AT2020ohl: its nature and probable implications

ASASSN-20hx, a.k.a AT2020ohl, is an ambiguous nuclear transient (ANT), which was discovered in the nearby galaxy NGC6297 by the All-Sky Automated Survey for Supernovae (ASAS-SN). We have investigated the evolution of AT2020ohl using a multi-wavelength dataset to explain the geometry of the system and the energy radiated by it between X-ray and radio wavelengths. Our X-ray, UV/optical, and radio observations of the object jointly clarify the association of AT2020ohl with the nuclear activity of NGC6297. We detected radio counterpart of AT2020ohl 111 days and 313 days after the discovery in Jansky Very Large Array X-band with flux densities 47$\pm$14 $\mu$Jy and 34$\pm$3 $\mu$Jy, respectively. Using multi-wavelength data analysis, we nullify the possibility of associating any stellar disruption process with this event. We found some evidence showing that the host galaxy is a merger remnant, so the possibility of a binary SMBH system can not be ruled out. The central SMBH has a mass of $\sim1.2\times10^7$ M$_\odot$. We propose the accretion disk activity as the origin of AT2020ohl $-$ it is either due to disk accretion event onto the central SMBH or due to the sudden accretion activity in a preexisting accretion disk of the system during the interaction of two SMBHs which became gravitationally bound during a merger process. However, we also admit that with the existing dataset, it is impossible to say definitively, among these two probabilities, which one is the origin of this nuclear transient.


INTRODUCTION
The region (≲ 100pc) close to the center of any galaxy is expected to be the harbor of various cosmic catastrophes, although it was hard to detect them observationally even a few decades ago.However, with the advent of all-sky survey programs in the recent past (e.g., Panoramic Survey Telescope & Rapid Response System (Pan-STARRS), Hodapp et al. 2004;Texas Supernova Search, Quimby 2006;Palomar Transient Factory (PTF), Rau et al. 2009; Catalina Real-time Transient Survey (CRTS), Drake et al. 2009; All-Sky Automated Survey for SuperNovae (ASAS-SN), Shappee et al. 2014;Zwicky Transient Facility (ZTF), Bellm et al. 2019) highly energetic explosions/flares have been discovered at the centers of many nearby and distant galaxies.Multi-channel origin of these explosions is quite obvious.Among these energetic phenomena, if we consider only those with peak radiated power more than 10 41 erg s −1 , there are three known physical processes responsible for these events.One possibility is the occurrence of luminous supernova (SN) from very massive stars located within ∼ 100 pc from the center (cf., Kankare et al. 2017).The supernova remnant Sgr-A East located within ∼ 2 pc from the center of our Milky Way galaxy (Sgr-A * ) is the nearest evidence in this regard (cf., Maeda et al. 2002 and references therein).The second possibility is the disruption of a star (roughly a ★ E-mail: rupakroy1980@gmail.com,rupak.roy@manipal.edusolar mass) during its sufficiently close approach to the Supermassive Black Hole (SMBH) at the center of a galaxy.This process, which was theoretically predicted more than 40 yrs ago (cf., Hills 1975;Rees 1988) although observationally found much later (cf., Komossa & Bade 1999;Gezari et al. 2009), is known as a Tidal Disruption Event (TDE).Both possibilities are connected to disruptions of stars, although the underlying physics is completely different.The third channel is related to the accretion phenomena onto the central SMBHs of the Active Galactic Nuclei (AGN) and the instability associated with these processes (cf., Beckmann & Shrader 2012;Padovani et al. 2017).
However, observationally distinguishing the electromagnetic features of these nuclear transients is still a challenge.As a consequence, the nature of several newly discovered nuclear transients are debated.For example, (1) the same event has been interpreted early as an SN and later as a TDE or vice versa (e.g., ASASSN-15lh, Dong et al. 2016;Leloudas et al. 2016;CSS100217, Drake et al. 2011;Blanchard et al. 2017;ASASN-17jz, Holoien et al. 2022 and references therein), (2) a few nuclear events have simultaneously shown TDE and AGN-like features, and in some cases detailed study could not fix their origin.These are now called as Ambiguous Nuclear Transients (ANT) (e.g., 1ES 1927+654/ASASSN-18el, Trakhtenbrot et al. 2019b;ASASSN-18jd, Neustadt et al. 2020), (3) a few of them did not show any significant spectral changes during their evolution (e.g., Dougie Vinkó et al. 2015; ASASSN-20hx, Hinkle et al. 2022), mak-ing their nature questionable.Some of these ambiguous phenomena may be related to the intrinsic properties of the central SMBH itself, especially associated with its dynamics and recurrent activity, which are not yet well understood.Discoveries have confirmed the presence of binary SMBH with inner separations as small as few pc (Kharb et al. 2017 and references therein) at the center of a merged system.Merging (or completely merged) galaxies and/or gas accumulation processes in a single, binary (or trinary) system of galaxies is a phenomenon with a time scale of millions of years.These are believed to be switching processes between inactive and active states of the SMBHs (e.g., Schoenmakers et al. 2001;Nandi et al. 2019 and references therein).However, for a given system, how these processes are initiated is yet unknown.Needless to say, advanced transient survey programs have provided a unique opportunity to probe the variety of SMBH triggering mechanisms and to search for their different accretion behaviours.Apart from the regular flaring activities of AGNs (which are more prominent in its Blazar subclass), long-term spectral and temporal evolutions have also been noticed in active SMBH systems (e.g., 'Changing Look' quasar, LaMassa et al. 2015).Recent observations of transients also demand the existence of 'rejuvenated' SMBHs (e.g., AT2017bgt, Trakhtenbrot et al. 2019a).While 'Changing Look' behaviours may be due to a significant change in the accretion rate onto the SMBH over a longer timescale, or due to the dust attenuation along the line of the sight; the 'rejuvenated' scenario has been claimed to be due to a sudden matter flow in the disk of active SMBH, causing a several-order-of-magnitude rise in the ultraviolet (UV) and X-ray flux, along with the appearance of Bowen fluorescence lines.Therefore, the 'rejuvenated' SMBH scenario is expected to be observable in AGN dominated systems.
However, the occurrence of transients due to 'rejuvenated' SMBH, in inactive or extremely mildly active systems, or association of such transient activity with the dynamics of a binary SMBH in a merged system is as yet unknown.In this work, revisit the nuclear transient event AT2020ohl/ASASSN-20hx, which was hosted by an mildly-active galaxy that might be a merger remnant.The event was extensively followed by Hinkle et al. (2022).The evolution of the transient in X-ray, near-ultraviolet (NUV) and optical wavelengths was reported, starting from −30 to 275 days relative to the maximum UV/optical brightness.The observed UV/optical lightcurves were fitted with TDE-models, although the shallow post-maximum decline of the lightcurves and non-thermal X-ray spectrum were indicators of the non-TDE origin of the object.However, it could not be characterized as a Tidal Disruption Event (TDE), or phenomena associated with Active Galactic Nuclei (AGN).Here we present Xray, near-ultraviolet (NUV), optical imaging and spectroscopic, and radio photometric observations of AT2020ohl and the properties of its host galaxy.The NUV and optical photometric data cover about 800 days since the discovery.The pre-transient optical monitoring of the object of a similar time span is also been presented in this work.
Section 2 describes the discovery and follow-up of the transient at different electromagnetic (EM) wavelengths.The nature of the host galaxy, the extinction along the line of sight and the distance to the system are discussed in section 3.In section 4, the evolution of the transient in different EM wavelengths is described.The interpretations of our observational results are discussed in the section 5 and conclusions are drawn in the same section.

