11institutetext:  European Southern Observatory, Karl-Schwarzschild-Strasse 2,
85748 Garching bei München, Germany<EMAIL_ADDRESS>22institutetext:
Millennium Institute of Astrophysics (MAS), Nuncio Monseñor Sotero Sanz 100,
Of. 104, Providencia, Santiago, Chile 33institutetext: Instituto de Física y
Astronomía, Facultad de Ciencias,Universidad de Valparaíso, Gran Bretaña No.
1111, Playa Ancha, Valparaíso, Chile 44institutetext: Millennium Nucleus on
Transversal Research and Technology to Explore Supermassive Black Holes
(TITANS), 4030000 Concepción, Chile 55institutetext: Instituto de Astrofísica,
Facultad de Física, Pontificia Universidad Católica de Chile, Casilla 306,
Santiago 22, Chile 66institutetext: Centro de Astroingeniería, Pontificia
Universidad Católica de Chile, Av. Vicuña Mackenna 4860, 7820436 Macul,
Santiago, Chile 77institutetext: Space Science Institute, 4750 Walnut Street,
Suite 205, Boulder, Colorado 80301 88institutetext: Instituto de Estudios
Astrofísicos, Facultad de Ingeniería y Ciencias, Universidad Diego Portales,
Av. Ejército Libertador 441, Santiago, Chile 99institutetext: Kavli Institute
for Astronomy and Astrophysics, Peking University, Beĳing 100871, China
1010institutetext: Max-Planck-Institut für Extraterrestrische Physik,
Gießenbachstraße, D-85748 Garching, Germany 1111institutetext: California
Institute of Technology, 1200 E. California Blvd, Pasadena, CA 91125, USA
1212institutetext: Gemini Observatory, NSF’s National Optical-Infrared
Astronomy Research Laboratory, Casilla 603, La Serena, Chile
1313institutetext: INAF - Osservatorio Astrofisico di Arcetri, Largo E. Fermi
5, 50125, Firenze, Italy 1414institutetext: Leibniz-Institut für Astrophysik
Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany 1515institutetext:
Departamento de Astronomía, Universidad de Chile, Casilla 36D, Santiago, Chile
1616institutetext: Astronomy Department, Universidad de Concepción, Barrio
Universitario S/N, Concepción 4030000, Chile 1717institutetext: Center for
Mathematical Modeling, University of Chile, AFB170001, Chile
1818institutetext: INAF – Institute for Space Astrophysics and Planetology,
via del Fosso del Cavaliere, 100, I-00133 Roma, Italy 1919institutetext: Data
and Artificial Intelligence Initiative (ID&IA), University of Chile, Santiago,
Chile 2020institutetext: Cahill Center for Astrophysics, California Institute
of Technology, 1216 East California Boulevard, Pasadena, CA 91125, USA

# SDSS1335+0728: The awakening of a $\sim 10^{6}M_{\odot}$ black hole††thanks:
Based on observations collected at the European Southern Observatory under ESO
programme 109.24F5.001. The SOAR/Goodman and Keck/LRIS spectra are available
in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr
(130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/

P. Sánchez-Sáez 112*  —2*  — L. Hernández-García 223*  —3*  — S. Bernal 3344
A. Bayo 1*  —1*  — G. Calistro Rivera 1*  —1*  — F. E. Bauer 5566227*  —7*  —
C. Ricci 889*  —9*  — A. Merloni 1010 M. J. Graham 11*  —11*  — R. Cartier
121288 P. Arévalo 334*  —4*  — R. J. Assef 8*  —8*  — A. Concas 111313 D.
Homan 1414 M. Krumpe 1414 P. Lira 15154*  —4*  — A. Malyali 1010 M.L.
Martínez-Aldama 1616 A. M. Muñoz Arancibia 2217*  —17*  — A. Rau 10*  —10*  —
G. Bruni 18*  —18*  — F. Förster 191922171715*  —15*  — M. Pavez-Herrera 22 D.
Tubín-Arenas M. Brightman 14*  —14*  —20*  —20*  —

###### Abstract

Context. The early-type galaxy SDSS J133519.91+072807.4 (hereafter
SDSS1335+0728), which had exhibited no prior optical variations during the
preceding two decades, began showing significant nuclear variability in the
Zwicky Transient Facility (ZTF) alert stream from December 2019 (as
ZTF19acnskyy). This variability behaviour, coupled with the host-galaxy
properties, suggests that SDSS1335+0728 hosts a $\sim 10^{6}M_{\odot}$ black
hole (BH) that is currently in the process of ‘turning on’.

Aims.  We present a multi-wavelength photometric analysis and spectroscopic
follow-up performed with the aim of better understanding the origin of the
nuclear variations detected in SDSS1335+0728.

Methods. We used archival photometry (from WISE, 2MASS, SDSS, GALEX, eROSITA)
and spectroscopic data (from SDSS and LAMOST) to study the state of
SDSS1335+0728 prior to December 2019, and new observations from _Swift_ ,
SOAR/Goodman, VLT/X-shooter, and Keck/LRIS taken after its turn-on to
characterise its current state. We analysed the variability of SDSS1335+0728
in the X-ray/UV/optical/mid-infrared range, modelled its spectral energy
distribution prior to and after December 2019, and studied the evolution of
its UV/optical spectra.

Results. From our multi-wavelength photometric analysis, we find that: (a)
since 2021, the UV flux (from _Swift_ /UVOT observations) is four times
brighter than the flux reported by GALEX in 2004; (b) since June 2022, the
mid-infrared flux has risen more than two times, and the W1$-$W2 WISE colour
has become redder; and (c) since February 2024, the source has begun showing
X-ray emission. From our spectroscopic follow-up, we see that (i) the narrow
emission line ratios are now consistent with a more energetic ionising
continuum; (ii) broad emission lines are not detected; and (iii) the [OIII]
line increased its flux $\sim 3.6$ years after the first ZTF alert, which
implies a relatively compact narrow-line-emitting region.

Conclusions. We conclude that the variations observed in SDSS1335+0728 could
be either explained by a $\sim 10^{6}M_{\odot}$ AGN that is just turning on or
by an exotic tidal disruption event (TDE). If the former is true,
SDSS1335+0728 is one of the strongest cases of an AGN observed in the process
of activating. If the latter were found to be the case, it would correspond to
the longest and faintest TDE ever observed (or another class of still unknown
nuclear transient). Future observations of SDSS1335+0728 are crucial to
further understand its behaviour.

###### Key Words.:

galaxies: active – accretion, accretion discs – galaxies: individual: SDSS
J133519.91+072807.4

##  1 Introduction

All-sky time-domain surveys, such as the Zwicky Transient Facility (ZTF; Bellm
et al. 2019) and the upcoming Vera C. Rubin Observatory Legacy Survey of Space
and Time (LSST; Ivezić et al. 2019), which are generating alert streams in
real time, are facilitating the identification of nuclear transients in
galaxies hosting supermassive black holes (SMBHs). These events include tidal
disruption events (TDEs); the disruption of a star when it approximates a
black hole (BH) with a mass of $\lesssim 10^{8}M_{\odot}$ (Rees 1988; van
Velzen et al. 2020); active galactic nuclei (AGN) with changing-state events
(CSAGNs, also known as changing-look AGNs); sources that drastically change
their accretion state, which could lead to changes in their classification as
type 1 or type 2 AGN (with or without broad emission lines) within a timescale
of months or years (Ricci & Trakhtenbrot 2023); AGNs with anomalous flaring
activity (e.g. Trakhtenbrot et al. 2019b; Frederick et al. 2021); and the
recently discovered ambiguous nuclear transients (ANTs; e.g. Hinkle et al.
2022 and references therein).

Nuclear transients could also correspond to AGN ignition events in formerly
quiescent galaxies (e.g. Arévalo et al. 2024). The duty cycle, meaning the
time an AGN is active, is estimated to range between 104 and 107 years (for
classical AGNs), and during this period, while a SMBH is switching on and off
(i.e., going through episodes of lower and higher activity), the expected
brightness change can be of several orders of magnitude (Hickox et al. 2014).
However, from the Soltan argument (Soltan 1982), we know most of the mass of
the SMBHs we observe has already been obtained through accretion, and hence we
do not expect to see many AGNs today in the process of turning on. However,
intermediate-mass BH (IMBHs) with masses of $10^{4}-10^{5}M_{\odot}$ or low-
mass SMBHs with masses of $10^{5}\sim 10^{6}M_{\odot}$ (Greene et al. 2020)
may still have masses close to those of their seeds (with masses of
$M\thickapprox 10^{2}\>M_{\odot}$ for light seeds, and masses of
$M\thickapprox 10^{3}-10^{5}M_{\odot}$ for heavy seeds) and, if so, they must
not have gone through many duty cycles and their AGN activity may be
incipient. Accordingly, we would expect to find IMBHs and low-mass SMBHs today
that are just starting to become active, and that will eventually grow to
become SMBHs.

Deciphering whether a nuclear transient corresponds to a TDE, a CSAGN, a
flaring AGN, or a turning-on AGN event could be challenging, as the number of
known sources presenting these behaviours is still low, and the timescales of
these processes are still not well understood. There are only a few dozen TDEs
known to date (e.g. van Velzen et al. 2020, 2021a; Hammerstein et al. 2023),
and a few hundred confirmed CSAGNs (e.g. MacLeod et al. 2019; Graham et al.
2020; López-Navas et al. 2022, 2023). Moreover, the identification of these
events could be contaminated by circumnuclear supernovae (SNe; e.g. Drake et
al. 2011). Zabludoff et al. (2021) proposed a comprehensive list of
photometric and spectroscopic properties that may separate TDEs from classical
AGNs and flaring AGNs, but these authors also note that no single TDE has been
known to present all of the described features.

