive –
http://archive.eso.org/eso/eso_archive_main.html.

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##  8 Affiliations

1 School of Physics and Astronomy & Institute for Gravitational Wave
Astronomy, University of Birmingham, B15 2TT, UK
2 University College London, Mullard Space Science Laboratory, Holmbury St.
Mary, Dorking, RH5 6NT, UK
3 Astrophysics Research Centre, School of Mathematics and Physics, Queens
University Belfast, Belfast BT7 1NN, UK
4 Astrophysics Science Division, NASA Goddard Space Flight Center, Greenbelt,
MD 20771, USA
5 Las Campanas Observatory, Carnegie Observatories, Colina El Pino, Casilla
601, La Serena, Chile
6 South African Astronomical Observatory, PO Box 9, Observatory 7935, Cape
Town, South Africa
7 Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch
7701, South Africa
8 Department of Physics, University of the Free State, PO Box 339,
Bloemfontein 9300, South Africa
9 Joint Space-Science Institute, Computer and Space Sciences Building,
University of Maryland, College Park, MD 20742, USA
10 University of Messina, MIFT Department, Polo Papardo, Viale F.S.
D’Alcontres 31, 98166 Messina, Italy
11 Australia Telescope National Facility, CSIRO Space and Astronomy, PO Box
76, Epping, NSW 1710, Australia
12 Astronomical Observatory, University of Warsaw, Al. Ujazdowskie 4, 00-478
Warszawa, Poland
13 Aryabhatta Research Institute of Observational Sciences (ARIES), Manora
Peak, Nainital-263002, India
14 Department of Physics, Deen Dayal Upadhyaya Gorakhpur University,
Gorakhpur-273009, India
15 Center for Space Science and Technology, University of Maryland Baltimore
County, 1000 Hilltop Circle, Baltimore, MD 21250, USA.
16 Center for Research and Exploration in Space Science and Technology,
NASA/GSFC, Greenbelt, Maryland 20771, USA
17 Department of Astronomy, University of Wisconsin, 475 N. Charter Str.,
Madison, WI 53704, USA
18 INAF–Osservatorio di Padova, vicolo dell Osservatorio 5, I-35122 Padova,
Italy
19 School of Physics and Astronomy, University of Leicester, LE1 7RH, UK
20 Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstraße 1,
85748 Garching, Germany
21 Department of Physics, University of Bath, Bath, BA2 7AY, UK
22 Australia Telescope National Facility, CSIRO Space and Astronomy, 1828
Yarrie Lake Road, Narrabri, NSW 2390, Australia
23 George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and
Astronomy, Mitchell Physics Building,
Texas A.&M. University, Department of Physics and Astronomy, College Station,
TX 77843, USA
24 Department of Astronomy and Astrophysics, The Pennsylvania State
University, University Park, PA 16802, USA
25 Institute for Gravitation and the Cosmos, The Pennsylvania State
University, University Park, PA 16802, USA
26 Department of Astronomy and Astrophysics, University of Toronto, Toronto,
ON, Canada
27 INAF – IASF Palermo, Via Ugo La Malfa 153, I-90146, Palermo, Italy
28 INAF – Osservatorio Astronomico di Brera, Via Bianchi 46, I-23807 Merate,
Italy
29 INAF-Osservatorio Astronomico di Roma, Via Frascati 33, I-00040 Monteporzio
Catone, Italy
30 University of Padova, Italy
31 Space Science Data Center (SSDC) - Agenzia Spaziale Italiana (ASI), I-00133
Roma, Italy
32 Department of Physics and Astronomy, Clemson University, Kinard Lab of
Physics, Clemson, SC 29634-0978, USA
33 National Science Foundation, Alexandria, VA 22314, USA
34 Department of Astrophysics/IMAPP, Radboud University, 6525 AJ Nijmegen, The
Netherlands.
35 Cosmic Dawn Center (DAWN), Denmark.
36 Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200
Copenhagen N, Denmark.
37 Los Alamos National Laboratory, B244, Los Alamos, NM, 87545, USA
38 Department of Physics and Mathematics, Aoyama Gakuin University,
Sagamihara, Kanagawa, 252-5258, Japan
39 Department of Physics, University of Rome Tor Vergata, via della Ricerca
Scientifica 1, I-00100 Rome, Italy
40 INAF, Via del Fosso del Cavaliere, 100, 00133, Rome 41 Key Laboratory of
Space Astronomy and Technology, National Astronomical Observatories, Chinese
Academy of Sciences, Beijing, 100101, China