AT2020OHL − DISCOVERY & FOLLOW-UP
AT2020ohl/ASASSN-20hx was discovered by the ASAS-SN survey (Brimacombe et al. 2020) on UT 2020-07-10.34 at g∼16.7 mag at the center of the nearby (z=0.0167)galaxy NGC 6297.It was initially classified as a TDE (Hinkle et al. 2020), and further analysis of the early Neil Gehrels Swift1 /XRT observations suggested it as a hard X-ray TDE candidate with a spectral index roughly between 2.2−2.6 (Lin 2020).The transient was extensively followed by Hinkle et al. (2022), mainly in the X-ray, near-UV (NUV), optical bands, and mid-IR in a few epochs.They also analysed the early data observed by TESS 2 during the rising phase of the transient.The rise in the TESS data was estimated to be on JD = 2459023.3+0.8  −0.6 (Hinkle et al. 2022), compared to the discovery date i.e., JD = 2459040.8,17.5 days later the initial explosion.This value is also consistent with the limit of non-detection by Brimacombe et al. (2020).In the present work, we use JD = 2459023.3as the epoch of explosion (t 0 ), and all the phases are measured with respect to t 0 .
Although the early bluish spectral energy distribution (SED) of the source, starting from UV to optical, along with X-ray detection, supported its TDE nature (as discussed in the subsequent sections, and also by Hinkle et al. 2022), the overall behaviour of AT2020ohl was not like canonical TDEs.Therefore detailed multi-wavelength follow-ups and analyses were necessary.To achieve this goal, we performed targeted observations of the transient in the X-ray, UV, Optical, and Radio bands.A brief overview of our observations has been discussed below.