In this work, we present the discovery and follow-up of a $\sim
10^{6}M_{\odot}$ BH candidate that started showing optical variations in
December 2019. The candidate was selected from the public ZTF alert stream by
making use of the classifications provided by the ALeRCE (Automatic Learning
for the Rapid Classification of Events) broker (Förster et al. 2021). The
source, SDSS J133519.91+072807.4 (hereafter SDSS1335+0728), presented a strong
jump in its optical emission, and was denoted ZTF object ID ‘ZTF19acnskyy’
from December 2019, but importantly exhibited no evidence of activity during
the preceding two decades. From December 2019 to present, SDSS1335+0728 has
shown stochastic optical variations. Based on our variability analysis and
photometric and spectroscopic follow up, we discuss different scenarios that
could explain the sudden flux jump and subsequent variability, and propose
that an AGN in the process of turning on or a TDE in a $\sim 10^{6}M_{\odot}$
BH could explain the observations. The paper is organised as follows. In
Section 2 we present the identification of the nuclear variations in
SDSS1335+0728. In Section 3 we present the archival and follow-up data used to
study the source. In Section 4, we present the variability, photometric, and
spectroscopic analysis performed to understand the origin of the variations
observed in SDSS1335+0728. Finally, in Section 5, we conclude and summarise
our findings.

##  2 Identification of the nuclear transient ZTF19acnskyy in SDSS1335+0728

The ALeRCE broker is actively processing the public ZTF alert stream (alerts
are generated when a $5\sigma$ detection is obtained in the difference image
of a source), providing light curve and stamp classifications of different
variable and transient objects. The ALeRCE light curve classifier (LCC;
Sánchez-Sáez et al. 2021) uses 152 variability features computed from the ZTF
alert stream, as well as average colours, to classify objects among 15
periodic, stochastic, and transient classes with high accuracy. Among the
stochastic classes, there are three distinct AGN classes: core-dominated,
host-dominated and blazars.

During the development of the AleRCE LCC, we started to test its efficiency in
identifying IMBHs and low-mass SMBH candidates from the ZTF alert stream.
Specifically, we selected all the sources in the NASA-Sloan Atlas (NSA)
catalogue111http://nsatlas.org v1_0_1 (Blanton et al. 2011) with stellar mass
(from K-correction fit for Sersic fluxes) below $10^{10}M_{\odot}$, and
searched in the ALeRCE database for cross-matches to LCC objects with a
classification of AGN (including the three AGN classes), using a radius of
$1\aas@@fstack{\prime\prime}5$. We found 45 candidates, of which 12 were
previously identified as IMBHs or low-mass SMBH candidates from Sloan Digital
Sky Survey (SDSS; York et al. 2000) spectroscopy (Greene & Ho 2004, 2007; Dong
et al. 2012; Chilingarian et al. 2018; Liu et al. 2018). Among the remaining
33 sources, we found a very particular source, ZTF19acnskyy222SDSS1335+0728
was associated later in 2022 with the ZTF source ZTF22abyhaut, also detected
in the nucleus of the galaxy., classified as core-dominated AGN by the LCC,
and coincident with the nucleus of SDSS1335+0728 ($\alpha_{\rm J2000}:$
203.833062°, $\delta_{\rm J2000}:$ 7.468791°, SDSS spectroscopic redshift
0.024, and stellar mass of $\sim 6\times 10^{9}M_{\odot}$ from the NSA
catalogue), which started producing alerts (with stochastic variations) in the
public ZTF stream since December 13th 2019 (MJD: 58830.567). SDSS1335+0728 was
previously observed by ZTF, and it has observations in the ZTF data releases
since March 2018 (MJD: 58198), but did not present $5\sigma$ detections in the
ZTF difference images (and therefore did not produced alerts) until December
2019.

From the stellar mass of SDSS1335+0728 reported in the NSA catalogue, the
predicted BH mass would be $\sim 1.5\times 10^{6}M_{\odot}$, assuming the
scaling relations presented in Equations 4 and 5 of Reines & Volonteri (2015),
for local AGNs. SDSS DR8333https://www.sdss4.org/dr17/spectro/galaxy_mpajhu/
provide stellar velocity dispersions, stellar masses, and star formation rates
(SFRs), obtained from the MPA-JHU catalogue (after the Max Planck Institute
for Astrophysics and the Johns Hopkins University; Kauffmann et al. 2003b;
Brinchmann et al. 2004; Tremonti et al. 2004). For SDSS1335+0728 the reported
stellar velocity dispersion is 74.71 km/s. Using the $M-\sigma$ relation of
McConnell & Ma (2013) for early type galaxies, we derive a BH mass of $\sim
1.5\times 10^{6}M_{\odot}$, consistent with the previous estimated value.
While the reported stellar mass and SFR are $1.3\times 10^{10}M_{\odot}$ and
0.228 $M_{\odot}/\text{yr}$, respectively. From this stellar mass, we can
predict a BH mass of $3.3\times 10^{6}M_{\odot}$. These predicted masses are
right at the limit of what can be considered a low-mass SMBH, and are within
the limits of BH masses expected for TDE hosts (Wevers et al. 2017).

The left panel of Figure 1 shows the difference-flux light curve at the
position of SDSS1335+0728, obtained by using the ZTF forced photometry service
(Masci et al. 2023), in the ZTF $g$ and $r$ bands. This corresponds to the
point-spread-function (PSF)-fit flux measured over all the difference images
of the target, which are constructed by subtracting a reference image from the
single-epoch science images of the source. The figure shows binned light
curves, constructed by measuring the median MJD and fluxes in bins of three
days. For the errors we assumed that they combine as the root-mean-
square444Following the implementation of Lightkurve v2.4. More details in
https://docs.lightkurve.org/index.html.

|
---|---
Figure 1: ZTF light curves of SDSS1335+0728. Left: ZTF forced photometry light
curve at the position of SDSS1335+0728 in the $g$ (green circles) and $r$ (red
stars) bands. The light curves have been binned using a bin of three days. The
black dashed line marks the day when SDSS1335+0728 started triggering alerts
in the ZTF alert stream. The green and red dashed lines show the results of
the power-law decay fit (with their respective uncertainties shown in light
grey). We obtain power-law indices $p$ of $-0.17\pm 0.03$ and $-0.14\pm 0.03$
in the $g$ and $r$ bands, respectively. Right: $g$ (green), and $r$ (red) band
light curves obtained after normalising by the power-law decay fit.

From the left panel of Figure 1, we can see that prior to December 2019, the
flux detected in the difference images was consistent with being non-variable
(difference-flux around zero), since the first observation of the source in
March 2018. In December 2019, SDSS1335+0728 increased its flux, reached its
peak in MJD 58991 (May 2020), and has been slowly decaying since, showing
stochastic variation for more than 1,550 days. The brightest absolute
magnitudes (AB) observed in the difference images were $-16.9$, $-16.8$, and
$-16.4$ in the ZTF $g$, $r$, and $i$ band, respectively.

Considering the temporal behaviour and the properties of SDSS1335+0728, we
hypothesised two possible scenarios for its origin: (a) it corresponds to a
$\sim 10^{6}M_{\odot}$ BH that was either not accreting or was accreting at
very low rates (a low-luminosity AGN; LLAGN; e.g. Ho 2008, 2009) that turned
on (i.e. a BH that increases its accretion rate enough as to form an accretion
disc, potentially for the first time); or (b) it corresponds to a TDE (or
another class of still unknown nuclear transient) in a $\sim 10^{6}M_{\odot}$
BH.

The decay timescale and maximum brightness of ZTF19acnskyy are much slower and
fainter than other known classical SNe events, respectively (e.g. Perley et
al. 2020; Grayling et al. 2023; Chen et al. 2023), and thus we rule out a
classical SN to explain the variations observed in the nucleus of
SDSS1335+0728. However, a potential origin for the behaviour of SDSS1335+0728
would be an exotic circumnuclear SN. iPTF14hls (Arcavi et al. 2017) and
SN2018ibb (Schulze et al. 2024) are two examples of SNe that evolve extremely
slowly compared to normal SNe events, with electromagnetic emission detected
during $\sim 1000$ days, similar to what we observe in ZTF19acnskyy. However,
the absolute magnitudes at the peak for iPTF14hls and SN2018ibb ($\sim-18.2$
and $-21.8$ in the $g$ band, respectively) are much brighter than that
observed for ZTF19acnskyy, and they decayed more than five magnitudes after
$\sim 1000$ days, while ZTF19acnskyy has only faded by $\sim 0.82$ magnitudes
in the $g$ band (in the difference-magnitude light curve), after 1550 days.
Hence, we ruled out the hypothesis of a potential circumnuclear SN to explain
the behaviour at the nucleus of SDSS1335+0728.

In case we confirm SDSS1335+0728 as a turning-on AGN, this source will allow
us to observe an AGN as it becomes active, which will be extremely relevant
for the understanding of the physics behind BH accretion, as well as AGN
formation and evolution. Recently, Arévalo et al. (2024) reported the
discovery of new AGN activity in a former quiescent galaxy. However, the
source was discovered several years after the transition event, and thus, it
was not possible to follow up the source while the broad line region (BLR) was
forming. Their work, though, provides an upper limit for the timescale of the
appearance of broad emission lines (BELs) in AGN ignition events ($\sim$18
years for a $\sim 10^{7}M_{\odot}$ BH).

Transitions from quiescent states to active AGNs have also been observed in
Low Ionization Nuclear Emission Line Regions (LINERs). For instance, Yan et
al. (2019) and Frederick et al. (2019) reported the discovery of LINERs that
transitioned into type 1 AGNs. SDSS1335+0728 is classified as composite (i.e.
a galaxy whose emission lines could be explained by both AGN and/or SF star
formation activity; also known as transition objects) in the MPA-JHU
catalogue. Composite galaxies are at the limit of what can be considered a
pure star-forming galaxy and galaxies with AGN activity (Ho et al. 1993; Veron
et al. 1997). Their origin is still a matter of debate, although it has been
proposed that some of them could host LLAGNs (e.g. Ho 2008). If we confirm the
AGN nature of the variations in SDSS1335+0728, it will correspond to the first
direct observation of a composite galaxy that transitioned into a classical
AGN.

On the other hand, the faintest TDE known up to date corresponds to eRASSt
J074426.3+291606 (Malyali et al. 2023), with an absolute magnitude in the
$g$-band peak of $-16.8$ (very close to the value observed for ZTF19acnskyy),
and an exponential decay timescale of $\sim 120$ days in the optical range.
Optically faint TDEs are normally observed to fade faster than their brighter
counterparts (van Velzen et al. 2020), and thus Malyali et al. (2023) proposed
that eRASSt J074426.3+291606 corresponds to the first example of a potential
new class of ‘faint and slow’ TDEs. ZTF19acnskyy has been showing stable
variations during more than 1550 days, and thus, if we confirm its TDE nature,
this will correspond to the slowest and faintest optical TDE yet observed.

##  3 Data collection for SDSS1335+0728

###  3.1 Archival data

To better understand the origin variations observed in ZTF19acnskyy and the
properties of SDSS1335+0728, we collected single epoch UV/optical/infrared
photometry for SDSS1335+0728 (including the whole galaxy), obtained prior to
December 2019 from the NASA/IPAC Extragalactic Database (NED; Helou et al.
1991), obtaining detections from the Galaxy Evolution Explorer (GALEX; Martin
et al. 2005; Kron flux in elliptical aperture), SDSS (Model flux), the Two
Micron All Sky Survey (2MASS; Skrutskie et al. 2006; extended object
magnitude), and the Wide-field Infrared Survey Explorer (WISE; Wright et al.
2010; Mainzer et al. 2014; profile-fit/extended object magnitude).

To investigate if SDSS1335+0728 presented previous variable activity, we
searched for archival light curves from the following datasets: ZTF $g$, $r$,
and $i$ bands (as mentioned previously); Gaia DR3 (Gaia Collaboration et al.
2023), which includes data from July 2014 to May 2017; the Catalina Real-Time
Transient Survey DR2 (CRTS; Drake et al. 2009), including data from April 2005
to April 2016; the All-Sky Automated Survey for Supernovae (ASASSN; Jayasinghe
et al. 2018, using data from January 2012 to March 2024; and WISE, using
observations from January 2014 to June 2023.

In terms of spectroscopic observations, SDSS1335+0728 was observed prior to
December 2019 by SDSS on February 2nd, 2007 (MJD 54156), using a
$3^{\prime\prime}$ fibre (R=1500 at 3800 $\AA$, R=2500 at 9000 $\AA$). More
recently, the source was observed by the Large Sky Area Multi-Object Fiber
Spectroscopic Telescope (LAMOST; Yan et al. 2022) on May 12th, 2015 (MJD
57155) using a $3\aas@@fstack{\prime\prime}5$ fibre (R=1800 at 5500 $\AA$).

We did not find archival radio or X-ray detections for SDSS1335+0728. The
source remained undetected by the Very Large Array Sky Survey in April 2019
and November 2021 (VLASS, RMS is 0.13 mJy/beam), and in the Rapid ASKAP
Continuum Survey (RACS, RMS is 0.2 mJy/beam) in April 2019. Moreover, it was
not detected by the German eROSITA (extended ROentgen Survey with an Imaging
Telescope Array; Merloni et al. 2012; Predehl et al. 2021) All-Sky Survey
(eRASS), including the five eRASS available observations (MJDs: 58848, 59030,
59214, 59399, and 59583) and the stacked images of the first four eRASSes.

###  3.2 Follow-up data

In mid-2021, we started a follow-up campaign to further understand the origin
of the variation in the nucleus of SDSS1335+0728. In July 2021 (MJD 59426),
July 2022 (MJD 59775), July 2023 (MJD 60139), and February 2024 (MJD 60344),
we requested target of opportunity (ToO) observations of SDSS1335+0728 with
_Swift_ (Burrows et al. 2005), observing the source with exposure times of 3,
4.6, 1.1, and 2 ksec, respectively. The data reduction was performed following
standard routines as described by the UK _Swift_ Science Data Centre (UKSSDC),
using the software in HEASoft version 6.30. In addition, a _Chandra_
(Weisskopf et al. 2000) Director’s Discretionary Time (DDT) observation was
obtained in April 2024 (MJD 60403). The data reduction was performed using
CIAO v4.16 and CALDB v4.11.0.

Moreover, we performed a spectroscopic follow-up of SDSS1335+0728. First, in
June 2021 (MJD 59372) and July 2021 (MJD 59396), we obtained optical spectra
with the Goodman spectrograph at SOAR (Clemens et al. 2004), using the Red
Camera with the 400M2 (total exposure time of 1800 seconds; wavelength range
$5100\AA-8950\AA$) and 400M1 (total exposure time of 3000 seconds; wavelength
range $3300\AA-7030\AA$) configurations, respectively. In both observations,
we used the $1^{\prime\prime}$ slit, with $2\times 2$ binning (R$\sim 1270$).
The spectra were reduced using our own custom IRAF routines. Since the source
did not vary drastically in the optical range between MJD 59372 and MJD 59396,
we combined both observations, to improve the wavelength coverage. The signal-
to-noise per resolution element below $3800\AA$ was $\sim 2$, and thus we cut
the spectrum at this wavelength.

In July 2022 (MJD 59789) we obtained 1.2 hours of DDT (programme ID
109.24F5.001) to observe SDSS1335+0728 with VLT/X-shooter (Vernet et al.
2011), using the following configuration: a) UBV arm with
0$\aas@@fstack{\prime\prime}$5 slit (R=9861), $1\times 1$ binning, and total
exposure time of 1500 seconds, b) VIS arm with 0$\aas@@fstack{\prime\prime}$4
slit (R=18340), $1\times 1$ binning, and total exposure time of 1050 seconds,
and c) NIR arm with 0$\aas@@fstack{\prime\prime}$4 slit (R=11424), $1\times 1$
binning, and total exposure time of 1500 seconds. The observations were done
using an OSSO pattern, with a fixed RA offset of $0^{\prime\prime}$ and a
Fixed DEC offset of $20^{\prime\prime}$. The data were reduced using the
standard ESO pipelines (Modigliani et al. 2010; version 3.6.3.1) within the
Reflex (Freudling et al. 2013) front
end.555https://ftp.eso.org/pub/dfs/pipelines/instruments/X-shooter/xshoo-
reflex-tutorial-3.6.3.pdf Unfortunately, the NIR observation had an overall
signal-to-noise of 4.9 and was dominated by telluric lines, and thus we did
not use the NIR arm in this work. For the case of the UVB arm, the signal-to-
noise at wavelengths shorter than 3800 $\AA$ was $\lesssim 5$, and therefore,
we cut this arm at 3800 $\AA$. In the VIS arm, we cut the spectrum at 5500
$\AA$ (where the signal-to-noise was $\lesssim 3$).

In July 2023 (MJD 60137), we obtained observations with the Low Resolution
Imaging Spectrometer (LRIS) at Keck I (Oke et al. 1995), with the 600/4000
Blue grism ($2\times 2$ binning, R=3495), and the 600/7500 Red grating
($2\times 1$ binning, R=8835). We obtained two observations, 300 second
exposures each, with 1$\aas@@fstack{\prime\prime}$5 and
0$\aas@@fstack{\prime\prime}$7 slits, respectively. The two adopted slit
widths will later allow us to compare aperture loss issues. The data were
reduced using the standard Keck lpipe
pipeline.666https://sites.astro.caltech.edu/~dperley/programs/lris/manual.html.

Finally, in January 2024 (MJD 60337), we obtained a new spectrum with the
Goodman spectrograph using the Red Camera with 400M1 (total exposure time of
3000 seconds). As for the 2021 observation, the spectrum was reduced using
IRAF. A summary of all the available spectroscopic observations is presented
in Table 1.

In the following sections, we use all the available photometric and
spectroscopic data to disentangle the nature of the variations currently
observed in SDSS1335+0728.

Table 1: Spectroscopic observations of SDSS1335+0728. Instrument | MJD | Aperture | Seeing | Resolution (R)
---|---|---|---|---
SDSS | 54156 | 3′′ | 1$\aas@@fstack{\prime\prime}$64 | 1500 at 3800 $\AA$, 2500 at 9000 $\AA$
LAMOST | 57155 | 3$\aas@@fstack{\prime\prime}$5 | 4′′ | 1800 at 5500 $\AA$
SOAR/Goodman | 59396 | 1′′ | 1$\aas@@fstack{\prime\prime}$5 | $\sim 1270$
VLT/X-shooter | 59789 | 0$\aas@@fstack{\prime\prime}$4 (0$\aas@@fstack{\prime\prime}$5 UBV) | 0$\aas@@fstack{\prime\prime}$7 | 9861 in the UBV arm and 18340 in the VIS arm
Keck/LRIS | 60137 | 0$\aas@@fstack{\prime\prime}$7 and 1$\aas@@fstack{\prime\prime}$5 | 0$\aas@@fstack{\prime\prime}$45 | 3495 in the Blue grism and 8835 in the Red grating
SOAR/Goodman | 60337 | 1′′ | 1$\aas@@fstack{\prime\prime}$3 | $\sim 1270$

##  4 Understanding the nature of the variations in the nucleus of
SDSS1335+0728

###  4.1 Optical variability

We used the ZTF optical light curves to explore the potential for ZTF19acnskyy
to be a TDE in the nucleus of SDSS1335+0728. We fitted the ZTF $g$ and $r$
difference light curves shown in the left panel of Figure 1 using the decaying
power-law model presented in Equation
3777$L_{\text{TDE}}(t)\propto[(t-t_{\text{peak}}+t_{0})]^{p}$ for
$t>t_{\text{peak}}$. of van Velzen et al. (2021a), which has been successfully
used to fit TDE light curves. TDEs are predicted to have light curves with a
smooth power-law decay with $p$ index of $-5/3$ (Rees 1988; Phinney 1989),
although some deviations from this value have been observed ($-3.82\lesssim
p\lesssim-0.78$; Hammerstein et al. 2023). For the case of ZTF19acnskyy, we
obtain much flatter power-law indices $p$ of $-0.17\pm 0.03$ and $-0.14\pm
0.03$ in the ZTF $g$ and $r$ bands respectively. These fits are shown in the
left panel of Figure 1 (dashed lines).