JVLA Radio Observation
Prior to AT2020ohl, the center of NGC6297 was not detected in radio wavebands.We analysed available archival radio data from the FIRST 5 , NVSS 6 , and GB6 7 radio sky surveys.The host was detected in neither the 1.4 GHz NVSS or FIRST surveys (with limiting skyrms 0.45 and 0.15 mJy/beam, respectively), nor in the 5 GHz GB6 survey (limiting sky-rms 3 mJy/beam).We did not find any pretransient radio emission in the 3 GHz radio sky survey VLASS8 as well (limiting sky-rms 0.12 mJy/beam).
Motivated by the detection of the X-ray counterpart of AT2020ohl, we observed the field using Jansky Very Large Array (JVLA) at three epochs in two frequencies − X-band (8−12 GHz), and C-band (4−8 GHz).The X-band observations were done on 2020-10-29 (proposal code VLA/20B-427) and 2021-05-19 (proposal code VLA/21A-420) using the B-array and D-array configurations, respectively.In Cband, it was done only in the D-array configuration on 2021-05-24 ( VLA/21A-420).The raw data were calibrated using the standard VLA pipeline tool9 which is based on CASA10 .After calibration the target data was extracted using the task split, and then imaged by tclean.The JVLA Synthesized Beam width in the B-array in X-band is 0. ′′ 6, whereas in D-array, they are 7. ′′ 2 and 12 ′′ for the X-band and C-band, respectively.A radio counterpart was detected from NGC 6297 in all of our observations.Details of the radio observations are given in Table A2.
Due to its higher spatial resolution, the X-band observation on 2020-10-29 revealed two radio sources near the center of NGC6297 − a central component with a second radio knot at a separation of 1. ′′ 5 from the center of the galaxy.This angular separation is extremely small compared to the projected dimension (of major axis11 ) of the host (Makarov et al. 2014) 12 , which is roughly 38 ′′ .However the aforementioned high resolution radio observation was inadequate to distinguish the transient from those two radio counterparts at the center of NGC 6297.Noteworthy, the optical spectra (acquired using a slit of width 1. ′′ 5) also contain both host and transient spectra.Therefore the present data set cannot spatially resolve the transient from its host's centre.Nevertheless, since the X-ray counterpart of AT2020ohl was detected by Swift/XRT, high-resolution X-ray imaging of the field using CHANDRA X-ray observatory13 was done to mark the transient position precisely.

Swift/UVOT Observation
Swift/UVOT data have also been used to study the near-UV (NUV) and optical counterparts of the transient14 .Early Swift/UVOT observations (till 275 days post-maximum) were reported by Hinkle et al. (2022).In this work, we have compiled the Swift NUV data till ∼800 days post-maximum.The public Swift data is available in the website of 'Heasarc'.The Swift data were reduced by using the standard pipeline available in the HEAsoft software package15 .UVOT observations at each epoch were conducted using one or several orbits.To improve the signal-to-noise ratio (SNR) in a given band at a particular epoch, all orbit data of that corresponding epoch have been co-added using the HEAsoft routine uvotimsum.The routine uvotdetect is used to determine the correct position of the transient (which is consistent with the ground-based optical observations).We used the routine uvotsource to perform aperture photometry.For source extraction, a small aperture of radius 3. ′′ 5 was used to minimize the host contamination, while an aperture of radius 50 ′′ was used to determine the background 16 .The host-subtracted UVOT photometry is presented in Table A3.

Swift/XRT Observation
Swift X-ray Telescope (XRT) operates in the energy range 0.3 -10 keV (Burrows et al. 2005).XRT monitored the source AT2020ohl over 800 days, starting from 2020-07-19 (MJD 59049).Like NUV observations, we have also used the publicly available Swift/XRT data of this event till ∼800 days post-maximum, while only the initial ∼275 days XRT observations were reported by Hinkle et al. (2022).The source was X-ray faint, and XRT observed the source in photon counting (PC) mode.In this study, we considered all observations with an exposure time greater than 700 seconds to achieve a good signal-to-noise.The XRT data reduction followed the standard procedure17 .We ran xrtpipeline to generate the Level 2 products.We select a circular region having a 20-pixel radius around the source and a larger circle with a radius of 50 pixels as the background region.No pile-up is detected in the source data.We use XSELECT to create the source and background spectrum.The xrtmkarf tool with the source spectrum and exposure map is used to generate the ancillary response function (ARF) file for each observation.We use the response files (RMF) from the CALDB version 20220803 18 .All the bad channels are ignored.We group the source spectrum with 'group min 1' to ensure at least one count per bin and use cstat in XSPEC for spectral modelling.The details of the XRT observations are tabulated in Table A4.