The right panel of Figure 1 shows the ZTF difference light curves, but
normalising them by the power-law decay fit obtained for each band. From this
figure we can see that the variations of ZTF19acnskyy differ from the smooth
power-law decay expected for TDEs, showing stochastic variations similar to
those observed in classical AGNs. The variations observed in both bands are
consistent, although the $r$-band light curve shows more scatter due to its
lower signal-to-noise. The excess variance
$\sigma^{2}_{\text{rms}}$888$\sigma^{2}_{\text{rms}}=(\sigma_{LC}^{2}-\overline{\sigma}_{x}^{2})/\overline{x}^{2}$,
with $\sigma_{LC}$ being the standard deviation of the light curve,
$\overline{\sigma}_{x}$ the average photometric error, and $\overline{x}$ the
average magnitude or flux (Sánchez et al. 2017 and references therein). of the
normalised flux light curve in the $g$ band is 0.017, while for the original
difference-flux light curve (using epochs after MJD 58830) is 0.039. Both
values are in agreement with values measured for classical AGNs (Simm et al.
2016).

Figure 2 shows the ZTF Forced Photometry total magnitude light curve of
SDSS1335+0728, obtained by correcting the difference-flux by the total flux
measured on its respective reference image. It can be seen that the optical
flux increased in all the ZTF bands in December 2019, with the largest
variation being observed in the ZTF $g$ band. After this increase in
luminosity, the source has shown stochastic variations (according to the LCC
top-level classification) for more than 1550 days ($\sim 4.2$ years), but its
flux has declined by a factor of $\sim 2$ since its peak.

Figure 2: Total magnitude light curves of SDSS1335+0728 in the ZTF $g$
(green), $r$ (red), and $i$ (yellow) bands, CRTS (blue), WISE W1 (gold), and
W2 (light green) bands, and in the ASAS-SN V (pink) and $g$ (brown) bands. The
black circles show the binned light curves constructed using bins of 180 days.
The black dashed line marks the day when ZTF19acnskyy started triggering
alerts in the ZTF alert stream. The red dashed lines mark the dates with
available spectra, and the grey dashed-dotted lines show the dates with
available _Swift_ /UVOT observations (more details on Section 3).

To investigate if SDSS1335+0728 presented previous optical variability, we
used the archival CRTS and ASSASN light curves. These light curves are shown
in Figure 2. We did not use the Gaia DR3 data since the source is classified
as non-variable, and thus its time series is not currently available. These
optical light curves cover almost 19 years of data, from April 2005 until
March 2024, but have low signal-to-noise ratios, and therefore we binned them
to have a better idea of any possible variations. We used the same procedure
described in Section 2. By visual inspection, we see that the source did not
vary in the optical range before December 2019. The ASASSN_V band light curve
shows modest evidence of an increase in the flux since December 2019,
especially after binning the light curve, although this light curve remains
quite noisy.

To constrain quantitatively the variability properties of the archival optical
light curves, following Sánchez et al. (2017), we calculated the probability
that the source is intrinsically variable ($P_{\text{var}}$; see Paolillo et
al. 2004 and references therein), and the excess variance
$\sigma^{2}_{\text{rms}}$ (in magnitudes), and classified as variable the
light curves showing $P_{\text{var}}>0.95$ and
$\sigma^{2}_{\text{rms}}-err(\sigma^{2}_{rms})>0$. For this, we used the
binned archival light curves shown in Figure 2. None of the CRTS and ASSASN
light curves is classified as variable according to the criteria, with all
having $P_{\text{var}}\lesssim 0.5$ and
$\sigma^{2}_{\text{rms}}-err(\sigma^{2}_{rms})<0$. On the other hand, all the
ZTF light curves are classified as variable. Consequently, we can confirm that
SDSS1335+0728 has not varied in the optical range in the last $\sim 2$ decades
until it started showing variations in December 2019.

From the optical variability, we conclude that the turning-on AGN scenario is
plausible, while the TDE scenario is less probable, especially considering the
slow temporal evolution of ZTF19acnskyy. For the case of a potential TDE in an
AGN, simulations still predict a smooth power-law decay (e.g. Chan et al.
2020), and available observations show that the timescales of these events are
similar to those of classical TDEs (e.g. Merloni et al. 2015; Ricci et al.
2020; Homan et al. 2023). However, given the current declining trend of the
optical light curve, it is still too soon to rule out a transient event.
Future observations from ZTF and LSST will be key to further understanding the
variability observed in SDSS1335+0728.

###  4.2 Infrared variability

To study the mid-infrared evolution of SDSS1335+0728, we used the archival
WISE light curve, which is shown in Figure 2. The W1 and W2 light curves show
a slow increase in their flux since $\sim$ June 2022 (i.e. $\sim 900$ days
since the first ZTF alert), which is more evident in the W2 band, where the
flux increased more than two times in $\sim$one year (between June 2022 and
June 2023).

As for the optical light curves (see Section 4.1), we binned the WISE light
curves using the procedure presented in Section 2 and measured the
$P_{\text{var}}$ and $\sigma^{2}_{\text{rms}}$ from them. When using the full
timespan of the WISE light curves, for both bands, we obtain
$P_{\text{var}}<0.95$ and $\sigma^{2}_{\text{rms}}-err(\sigma^{2}_{rms})<0$.
However, these full light curves are dominated by the observations prior to
2022. If we only include observations obtained after the first ZTF alert, the
W2 light curve is classified as variable.

We show in Figure 3 the W1-W2 WISE colour evolution of SDSS1335+0728. From the
figure, we can see that the W1-W2 colour changed from $\sim 0.0$ to $\sim
0.14$ at MJD 59958 (1128 days after the first ZTF alert).

Figure 3: W1-W2 WISE colour evolution for SDSS1335+0728. The red and black
dashed lines show the mean and median W1-W2 colour, respectively.

The delayed response of the MIR emission is much larger than what is expected
from the UV/optical emission reprocessing by a potential preexisting dusty
torus around the SMBH (i.e. infrared dust echo). For classical AGNs, from
previous dust reverberation mapping studies, we know that delays of the order
of 900 days (or 879 days in rest frame for SDSS1335+0728) in the 3.6–4.5
$\mu$m wavelength range are expected for AGNs with bolometric luminosities
larger than $10^{46}$ erg/s (Lyu et al. 2019; Yang et al. 2020). For instance,
by using equation 16 from Lyu et al. (2019), a time delay of 879 days in W2,
implies a bolometric luminosity of $\sim 2.4\times 10^{46}$ erg/s, which is
much brighter than the nuclear emission observed from SDSS1335+0728 (see
Section 4.5). Moreover, TDEs with known MIR variations have shown much shorter
MIR lags (e.g. van Velzen et al. 2016; Jiang et al. 2019; van Velzen et al.
2021b), with typical estimated timescales of a few months, for the
reprocessing of the X-ray/UV/optical signal by the dusty torus.

Evolution towards redder MIR colours has been observed before in CSAGNs (e.g.
Lyu et al. 2022; López-Navas et al. 2023 and references therein) and in the
newborn AGN reported by Arévalo et al. (2024), and it has been associated with
reprocessed emission from the AGN dusty torus. Classical AGNs have values of
W1-W2 larger than 0.6 (Wright et al. 2010; Comparat et al. 2020), while values
lower than 0.4 are normally associated with quiescent or star-forming
galaxies. However, low-luminosity AGNs (LLAGNs) can have W1-W2 colours of
$\sim 0.08$ (Lyu et al. 2022), as the host galaxy dominates their MIR
emission. For the case of SDSS1335+0728, the recent MIR flux and colour
evolution could indicate that a new dusty torus is just being formed in an
LLAGN, or that the dust reprocessing is happening in a much larger dusty
structure than the ones typically observed in classical AGNs or TDEs, with a
size of few light years.

###  4.3 Ultraviolet photometry

GALEX detected SDSS1335+0728 on April 16, 2004 (MJD 53111) in both NUV and FUV
filter bands. Around 17 years later, we started observing the source with
_Swift_ /UVOT in the UVW1, UVW2, UVM2, U, B, and V bands. The uvotsource task
was used to perform aperture photometry on the _Swift_ /UVOT images, using a
circular aperture radius of $5^{\prime\prime}$ centred on the source. We
excluded from the analysis the _Swift_ /UVOT observation from MJD 60344, since
the UVW1, UVW2, and UVM2 images presented tracking issues.

We used the available photometry described in Section 3, obtained prior to and
after the event (excluding the MJD 60344 _Swift_ /UVOT observation), to
construct a spectral energy distribution (SED) of SDSS1335+0728, which is
shown in Figure 4. We show the four _Swift_ /UVOT measurements independently
and complement them with contemporaneous WISE data, except for the MJD 60139
observation, for which there is no contemporaneous data publicly available
(but we show the most recent WISE observation).

Figure 4: Spectral energy distribution of SDSS1335+0728 before December 2019
(pink circles) constructed using data from WISE, 2MASS, SDSS, and GALEX; and
at MJD 59426 (blue stars), MJD 59775 (green squares), MJD 60113 (grey
triangles), and MJD 60139 (orange triangles) constructed using data from WISE
and _Swift_ /UVOT.

From Figure 4, we can see that the UV flux increased by a factor of $\sim$4
between archival GALEX observations (2004) and the first _Swift_ /UVOT
measurement (2021). It is worth remarking that this difference is still robust
even if one takes into account the difference in the filters involved.

The _Swift_ /UVOT measurements show that SDSS1335+0728 has been varying in the
UV since its outburst, with the highest flux being detected in the July 2021
observation and the lowest one in the July 2023 observation. In the UVW2 band
the source varied 0.43 magnitudes between the 2021 and the 2023 observations,
while in the UVM2, the source varied 0.28 magnitudes in the same period. The
recent UV variability exhibits a bluer when brighter behaviour, with observed
_Swift_ colour UVW2-UVM2 of $-0.208$, $-0.047$, and $-0.057$, and UVM2-U of
0.484, 0.527, and 0.554, in July 2021, July 2022, and July 2023, respectively.
In comparison, the $g-r$ optical colour measured from the difference forced
photometry has mean, median, and standard deviation values of $-0.54$,
$-0.52$, and 0.25, respectively. This difference in the behaviour of the UV
and optical colours of SDSS1335+0728 could indicate that a large fraction of
the transient event energy is being released in the UV (presumably near to the
SMBH).