CHANDRA X-ray Observation
We observed the AT2020ohl with the CHANDRA X-ray space telescope on 2021-04-16, using the High Resolution Camera for Imaging (HRC-I) instrument to obtain an image with a high spatial resolution (∼ 0. ′′ 4).This observation was also necessary to localize the nonthermal X-ray emission and its association with the possible radio counterparts.The black curve shows the resulting model fit for the SDSS -band data from the combination of Sersic (blue dash-dot line), inner exponential-disk (green dash line), and outer exponential-disk (red dotted line) profiles.The The dotted-dashed purple line is the background, while the inset shows a ∼ 1.5 ′ × 1.5 ′ SDSS colour composite image of NGC6297.Right Panel: The UV-optical spectrum of the center of NGC6297.The blue spectrum shows the original flux calibrated SDSS observation (using an optical fiber of radius 1. ′′ 5).The green spectrum is obtained after re-scaling the SDSS spectrum with respect to the central 'g' and 'r' band fluxes of the host computed within an aperture of radius 3. ′′ 5 (red points).These photometric measurements have been done on the pre-flare SDSS images.The magenta curve is the low-order polynomial fit over the available pre-flare UV data.The inset shows the region around the Na i D5890, 5896 doublet in the calibrated observed spectrum.The cyan shaded region shows the contribution due to the Milky Way, while the red region marks that due to the host galaxy.

Other archival data
The transient occurred at the center of the host.Therefore, it is essential to remove the host contribution from the observed flux, to compute the actual flux of the transient at different epochs.We used the pre-explosion archival SDSS spectrum of the host center (Smee et al. 2013), which was acquired much before the occurrence of AT2020ohl.We have also used the archival photometric data from the SDSS survey to compute the central flux of the host.The procedure is described in §3.The ATLAS sky survey (Tonry et al. 2018) observed the field more than 800 days before the event and continued the monitoring afterword.We use ATLAS forced-photometry on the host subtracted frames 19 where there are 5 detections 20 (see §4.1).

THE HOST − NGC6297
Morphologically, the host is an S0 galaxy 12 .The absence of starforming lines and AGN lines in the pre-transient archival SDSS spectrum indicate that the AGN activity in this galaxy has been quenched (or at a very low-level).This is also consistent with its X-ray faintness in the pre-transient ROSAT 21 X-ray observations.However, 19 https://fallingstar-data.com/forcedphot/ 20 ATLAS usually observes in a quad, i.e. 4 images of the same sky area during a given night.To improve the SNR, the fluxes of the object obtained from the host subtracted photometry in a given night have been stacked.Further, only the 5 detections have been considered for this analysis, and corresponding AB magnitudes have been computed. 21https://www.dlr.de/content/en/articles/missions-projects/past-missions/rosat/rosat-mission.htmlanalysis of pre-transient   /XRT observations reveal a faint Xray luminosity ∼ 3.4 × 10 41 erg  −1 , which points to the existence of a low-luminous AGN (LLAGN) at the centre of NGC6297 (Hinkle et al. 2022 and references therein).S0 galaxies are found in various environments that indicate multiple pathways of their formation (e.g., Deeley et al. 2020).Various possible channels have been proposed for their formation − either through gas stripping of spirals inside dense galaxy clusters (e.g., Quilis et al. 2000) or, by merging of disk-dominated galaxies (e.g., Querejeta et al. 2015) or, due to merging of compact ellipticals with their gas-rich irregular companions (Diaz et al. 2018).There are also observational evidence suggesting that galaxies with 'bars/lenses' make a significant contribution to the formation of S0 galaxies, which is difficult to explain with these formation mechanisms (Laurikainen et al. 2009).
We used the SDSS r-band image to measure the surface brightness properties of NGC6297.The left panel of Figure 1 shows the radial profile of the r-band intensity of the galaxy in terms of mag/arcsec 2 found by fitting concentric isophotes.The outermost isophote is at a distance of ∼12 kpc from the center of the galaxy.The surface brightness of massive galaxies is commonly modelled with two components − a Sersic profile for the central bulge and an exponential profile for the disk (Graham & Driver 2005).We found that along with the Sersic profile, two exponential profiles are required to model the surface intensity distribution of NGC6297.The value of the reduced chisquare ( 2 ) for the 'Sersic and 'single-exponential' profile is 12.3, while that for the 'Sersic and double-exponential' profile is 0.9.This implies that the disk of NGC6297 may have two components − an inner disk with a steeper intensity profile and an outer disk with shallower intensity profile.The presence of an extended disk with multiple components probably supports the merging scenario as the origin of the host of AT2020ohl (see §5).
To estimate the true transient flux (and hence the magnitude) in UVOT bands, the flux of the host at different UVOT bands is needed.However as the system remained variable over a long timescale in the post-flare epochs, we computed synthetic magnitudes of the host-center from the archival data.The archival SDSS spectrum (recalibrated using the 3. ′′ 5 aperture-photometry of the host-center) and the SED produced from the archival GALEX measurements were used.This is shown in the right panel of the Figure 1 22 .The synthetic magnitudes of the host-centre, in the UVOT uvw2, uvm2, uvw1, U, B, & V bands, that correspond to flux within an aperture of radius 3. ′′ 5 are respectively ∼18.28, ∼18.75, ∼17.79, ∼17.39, ∼16.05, and ∼15.00 mag 23 .For the rest of the work, we use these magnitudes as the measures of NUV-optical fluxes of the quiescent galaxy.