These results support the hypothesis of a turning on BH in the nucleus of
SDSS1335+0728; as a bright UV emission and a bluer when brighter behaviour is
normally observed in accretion discs of classical AGNs, while for the case of
TDEs, a rather constant colour is expected (Zabludoff et al. 2021).

###  4.4 X-ray photometry and spectroscopy

As mentioned in Section 3.1, eROSITA did not detect SDSS1335+0728 in any of
the five available eRASS epochs in the same field of the source. We note that
the first epoch was obtained on December 31, 2019, after the first ZTF alert.
We obtained eROSITA upper limits from the stacked images (Tubín-Arenas et al.
2024), which are calculated using X-ray photometry on the eROSITA standard
calibration data products (counts image, background image, and exposure time),
following the Bayesian approach described by Kraft et al. (1991). We consider
a circular aperture with a radius given by a PSF encircled energy fraction of
EEF = 0.75 ($\sim 30\arcsec$) and computed upper limits with a confidence
interval of 99.87% (this corresponds to a one-sided $3\sigma$ level). To
calculate energy conversion factors (ECFs), we assumed an absorbed power-law
model with spectral index $\Gamma=2$ and Galactic absorption $3\times$1020
cm-2. We computed upper limits for the most sensitive eROSITA band with
$0.2-2.3$ keV, as well as in nominal soft (0.2–0.6 keV), medium (0.6–2.3 keV),
hard ( 2.3–5.0 keV), and total (0.2–5.0 keV) bands, obtaining values of
$3.98\times 10^{-14}$, $3.09\times 10^{-14}$, $3.06\times 10^{-14}$,
$2.16\times 10^{-13}$, and $4.00\times 10^{-14}$ erg s-1 cm-2, respectively.
From this, we estimate a $3\sigma$ upper limit in the luminosity of
SDSS1335+0728 in the $0.2-2.3$ keV energy band of $5.17\times 10^{40}$ erg
s-1.

Additionally, in the first three _Swift_ observations (MJD 59426, 59775, and
60139), the source was not detected by the _Swift_ /XRT instrument. We used
the longest observation (4.6 ksec in MJD 59775) to get an idea of the
potential 0.5-10 keV band flux limit. We used
WebPIMMS999https://heasarc.gsfc.nasa.gov/cgi-bin/Tools/w3pimms/w3pimms.pl and
estimated a count-rate of 1.3$\pm$0.5 $\times$10-3 cts s-1 from a circular
region of 20′′ centred on the source. Then, assuming a power-law model with
spectral index $\Gamma=2$ and Galactic absorption $3\times$1020 cm-2, we
obtain estimated flux limits of $4.76\times$10-14 erg s-1 cm-2 in the $0.5-10$
keV band, and $3.08\times$10-14 erg s-1 cm-2 in the $0.2-2.3$ keV band,
consistent with the upper limits provided by the eROSITA observations. If we
instead use a black body model with kT=100 eV, we obtain estimated flux limits
of $3.11\times$10-14 erg s-1 cm-2 in the $0.5-10$ keV band, and
$7.74\times$10-14 erg s-1 cm-2 in the $0.2-2.3$ keV band.

Notably, on February 4 2024 (1514 days after the first ZTF alert), a source
was detected in the 2 ksec _Swift_ /XRT observation at a distance of
6$\aas@@fstack{\prime\prime}$0$\pm$3$\aas@@fstack{\prime\prime}$6 from the
nucleus of SDSS1335+0728101010This detection was also reported in (Hernandez-
Garcia et al. 2024). To extract the spectrum, we used the swifttools v3.0,
following the prescriptions of Evans et al. (2009). The obtained spectrum
shows very soft emission, which is shown in Figure 5. We modelled the spectrum
with a black body model with a temperature of kT$=104\pm 9$ eV. We obtained a
flux of $7.4\times$10-12 erg s-1 cm-2 in the $0.3-2$ keV band, which is two
orders of magnitude larger than the eROSITA and _Swift_ upper limits for
previous epochs. Using the redshift of the host galaxy, we obtain a luminosity
of $9.62\times$1042 erg s-1 in the $0.3-2$ keV band.

Figure 5: _Swift_ /XRT spectrum of the source detected at a distance of
6$\aas@@fstack{\prime\prime}$0$\pm$3$\aas@@fstack{\prime\prime}$6 from the
nucleus of SDSS1335+0728, obtained on February 4 2024. Top: The blue circles
show the data, and the black dashed line shows the black body model with
kT$=104\pm 9$ eV. Bottom: Residual of the modelling (the difference between
the original spectrum and the fit).

_Swift_ entered in safe mode on March 15 2024, due to the failure of a
gyroscope, and therefore we could not confirm if the X-ray detection was
nuclear or off-nuclear. To confirm the position of the X-ray emission, we
requested a DDT observation with _Chandra_. The source was observed on April 3
2024, during 2 ksec. Figure 6 shows the _Chandra_ image in the $0.3-2$ keV
energy band, highlighting the position of the ZTF alert (ZTF19acnskyy) and the
position of the first _Swift_ /XRT detection. The csmooth task included in
CIAO was used to adaptatively smooth the image, using a fast Fourier transform
algorithm and a minimum and maximum significance level of the signal-to-noise
of 3 and 4, respectively. The image shows a source located at RA: 13:35:19.93
DEC: +7:28:07.48, that is, coincident with the nucleus of SDSS1335+0728 and
the position of ZTF19acnskyy. From this, we confirm that the previous _Swift_
/XRT detection comes from the nucleus of the source. We used the routine
specextract to extract the spectral products, using circles of 2′′ and 10′′
radii for the source and background, respectively. We modelled the spectrum
with a black body model with kT$=80\pm 7$ eV, obtaining a flux of
$9.7\times$10-12 erg s-1 cm-2 in the $0.3-2$ keV band.

Figure 6: _Chandra_ image of SDSS1335+0728 in the $0.3-2$ keV energy band,
obtained on April 3 2024. The blue circle (with a radius of 1′′) shows the ZTF
alert position (ZTF19acnskyy), and the red circle (with a
3$\aas@@fstack{\prime\prime}$6 radius) is centred on the centroid of the first
_Swift_ /XRT detection.

The _Swift_ /XRT spectrum shows a shape similar to that observed in 1ES
1927+654 while its X-ray corona was being reformed (Ricci et al. 2020), which
could indicate that an AGN corona has started to form in the nucleus of
SDSS1335+0728. Moreover, the spectra of X-ray-detected TDEs tend to show soft
black body-like emission and temperatures of kT $\sim 0.04-0.12$ keV
(Zabludoff et al. 2021; Saxton et al. 2021), and thus, the recent _Swift_ /XRT
detection could point to a TDE origin. Delayed X-ray brightening has been
observed in optically detected TDEs, but the observed timescales for these
events are a few hundred days (Guolo et al. 2024; Wevers et al. 2024), thus
much shorter than the ones observed for SDSS1335+0728. We have initiated a
more intensive follow-up campaign with _Swift_ /XRT observations to confirm
which of these hypotheses is correct. The analysis of this campaign, including
a more detailed assessment of the _Chandra_ observation, will be presented in
a forthcoming paper.

###  4.5 Modelling the spectral energy distribution

To better understand the properties of the galaxy SDSS1335+0728 prior to and
after December 2019, we modelled the SED presented in Figure 4 using the
Bayesian Markov chain Monte Carlo based code AGNFITTER (Calistro Rivera et al.
2016). This code models the SED of AGNs by combining multiwavelength models of
the host-galaxy and AGN emission. In particular, four components are included:
AGN accretion disc emission (big blue bump or BBB), the hot dust emission from
the AGN torus, the stellar population of the host-galaxy, and the emission
from the cold dust in galactic star-forming regions. The model returns the
following parameters for the host-galaxy: stellar mass ($M_{\star}$), age,
SFR, star-formation history timescale (SFH $\tau$), galaxy reddening
[$E(B-V)_{\text{gal}}$], among others. For the AGN component, the code returns
the torus and the BBB emission. From this, we can obtain integrated
luminosities for the accretion disc and the torus. We run AGNFITTER using
10,000 walkers. Following Calistro Rivera et al. (2021), we sampled each
parameter from the model using 100 random realisations from the posterior, and
we measured the luminosity of the accretion disc [$L_{\text{BBB}}$(0.05-1)] by
integrating the BBB emission in the wavelength rage $0.05-1\mu$m.

We first used AGNFITTER to model the SED of SDSS1335+0728 prior to December
2019 (preSED-model), using the photometry obtained from WISE, 2MASS, SDSS, and
GALEX. Then, to model the SED after December 2019 (afterSED-model), we
combined the first _Swift_ /UVOT observation obtained in July 2021 (MJD
59426), with the photometry from 2MASS and WISE obtained prior to December
2019, assuming that the source did not vary in the infrared range up to that
date. This allowed us to cover a larger range in wavelength. We assume
throughout that the host-galaxy properties are constant, and fixed the stellar
age, SFH $\tau$, and $E(B-V)_{\text{gal}}$ parameters, using the output of the
preSED-model within $1\sigma$. The most recent WISE observations show a new
MIR component, which could be associated with dust echo (torus reprocessing).
Unfortunately, we do not have access to recent near-infrared (NIR)
observations, and we cannot assume anymore that the source does not vary in
this range. Thus, we cannot properly model an SED that includes this new
component. Therefore, we just modelled the SED for the July 2021 observation.