Distance and Extinction toward NGC6297
The spectrum of NGC6297 does not have any strong emission lines.The redshift () of the host computed from its narrow Balmer lines is ∼ 0.01671 (Adelman-McCarthy et al. 2006).This corresponds to a luminosity distance (  ) of ∼ 72.9 Mpc, adopting a standard cosmology 24 corresponding to a distance modulus () ∼ 34.3.
Estimation of the reddening toward the center of NGC6297 (particularly the contribution due to the host) is non-trivial.The Galactic contribution derived from Schlafly & Finkbeiner (2011) is 0.0203±0.0011.The inset of the right panel of Figure 1, shows the region around the Na i D5890, 5896 absorption lines in the SDSS spectrum of the host.The lines from the Milky-Way and host have been marked with cyan and reddish regions.Impression of Na i D 22 To calculate the central flux of the host, the pre-flare images of NGC6297 in FUV & NUV bands (from GALEX observation), and in NUV-optical bands (from UVOT & SDSS observations) have been used.As discussed in §2.3, for UVOT data analysis, an aperture of radius of 3. ′′ 5 has been used.To calculate the central flux of the host in UVOT-filters within an aperture of 3. ′′ 5 radius, first we re-calibrated the observed flux density of the pre-flare SDSS spectrum of the centre (acquired using optical fiber of radius 1. ′′ 5) with respect to the magnitudes of the host-centre in SDSS  (15.85±0.05mag) &  (15.07±0.04)bands, computed from the pre-flare SDSS images taking an aperture of radius 3. ′′ 5. Here, we have assumed that the relative flux calibration of the SDSS-spectrum is correct.The original (in blue) and the re-calibrated (in green) SDSS spectra are shown in the right panel of Figure 1.Further, to construct the SED of the host-centre in UV wavelengths, pretransient observations from GALEX (in FUV & NUV bands) and Swift (in 1,  bands) have been used and computed by fitting a lower-order (order 3) polynomial to these pre-transient UV data.It is shown with the magenta line in the Figure .The magnitudes of the host-center are, respectively 20.85±0.29,18.89±0.07,18.58±0.20,and 17.56±0.01 in FUV, NUV, 2, and  bands.After constructing the entire re-calibrated UV-optical spectrum of the hostcentre (i.e., magenta+green lines in the above mentioned Figure), we have computed the synthetic magnitudes of the host center (that corresponds to the flux-density within an aperture of 3. ′′ 5 radius) in   /UVOT filters (viz.2, 2, 1, , , ) by using their corresponding response curves and following equation 1 of Koornneef et al. (1986). 23 Hinkle et al. (2022) computed the synthetic-magnitudes of the host-center considering an aperture of radius 5 ′′ and using the synthetic spectrum produced from the code Fitting and Assessment of Synthetic Templates (FAST, Kriek et al. 2009).We have reproduced their results by taking an aperture of radius 5 ′′ .However, in the present work, to reduce the host contamination from the beginning, we have used 3. ′′ 5 aperture to extract the source, and applied aperture-correction. 24 Throughout this paper the cosmology model with Ω M = 0.3, Ω Λ = 0.7 and H 0 = 69.6 km s −1 Mpc −1 has been assumed (Wright 2006).doublet in the spectrum has been considered as a moderate tracer of reddening toward the line of sight over the last couple of decades (e.g., see Barbon et al. 1990;Richmond et al. 1994;Turatto et al. 2003;Poznanski et al. 2012), although recent studies suggest a significant deviation from the previously proposed correlations, particularly using low-resolution spectra (e.g., see Phillips et al. 2013).The equivalent width (EW) of the Na i D doublet due to Milky-Way is ∼ 0.1795Å , that corresponds to a reddening of ∼ 0.04 according to Barbon et al. (1990), and 0.023 +0.005 −0.004 according to Poznanski et al. (2012).Note that both measurements are consistent with the abovementioned value of the Galactic reddening.Therefore, the average Galactic reddening along the direction of the transient is ≈ 0.028.Na i D absorption dip of the host is more prominent (although the components of the doublet are unresolved) than that of Milky-Way.The EW of this unresolved line is 2.63±0.22Å(the error in EW is estimated using Equation 6of Vollmann & Eversberg 2006).This corresponds to a reddening from the host galaxy 0.66±0.05according to Barbon et al. (1990) and to Poznanski et al. (2012) corresponding value of reddening is ∼16, which is essentially infeasible.
Therefore, in this work, we presume that the contribution of the host in reddening toward the center of NGC6297 is negligibly small.This is mainly supported by the following attributes − (a) the transient remains highly luminous in the NUV for a long time (see §4), (b) the host is a S0 galaxy, therefore the star formation in its centre may have been quenched, causing a very less abundance of dust along the center of the host.Therefore, considering only the Galactic absorption, the total reddening toward the transient is  ( − ) ≈ 0.028.This implies the total extinction toward AT2020ohl in V-band is   ≈0.09 mag, adopting the uniform value of total-to-selective extinction ratio (  ) = 3.1 of the Milky-Way.

EVOLUTION OF AT2020OHL
Using the high-cadence pre-transient data from TESS Hinkle et al. (2022) found a smooth rise in flux from the pre-transient to the transient state, following a power-law with temporal index ∼1.05.In this work, we will mainly focus on the post-maximum evolution of AT2020ohl.Figure 2 shows the X-ray, UV and optical light curves of the transient.The red and blue vertical lines mark the onset of the event (from TESS observation) and the first detection reported by the ASAS-SN survey (see §2).

UV-optical lightcurve
Flat/quasi-flat lightcurves are prominent during the first 120 days in all UVOT-bands, followed by a gradual decay until ∼ +400 days in the rest frame of the transient.Assuming a power-law decay of the flux with time (  ∝  −  ), two distinct decay profiles were seen before and after +120 day in each lightcurve.Before +120 day, the temporal index ( 1 ) remained almost constant in all UVOT bands (0.07 ± 0.01 in 2, to 0.01±0.01 in -band).Between 120−400 days, a steeper decay (with index  2 ) is observed.It is more prominent in the NUV bands, with a larger value of  2 (0.41±0.02), while shallower in band (roughly 0.3) and even shallower in  & -bands (roughly 0.1).Beyond +400 day, a shallow rise is seen for all of the NUV bands for the next 150 days followed by a decay until +700 day.In the  and  bands, the corresponding variations are almost negligible, although a decay of the -band flux beyond +550 day is also noticeable.
The left panel of Figure 3 shows the ATLAS forced-photometry of the center of NGC6297, performed on the host subtracted images in the ATLAS Orange (o) & Cyan (c)-bands and covering a timespan from ∼ −800d to +800d with respect to the onset of the event (i.e., t 0 ).Prior to that, fluctuations around the median of 19 mag are noticeable in both ATLAS bands.However, from this data, it would be exaggerated to say that those fluctuations are precursors of AT2020ohl.Nevertheless, it is noteworthy that on MJD = 59000.4(i.e., 22 days before the onset of the event in the source frame), the object became brighter than the median by 0.7 mag in the o-band.This flux enhancement is within 2 from the median-value (where standard deviation for the pre-event o-band measurements is ∼ 0.5 mag).The trend in the enhancement of flux in the o-band during that period is more prominent in 3 clipping analysis and, to some extent, is also noticeable in TESS observations (see Figure 4 of Hinkle et al. 2022).This may be a precursor of AT2020ohl.However, a rise in the c-band was not observed in either of the 5 or the 3 clipped photometry.So, if it is indeed a precursor of AT2020ohl, the corresponding spectrum must be very reddish.
On the other hand, beyond +400 day a little rise in both o-band and c-band fluxes can be seen.As the host galaxy is faint in NUV and -bands, a flux enhancement during that period has also been found from the analysis of Swift data.The enhancement in NUV is noticeably higher than other optical bands.
A close inspection of the first 120 days of photometric evolution after the peak reveals significant variation in the UV-optical light curves (right panel of Figure 3).The solid lines with periodicity ∼11 days are over-plotted with the data.Similar behaviour has also been noticed in X-ray lightcurve (with a periodicity of ∼8 days) during this timespan.However, due to the lack of high-cadence data with regular intervals, we cannot perform any quantitative analysis to conclusively establish the periodic variation in the light curves.