The results of the SED fitting are shown in Figure 7. The top panel shows the
results obtained for the preSED-model, while the bottom panel shows the
results for the afterSED-model. From the figure, we note that none of the SED
models require a torus component. Interestingly, we also note that most of the
realisations of the preSED-model require a BBB component, although much
fainter than the one required for the afterSED-model; however, we note that
some realisations do not include this component, and instead, a younger
stellar population is required. The obtained BBB components for the preSED-
model and the afterSED-model are more dominant in the UV. According to the
standard thin disc model (Shakura & Sunyaev 1973), accretion discs around BHs
are expected to have maximum effective temperatures proportional to
$(R_{\text{Edd}}/M_{\text{BH}})^{0.25}$ (where $R_{\text{Edd}}$ is the
Eddington rate of the source), and therefore for IMBHS and low-mass SMBHs, the
accretion disc emission should peak in the far/extreme UV, and should
contribute less flux in the optical range, compared to more massive BHs (see
for instance Figure 18 in Zou et al. 2023). This is consistent with what we
observe in the modelled SEDs of SDSS1335+0728.

Figure 7: Modelling of the SED of SDSS1335+0728 obtained using AGNFITTER. The cold gas component is shown in green, the stellar population component in yellow, the AGN accretion disc in blue, the AGN torus in purple (note that no torus component is needed in the model), and the total model in red. Top: SED model obtained using the photometry of SDSS1335+0728 before the ZTF19acnskyy started generating alerts (prior to December 2019) from WISE, 2MASS, SDSS, and GALEX. This model (preSED-model) does not require an AGN torus component, but in most of the realisations it requires a faint AGN accretion disc component. Bottom: SED model obtained using the first _Swift_ /UVOT observation obtained in July 2021 (MJD 59426). For the infrared range we used the photometry from 2MASS and WISE obtained prior to December 2019, which allowed us to cover a larger range in wavelength. We also fixed the stellar age, SFH $\tau$, and $E(B-V)_{\text{gal}}$ parameters, using the results obtained in the preSED-model, letting the parameters of afterSED-model vary within the $1\sigma$ values obtained for preSED-model. This model (afterSED-model) does not require an AGN torus model, while it requires a much brighter AGN accretion disc component.  Table 2: Output from AGNFITTER for SDSS1335+0728 before (preSED-model) and after (afterSED-model) ZTF19acnskyy started producing alerts. The reported values correspond to the median and $2\sigma$ errors. SED model | SFH $\tau$ [Gyr] | age [log yr] | $E(B-V)_{\text{gal}}$ | SFR [$M_{\odot}$/yr] |  $M_{\star}$ [log${}_{10}M_{\odot}$] |  $L_{\text{BBB}}$(0.05-1) [log10 erg s-1]
---|---|---|---|---|---|---
preSED-model | $1.00_{-0.95}^{+0.96}$ | $10.02_{-0.32}^{+0.10}$ | $0.07_{-0.04}^{+0.07}$ | $0.34_{-0.13}^{+0.48}$ | $10.20_{-0.23}^{+0.14}$ | $42.21_{-4.37}^{+0.16}$
afterSED-model | $0.85_{-0.52}^{+0.78}$ | $10.05_{-0.17}^{+0.05}$ | $0.10_{-0.02}^{+0.01}$ | $0.38_{-0.19}^{+0.57}$ | $10.15_{-0.14}^{+0.07}$ | $43.09_{-0.09}^{+0.05}$

Table 2 shows the preSED-model and afterSED-model output parameters, including
SFH $\tau$, age, $E(B-V)_{\text{gal}}$, SFR, the stellar mass ($M_{\star}$),
and $L_{\text{BBB}}$(0.05-1). The table shows the median and the $2\sigma$
errors (computed using the 2.5 and 97.5 percentiles of the different SED
realisations) for each parameter. The obtained stellar masses are in agreement
with the value reported in the MPA-JHU catalogue. The median BBB luminosity in
the preSED-model is $\sim 1.62\times 10^{42}$ erg s-1s around one order of
magnitude smaller than the one obtained for the afterSED-model ($\sim
1.23\times 10^{43}$ erg s-1). However, we note that the measured BBB
luminosity in the preSED-model for a younger stellar population is $7.84\times
10^{37}$ erg s-1 (which corresponds to the percentile 2.5 of the preSED-model
realisations). If we assume that $L_{\text{BBB}}$(0.05-1) is a good
representation of the bolometric luminosity (since the upper limit of the
X-ray emission is much fainter than the accretion disc luminosity), we can see
that the bolometric luminosities measured from both SED models are consistent
with previous observations of LLAGNs, including composite (or transitioning)
galaxies, LINERs, and Seyfert 1s (e.g. Ho 2009).

Assuming the BH masses derived in Section 2, we can estimate the Eddington
rate $R_{\text{Edd}}$ of the source, using the definition presented in Netzer
(2013). By assuming a BH mass of $3.3\times 10^{6}M_{\odot}$, prior to
December 2019 we obtain $R_{\text{Edd}}$ of 0.003 ($1\times 10^{-7}$ if we
assume the lowest BBB luminosity), and after a value of 0.025, while if we
assume a BH mass of $1.5\times 10^{6}M_{\odot}$, we obtain $R_{\text{Edd}}$ of
0.007 ($3\times 10^{-7}$ if we assume the lowest BBB luminosity) and 0.055
before and after December 2019. These $R_{\text{Edd}}$ estimated values are
also consistent with previous observations of LLAGNs (e.g. Ho 2009). Moreover,
the difference among these values is similar to what has been recently
observed for the transition of CSAGNs (Ruan et al. 2019; Graham et al. 2020;
Guolo et al. 2021; Temple et al. 2022; López-Navas et al. 2023), and in the
differences between LINERs and Seyfert 2s (e.g. Hernández-García et al. 2016).

The potential detection of a BBB component prior to December 2019, and the SED
and $R_{\text{Edd}}$ evolution of SDSS1335+0728, support the hypothesis of an
SMBH that was accreting at very low accretion rates and that is currently
turning on (AGN ignition event). However, the BBB luminosity measured in the
afterSED-model is consistent with previous observations of TDEs (Hammerstein
et al. 2023), and thus the TDE scenario cannot be discarded.

###  4.6 Spectroscopic analysis

SDSS1335+0728 was observed prior to December 2019 by SDSS in MJD 54156.
According to the SDSS pipeline classification, the source corresponds to a
star-forming galaxy. From its emission line ratios, the source was classified
as composite in the MPA-JHU catalogue and by Toba et al. (2014). Moreover,
this spectrum has no coronal lines detected in the Coronal Line Activity
Spectroscopic Survey (CLASS; Reefe et al. 2023) catalogue. More recently, the
source was observed by LAMOSTmin MJD 57155. The Reference Catalogue of
Spectral Energy Distributions version 2
(RCSED2111111https://rcsed2.voxastro.org; Chilingarian & Zolotukhin 2012;
Chilingarian et al. 2017) classifies this spectrum as star-forming galaxy.
From these observations, we can conclude that SDSS1335+0728 had either no
nuclear activity or very low AGN activity before it started showing variations
in late 2019.

As mentioned in Section 3.2, in mid-2021 we performed a spectroscopic follow-
up of SDSS1335+0728, observing the source with SOAR/Goodman (MJD 59372 and
60337), VLT/X-shooter (MJD 59789), and Keck/LRIS (MJD 60137).

Six of the seven available spectra are shown in Figure 8 (the LRIS
1$\aas@@fstack{\prime\prime}$5 slit spectrum is not presented, but is similar
to the 0$\aas@@fstack{\prime\prime}$7 slit observation). The top panel shows
the full spectra, while the bottom left and right panels show a zoom around
the Hβ and Hα lines, respectively.

|
---|---

Figure 8: Spectra of SDSS1335+0728. Top: Full spectra observed by SDSS (2007;
blue), LAMOST (2015; red), Goodman (2021 and 2024; yellow and pink), X-shooter
(2022; green), and LRIS (2023, 0$\aas@@fstack{\prime\prime}$7; purple). The
original X-shooter spectrum is shown in grey and the box smooth (with a window
of 10 pixels) binned spectrum is shown in green. Bottom: The left panel shows
a zoom around the Hβ line region, and the right panel shows a zoom around the
Hα line region.

From a visual inspection of Figure 8, we note that most of the observations
are dominated by the host-galaxy component of SDSS1335+0728 in the optical
range, while the Keck LRIS spectrum shows a much more prominent UV emission
(the same is observed in the 1$\aas@@fstack{\prime\prime}$5 slit spectrum),
which is consistent with the observations obtained from _Swift_ /UVOT. The
X-shooter and Goodman 2021 observations do not allow us to study wavelengths
shorter than 3800 $\AA$ (because of the low signal-to-noise ratio at this
wavelength range), and thus we cannot confirm spectroscopically if this UV
emission was also present in 2021 and 2022; however, this is confirmed by the
_Swift_ /UVOT observations. Moreover, we note that the relative strength of
the [OIII] and [NII] forbidden lines with respect to the Balmer lines has
changed with time. This change in the line fluxes could be due to changes in
the fibre/slit sizes or to the evolution of the source (this is further
discussed in Section 4.6.2).

####  4.6.1 Spectral modelling

We modelled each spectrum using the Penalized PiXel-Fitting (pPXF) software
(Cappellari & Emsellem 2004, Cappellari 2017). Each model includes stellar
populations with a wide range of metallicity and age (Vazdekis et al. 2016),
emission lines (narrow and broad) and a set of power laws of the form
$(\lambda/\lambda_{N})^{\alpha}$, where $\lambda$ is the wavelength,
$\lambda_{N}=5000\AA$ is a normalization factor and $\alpha$ the slope, that
goes from -4 to 0 in steps of 0.1. All the spectra, except that obtained by
X-shooter, were fitted over the whole wavelength range that includes the Hβ
and Hα regions. For the case of the X-shooter spectrum, to reduce the noise
(see grey spectrum in the top panel of Figure 8), we used a box
smooth121212Using specutils:
https://specutils.readthedocs.io/en/stable/index.html. with a window of 10
pixels (equivalent to $\sim$5 resolution elements at 7750 Å). This smoothed
spectrum is shown in green in Figure 8. Then, we fitted the optical range that
includes the H-$\alpha$ region and used these results to constrain the range
of slopes for the set of power laws to fit the UV spectrum with the H-$\beta$
region. The range of slopes was limited to values from $-1.1$ to 0 and no
other constraints were applied. The results of the pPXF modelling for all the
spectra are shown in Appendix A.