X-ray lightcurve and spectrum
The upper panel of Figure 2 shows the X-ray lightcurve.Like UVOT data, all of the orbit data of a given Swift/XRT observation were stacked to improve the SNR.We fit the individual, as well as the time-averaged X-ray spectra with the ztbabs*powerlaw model with a line of sight Hydrogen column density   = 1.3 × 10 20  −2 .The photon index of the time-averaged X-ray spectrum is ∼ 2.1 (see the right panel of Figure 4).From the spectral modeling, we estimated the hardness ratio (HR−defined as the ratio of 2-10 keV and 0.3-2 keV flux) and the spectral index for all the Swift/XRT observations.The HR (see lower plot of the left panel of Figure 4) and spectral index ( upper plot of the left panel of Figure 4) remain roughly constant at around 0.2 and 2, respectively.This indicates the soft nature of the X-ray emission.These results are consistent with the findings of Hinkle et al. (2022).The X-ray emission from AT2020ohl was completely power-law dominated and plausibly generated through non-thermal processes, while as described in §5.1 and §5.3) the UVoptical photons were produced due to black body emission.This is also evident from the uncorrelated nature of X-ray and UV-optical light curves.

Optical spectra
The spectral evolution of the transient in optical wavelength is shown in Figure 5 along with the SDSS pre-transient spectrum of the host.From the upper panel of Figure 5, it is clear that during its entire evolution (which corresponds to a timescale of ∼ 500 days post disruption in the rest frame), no new line was produced due to the transient activity.This may rule out the possibility of a TDE or SN as the origin of AT2020ohl (see §5).As shown in the middle and bottom panels, the +68d spectrum of the transient(+host) can be reproduced by adding a black body component to the host spectrum, where the black body emission is produced by a photosphere with a radius of R 68 ∼ 3.1 × 10 15 cm and having an effective temperature T 68 = 8966 ± 50K.Similarly, the +160d spectrum of the transient(+host) can be reproduced by adding a black body component with R 160 ∼ 1.9 × 10 15 cm and T 160 = 9100 ± 115K to the host spectrum 25 .Therefore, the optical emission produced by the transient had a black body origin, and the nature of the source did not change appreciably during the first 200 days.Later the emitter became cooler, and slowly the transient intensity started to decline (see the bolometric lightcurve in §5).