The best-fits from pPXF yield consistent ages of the stellar populations in
all the spectra, which are also consistent with what we obtain from the SED
modelling. The SDSS, LAMOST and SOAR 2021 spectra do not require an AGN power-
law component. For the LRIS spectrum a power-law component with a slope of
$-2.901$ ($-3.274$ for the 1$\aas@@fstack{\prime\prime}$5 slit) is needed. For
the case of the X-shooter data, we modelled the spectrum with and without a
power-law component, resulting in similar stellar population ages, and with
similar residual sum of squares (RSS), with RSS of $4.15\times 10^{-30}$ for
the model with a power-law component and $4.13\times 10^{-30}$ for the model
without a power-law component. This prevented us from identifying the best
model for the X-shooter spectrum. Thus, we decided to include the model with a
power-law component in the analysis, as the _Swift_ /UVOT contemporaneous
image shows a strong UV emission. For the SOAR 2021 spectrum, the model
requires a power-law component with a slope of $-1.22$.

We measured the uncertainties of the fits by performing Monte Carlo
simulations. For this, we divided the best-fit residuals into five (six for
the case of X-shooter) wavelength ranges limited by the Hβ and Hα regions. For
each range, we compute the standard deviation and then use it to generate a
set of random values from a Gaussian with the centre at zero, with these
values we build a random simulated noise. Then, we added the simulated noise
to the best-fit model and performed the fit with the same setting used to fit
each spectrum. We used 1000 simulations and the 16 and 84 percentiles for the
errors of measured values.

####  4.6.2 Emission line diagnostics

None of the available spectra of SDSS1335+0728 require a broad line emission
component in the pPXF fits. Moreover, none of them require Bowen fluorescence
(BF; Netzer et al. 1985) emission lines, which have recently been found in
flaring AGNs (Trakhtenbrot et al. 2019b; Frederick et al. 2021) and TDEs (van
Velzen et al. 2020; Zabludoff et al. 2021).

To have a better idea of any possible AGN-like activity in SDSS1335+0728, we
constructed BPT diagrams (after Baldwin, Phillips, and Terlevich; Baldwin et
al. 1981), consisting of indexes that relate the strengths of some narrow
emission lines (NELs; Hβ, [O III]$\lambda$5007, Hα, [N II]$\lambda$6583, and
[S II]$\lambda$6716.47,6730.85) as proxies for the state of ionization of a
region. From the best-fit pPXF results and the Monte Carlo simulations, we
obtained the emission lines intensities (with their respective errors) to
compute the ratios [OIII]$\lambda$5007/Hβ, [N II]$\lambda$6583/Hα, and
[SII]$\lambda$6716.47,6730.85/Hα. These are shown in Figure 9. We used the
theoretical relations from Kewley et al. (2001), Kauffmann et al. (2003a),
Kewley et al. (2006) and Schawinski et al. (2007), to separate star-forming,
composite and AGN (Seyfert and LINER) regions of the two diagrams.

|
---|---
Figure 9: BPT diagrams for all the modelled spectra. The left panel shows the
[OIII]$\lambda$5007/Hβ versus [N II]$\lambda$6583/Hα diagnostic, while the
right panel shows the [OIII]$\lambda$5007/Hβ versus
[SII]$\lambda$6716.47,6730.85/Hα diagnostic. The solid black lines show the
theoretical relations to separate the star-forming, composite and AGN (Seyfert
and Liner) regions in both diagrams.

For the SDSS and LAMOST spectra, we obtain results in agreement with
RCSED2131313https://rcsed2.voxastro.org/data/obj/450785. The LAMOST spectrum
is classified as star-forming, and the SDSS spectrum as composite. Differences
in the fibre aperture could explain this difference in the classifications of
the SDSS and LAMOST spectra, and worse seeing conditions normally observed at
the location of LAMOST. For Goodman 2021 and X-shooter, the results show a
preferred position near the limit between composite-AGN and star-forming-AGN,
indicating a more energetic ionization state than the SDSS and LAMOST results.
While for the case of the LRIS observations, a drastic evolution towards the
AGN region of the BPT diagram is observed for both slits. The same is observed
for the Goodman 2024 spectrum, but we see an evolution in the BPT location
with respect to the LRIS observations. This suggests that the Narrow Line
Region (NLR) of SDSS1335+0728 has had enough time to react to the increased
ionising continuum after more than 3.6 years of activity (the time between the
first ZTF alert and the LRIS spectra).

Figure 10: Light curve of the [OIII] NEL of SDSS1335+0728 for observations
obtained after December 2019. We include the measurements obtained from
Goodman (yellow and pink stars), X-shooter (green square), and LRIS (magenta
and brown diamonds). The horizontal dashed lines show the flux level of the
SDSS and LAMOST observations.

Figure 10 shows the evolution of the [OIII] NEL after December 2019, which is
the line that shows the most evident evolution among the different epochs. We
include as a reference the flux level of the line in the SDSS and LAMOST
spectra. These fluxes are highly dominated by the host-galaxy as they were
obtained using $3^{\prime\prime}$ and $3.^{\prime\prime}5$ fibres,
respectively. All the observations were obtained with different instruments,
slit apertures, and different seeing qualities (see Table 1). Thus, the
observed evolution should be considered with caution. We see a drastic
increment in the [OIII] flux in both LRIS observations (of $\sim$ two to four
times the flux observed by Goodman 2021 and X-shooter) and in the Goodman 2024
spectrum. The level of the LRIS and Goodman 2024 [OIII] flux is also larger
than the ones obtained from SDSS and LAMOST prior to 2019. Thus, this increase
in the [OIII] flux is difficult to explain by variations in the slit size or
by the use of a different instrument.

The evolution of the [OIII] line can be explained by the delayed reaction of
the NLR to the increase in the ionising flux since December 2019. From the BPT
diagram evolution, and the [OIII] line flux variability, we can estimate an
upper limit for the innermost radius of the NLR of 3.6 light years (or 1.1
pc). This is consistent with the results of Peterson et al. (2013), who
measured a radius of 1–3 pc for NGC 5548 (BH mass of $7\times
10^{7}M_{\odot}$; bolometric luminosity of $2.82\times 10^{44}$ erg s-1 Ebrero
et al. 2016). Generally, the NLR size is thought to span hundreds to thousands
of parsecs (Netzer 2013). However, when a source is just activating, the inner
parsec of ionised gas should react first. This would be possible if there is
an extensive zone in the NLR of relatively low filling factor located close to
the nucleus, from which most of the nuclear OIII emission of SDSS1335+0728 is
coming from. Such inhomogeneous density in the NLR has been previously
observed in other objects, like NGC1068 (Kraemer et al. 1998).

The spectral analysis supports the hypothesis of an AGN that is turning on,
whose NLR is starting to react to the X-ray/UV/optical emission. However,
significant NEL variability has also been detected in TDEs (e.g.
Charalampopoulos et al. 2022). The lack of BELs, along with a strong rise in
the UV, could be explained if there are few BLR clouds able to produce strong
enough broad-emission lines (Osterbrock & Ferland 2006). As mentioned in
Section 4.2, one of the possible scenarios to explain the MIR flux evolution
is that a torus component is being formed. This implies that clouds may be
falling in, populating the torus first and eventually the BLR; thus, we might
detect BELs in the future. Further monitoring will be necessary to confirm
this hypothesis.

##  5 Conclusions

SDSS1335+0728 started triggering alerts in the ZTF alert stream in December
2019 (object ID ZTF19acnskyy), and was classified as an AGN by the ALeRCE LCC.
The source did not show variations in the UV/optical/infrared range for almost
two decades before this date. From the behaviour of SDSS1335+0728, we
considered two hypotheses for its origin: (a) a $\sim 10^{6}M_{\odot}$ BH that
has now turned on as an AGN; and (b) a TDE (or another class of still unknown
nuclear transient) in a $\sim 10^{6}M_{\odot}$ BH. In 2021, we started a
photometric and spectroscopic follow-up campaign to better understand the
origin of the observed variations.

The classical TDE scenario is less favoured to explain the origin of the
nuclear variation in SDSS1335+0728 based on the following findings: (a) The
light curve of ZTF19acnskyy does not follow the smooth power-law decay
typically observed in TDEs; (b) ZTF19acnskyy had an absolute peak in the $g$
band of -16.9, and has been varying stochastically for more than 1550 days,
contrary to what is expected from faint TDEs; (c) the SED shows a bluer-when-
brighter behaviour, while TDEs are expected to show no colour variations; (d)
there is no evidence of broad emission lines; and (e) the [OIII] line flux is
quite prominent, and has increased with time, contrary to other TDEs observed
in the optical range.

Despite this, we cannot rule out the TDE hypothesis, as the variations
observed in the nucleus of SDSS1335+0728 could have originated from an exotic
TDE. In particular, some TDEs have shown a lack of broad emission lines, slow
temporal evolution, and faint optical emission, such as eRASSt
J074426.3+291606 (Malyali et al. 2023). Others have shown optical, UV, or
X-ray rebrightening (e.g. Soraisam et al. 2022; Malyali et al. 2023; Wevers et
al. 2023; Liu et al. 2024). Delayed X-ray emission is also common in TDEs,
although not with the same timescales observed in SDSS1335+0728 (Guolo et al.
2024; Wevers et al. 2024). Moreover, we still do not clearly understand the
behaviour of TDEs in already active AGNs (Chan et al. 2019); however, in the
last few years, some of these events have been discovered. For instance,
Merloni et al. (2015) proposed that the CSAGN discovered by LaMassa et al.
(2015) could be explained by a TDE in an accretion disc. Later, Trakhtenbrot
et al. (2019a) and Ricci et al. (2020) presented the discovery of 1ES
1927+654, a CSAGN, and claimed that a TDE in a type 2 AGN provoked an increase
in the accretion rate at the innermost regions of the accretion disc. More
recently, Homan et al. (2023) presented the discovery of eRASSt
J234402.9-352640 using eROSITA observations and concluded that a TDE within a
turned-off AGN best matches their observations. All these examples, however,
show temporal evolution similar to that observed in other TDEs.

We propose that a plausible origin for the nuclear activity observed in
SDSS1335+0728 would be a $\sim 10^{6}M_{\odot}$ BH that is now turning on as
an AGN. This is supported by: (a) its stochastic variable emission; (b) the
bluer-when-brighter behaviour; (c) the SED modelling, which is consistent with
a BBB component that increased its flux by at least one order of magnitude,
with a BBB luminosity of $\sim 10^{43}$ erg s-1 after December 2019,
consistent with an AGN; (d) the estimated $R_{\text{Edd}}$, which has varied
from less than $10^{-3}$ to $\sim 10^{-2}$, consistent with previously
observed CSAGNs; (e) the BPT diagram evolution from star-forming to AGN-like
emission line ratios; (f) the [OIII] line evolution, which is responding to
the UV/optical variations, and suggests that the NLR is starting to react to
the increased ionising flux $\sim 3.6$ years after the first ZTF alert, which
implies a compact NLR; (g) a delayed MIR flux increase from about June 2022,
and the redder W1-W2 colour, which could imply that a dusty torus is now being
formed; and (h) the recent X-ray detection, 1514 days after the first ZTF
alert, which shows very soft emission, similar to that observed in 1ES
1927+654 when its X-ray corona began to reform (Ricci et al. 2020), which
could imply that an X-ray corona is establishing itself in the nucleus of
SDSS1335+0728.

The lack of BELs suggests that SDSS1335+0728 does not yet have a BLR, as we do
see a strong new UV continuum and relatively rapid optical variability, and
therefore cannot associate the lack of BELs with obscuration effects. The
recent MIR evolution cannot be explained by a preexisting (AGN-like) dusty
torus, which suggests that a torus could be under formation, implying that
clouds are falling in from the outside, populating the torus first and then
the BLR. If this hypothesis is correct, we may eventually see BELs. If the
source forms these structures in the future, it would be a perfect laboratory
with which to study and test the different physical mechanisms proposed to
explain the origin and evolution of the BLR and the torus in AGNs (e.g. Czerny
& Hryniewicz 2011; Elitzur et al. 2014; Czerny et al. 2017).

Unfortunately, disentangling the origin of the variability detected in the
nucleus of SDSS1335+0728 is not yet possible. Future observations will allow
us to understand whether SDSS1335+0728 will eventually fade or will continue
evolving to become an AGN. In particular, multi-wavelength monitoring of the
source is needed to see if a typical corona, BELs, and a dusty torus are
eventually established, and to rule out a potential TDE or nuclear transient.
X-ray monitoring and millimetre observations will be crucial to understand
whether or not the recently detected X-ray emission originates from an X-ray
corona under formation (Ricci et al. 2020, 2023). If this is the case, we may
expect to detect hard X-ray emission in the future. High-angular-resolution
integral field unit (IFU) spectroscopy would be ideal for studying the
evolution of AGN signatures in spatially resolved emission-line diagnostic
diagrams (e.g. Mezcua & Domínguez Sánchez 2020) and for understanding the
connection between the nuclear variations and the host properties. Moreover,
observations in the $\sim 800$ to $\sim 3500$ $\AA$ UV range would help us to
better understand the evolution of the ionising continuum and its connection
with the evolution of the NELs. Finally, photometric and spectroscopic
observations in the optical and infrared regimes would be key to studying the
potential formation of BELs or a dusty torus, respectively.

###### Acknowledgements.

The authors acknowledge support from the National Agency for Research and
Development (ANID) grants: Millennium Science Initiative Program ICN12_12009
(PSS,LHG,FEB,FF,MPH,AMMA), Millenium Nucleus NCN19_058 (SB,PA,PL,MLMA), CATA-
BASAL - FB210003 (FEB,CR,RJA), BASAL FB210005 (AMMA), FONDECYT Regular -
1241005 (FEB), 1230345 (CR), 1201748 (PL), 1200710 (FF), 1231718 (RJA). SB
acknowledges financial support from ANID Becas 21212344, and the hospitality
of ESO under the SSDF Project 28/23D. AB acknowledges financial support from
Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under
Germany’s Excellence Strategy – EXC 2094 – 39078331. DH acknowledges support
from DLR grant FKZ 50 OR 2003. MK acknowledges support from DLR grant FKZ 50
OR 2307. DT acknowledges support by DLR grant FKZ 50 OR 2203. AM acknowledges
support by DLR under the grant 50 QR 2110 (XMM_NuTra). PA and SB acknowledge
support from the Max-Planck Society through a Partner Group grant. MJA
acknowledges support from the NSF Grant #2108402. This work has been possible
thanks to the use of AWS-U.Chile-NLHPC credits. Powered@NLHPC: This research
was partially supported by the supercomputing infrastructure of the NLHPC
(ECM-02). Based on observations obtained with the Samuel Oschin 48-inch
Telescope at the Palomar Observatory as part of the Zwicky Transient Facility
project. ZTF is supported by the National Science Foundation under Grant No.
AST-1440341 and a collaboration including Caltech, IPAC, the Weizmann
Institute for Science, the Oskar Klein Center at Stockholm University, the
University of Maryland, the University of Washington, Deutsches Elektronen-
Synchrotron and Humboldt University, Los Alamos National Laboratories, the
TANGO Consortium of Taiwan, the University of Wisconsin at Milwaukee, and
Lawrence Berkeley National Laboratories. Operations are conducted by COO,
IPAC, and UW. Based on observations obtained with the Samuel Oschin Telescope
48-inch and the 60-inch Telescope at the Palomar Observatory as part of the
Zwicky Transient Facility project. ZTF is supported by the National Science
Foundation under Grants No. AST-1440341 and AST-2034437 and a collaboration
including current partners Caltech, IPAC, the Weizmann Institute for Science,
the Oskar Klein Center at Stockholm University, the University of Maryland,
Deutsches Elektronen-Synchrotron and Humboldt University, the TANGO Consortium
of Taiwan, the University of Wisconsin at Milwaukee, Trinity College Dublin,
Lawrence Livermore National Laboratories, IN2P3, University of Warwick, Ruhr
University Bochum, Northwestern University and former partners the University
of Washington, Los Alamos National Laboratories, and Lawrence Berkeley
National Laboratories. Operations are conducted by COO, IPAC, and UW. We
acknowledge the use of public data from the _Swift_ data archive (Obs IDs
00014350003, 00014350004, 00014350005, 00014350007). The scientific results
reported in this article are based in part on observations made by the
_Chandra_ X-ray Observatory, observation ID: 29355. Based on observations
collected at the European Southern Observatory under ESO programme
109.24F5.001. Based on observations obtained at the Southern Astrophysical
Research (SOAR) telescope, which is a joint project of the Ministério da
Ciência, Tecnologia e Inovações (MCTI/LNA) do Brasil, the US National Science
Foundation’s NOIRLab, the University of North Carolina at Chapel Hill (UNC),
and Michigan State University (MSU), through CNTAC proposal CN2021A-17. Some
of he data presented herein were obtained at the W. M. Keck Observatory, which
is operated as a scientific partnership among the California Institute of
Technology, the University of California and the National Aeronautics and
Space Administration. The Observatory was made possible by the generous
financial support of the W. M. Keck Foundation. The authors wish to recognize
and acknowledge the very significant cultural role and reverence that the
summit of Maunakea has always had within the indigenous Hawaiian community. We
are most fortunate to have the opportunity to conduct observations from this
mountain. This research has made use of the NASA/IPAC Extragalactic Database
(NED), which is funded by the National Aeronautics and Space Administration
and operated by the California Institute of Technology. Funding for SDSS-III
has been provided by the Alfred P. Sloan Foundation, the Participating
Institutions, the National Science Foundation, and the U.S. Department of
Energy Office of Science. The SDSS-III web site is http://www.sdss3.org/.
SDSS-III is managed by the Astrophysical Research Consortium for the
Participating Institutions of the SDSS-III Collaboration including the
University of Arizona, the Brazilian Participation Group, Brookhaven National
Laboratory, Carnegie Mellon University, University of Florida, the French
Participation Group, the German Participation Group, Harvard University, the
Instituto de Astrofisica de Canarias, the Michigan State/Notre Dame/JINA
Participation Group, Johns Hopkins University, Lawrence Berkeley National
Laboratory, Max Planck Institute for Astrophysics, Max Planck Institute for
Extraterrestrial Physics, New Mexico State University, New York University,
Ohio State University, Pennsylvania State University, University of
Portsmouth, Princeton University, the Spanish Participation Group, University
of Tokyo, University of Utah, Vanderbilt University, University of Virginia,
University of Washington, and Yale University.

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##  Appendix A Spectral modelling

The following figures (from 11 to 14) show the results of the pPXF spectral
modelling. Each figure shows in the top panel the original spectrum in blue,
the best fit (including all the components) in orange, the stellar component
in green, the NELs in red, and the power law component in purple. The bottom
panel of each figure shows the residual of the modelling (the difference
between the original spectrum and the best fit).

|
---|---
Figure 11: Spectral modelling of the 2007 SDSS spectrum (MJD 54156; left) and of the 2015 LAMOST spectrum (MJD 57155; right). Top: The original spectrum is shown in blue, the best fit (including all the components) in orange, the stellar component in green, the NELs in red, and the power law component in purple. Bottom: The modelling residual is shown in grey. In both panels, the grey dashed lines show the location of the N III $\lambda 4640$ and the He II $\lambda 4686$ emission lines.  |
---|---
Figure 12: Same as Figure 11 but for the 2021 Goodman spectrum (MJD 59396; left) and the 2022 X-shooter spectrum (MJD 59789; right).  |
---|---
Figure 13: Same as Figure 11 but for the 2023 LRIS (MJD 60137) 0′′.7 slit
spectrum (left) and the 1′′.5 slit spectrum.  Figure 14: Same as Figure 11 but
for the 2024 Goodman spectrum (MJD 60337).