Evolution in Radio-band − connection with X-ray
As described in §2.2, the transient was detected in the JVLA X and C bands, at two epochs separated by 7 months.The radio flux contours 25 Noteworthy, the resolution of SDSS spectrum is comparable to that of the observed spectra of the transient.To perform the fitting, we first calculated the flux values of every observed spectrum at the wavelength bins of the SDSS spectrum using cubic spline interpolation.Further, we added a black body component with the SDSS spectrum and fitted it to individual observed spectrum using LMFIT package by implementing  2 minimization technique while varying the radius and temperature of the black body.
superimposed on the SDSS optical image for these two observations are shown in Figure 6.The X-band observation in the higher resolution under B-array configuration (synthesized beamwidth  = 0. ′′ 6), resolves the central emission (O) from the nearby radio knot 'K' to the 1. ′′ 5 north-east, as well as the nearby galaxy with compact radio emission ∼16 ′′ to the south and marked as 'G'.The integrated X-band flux density of the central core on +111 day is 33.5±3.7 Jy.The corresponding measurement for the knot (K) is 18.4±3.7 Jy.
Although the event was X-ray luminous, the transient can not be well localized due to the low resolution of the Swift/XRT (18 ′′ at 1.5 keV).To localize the X-ray transient, we observed it with ).The addition of the Blackbody spectrum (characterized by a single temperature 'T') emitted by a spherical region of radius 'R', with the host spectrum, can reproduce the transient spectrum at that epoch.For the +68d optical spectrum, the temperature and radius are, respectively T 68 = 8966 ± 50K & R 68 ∼ 3.1 × 10 15 cm, whereas the +160d spectrum can be reproduced with a blackbody surface at T 160 = 9100 ± 115K & R 160 ∼ 1.9 × 10 15 cm.These show there is not much evolution of the transient between these two epochs.The implication of this analysis has been discussed in §5.
CHANDRA/HRC-I.The upper-right inset shows the central region, and the centroid of the X-ray source, is consistent with the location of the central radio counterpart, confirming the nuclear-origin of AT2020ohl, and that the nearby radio-knot is not associated with the present transient activity (see §5).
The lower resolution D-array X-band images ( = 7. ′′ 2) could resolve the emission of NGC6297 (O + K) from the nearby galaxy G (see the green dot-dashed contour), but cannot the emission from the center of the host.The integrated X-ray flux density in the Darray data is 65.08±14 Jy.Since the nearby knot is not associated with the transient activity, its X-band luminosity is expected to be constant and we can conclude the transient X-band flux had been increased to ∼ 47 ± 14.7 Jy by +313 day.This is nearly 1.4 times higher than the X-band flux of the transient at +111 day, although the two values are consistent given their errors.This shows that over 200 days of evolution, there was no significant change in the X-band luminosity of AT2020ohl.On the other hand, the D-array C-band observation ( = 12 ′′ ) on +318 day could resolve none of the three sources therefore gives only the upper limit on the flux of the transient (see the magenta dashed contour).The integrated flux density of the entire C-band contour shown in the Figure is 132.78±20Jy.It is also noticeable from this image that the centroid of this contour coincides with the position of the nearby galaxy (G).It implies that G mostly dominates the radio flux.The peak flux density of this contour is 101.41±10Jy/beam.Therefore the upper limit of the central (i.e., transient) C-band flux of NGC6297 is roughly 30 Jy on +318 day (we consider it as the upper limit as this flux is a combination of transient flux and that of the radio-knot in C-band).Hinkle et al. (2022) pointed out that AT2020ohl/ASASSN-20hx simultaneously exhibited the characteristics of TDEs and AGN.Substantial emission in NUV, the smooth rise of the TESS light curve in TESS-band and the overall UV/optical evolution make the object comparable to TDE-like events.On the other hand, the linear rise of the flux, the absence of emission lines (also described in §4.3) differentiate AT2020ohl from canonical TDEs.Moreover, the nonthermal origin of the X-ray and the location of NGC6297 in the NIR color-color diagram is evidence for a possible AGN-origin of this event (also see Auchettl et al. 2018), although strong AGN lines were absent until the +483d spectrum (see Figure 5, and also Hinkle et al. 2022).In this work, we have revisited this problem to understand the origin of AT2020ohl using multi-wavelength data.This is also  to explore the probable mechanisms behind some of the ambiguous nuclear transients.

The nature of NUV & optical radiation
Although the rise of AT2020ohl was smooth (a monotonic function of time, Hinkle et al. 2022), its post-maximum evolution in different UV-optical bands were significantly different from those of canonical stellar disruptions (see §4.1).The Swift NUV/optical lightcurves showed quasi-periodicity for nearly 120 days after the maximum (lower panel of Figure 2).This indicates that the origin of these photons is roughly the same (although not precisely as discussed below).
The upper panel of Figure 7 shows the UV/optical bolometric lightcurve of the object along with the variation of the photospheric radius and temperature of the transient.To compute the bolometric lightcurve, we used the host-subtracted photometry of the transient obtained from the UVOT and ATLAS observations.The lightcurve was computed at the epochs of the Swift observations, and the observations from ATLAS were linearly interpolated to the Swift epochs.The observed SED of every epoch can be explained with a single black body.The temporal variation of black body radius and temperature have been shown in the second and third panels of the Figure 7.Although the peak luminosity is consistent with canonical SNe, or TDEs, the post-peak decline rate is extremely shallow − maintaining a power law with temporal index ∼ −0.07 during the first 120 days, and thereafter a steeper index of ∼ −0.3 has been observed (where negative sign indicates a decrease in flux with time, §4.1).Among nuclear transients, similar slow bolometric evolution has been observed in a few cases like ASASSN-17cv, ASASSN-18el (Trakhtenbrot et al. 2019a,b), and ASASSN-18jd (Neustadt et al. 2020).All these are ambiguous transients − either changing-look AGN (ASASSN-18el) or rejuvenated SMBH (ASASSN-17cv) or some unknown SMBHdriven transient in an AGN system (ASASSN-18jd).
The temperature obtained from the spectra are relatively lower than that computed from the SED-fitting (however, it is not an order of magnitude higher).This is mainly because SED contains spectral information over a larger wavelength range, and the photometric flux calibrations are better than spectroscopic flux calibrations.The higher value of inferred temperature, while using the UV-optical photometry was noticed previously as well in nuclear transients like TDEs (e.g., ASASSN-14ae, Holoien et al. 2014).One probable reason may be, that although we are assuming the origin of UV-optical photons is the same, actually UV photons are coming from the relatively inner hotter part of the disk, while the optical photons are generated at the relatively outer cooler part of the disk.The temperatures computed from the optical spectra are therefore lower than that calculated from the UV dominated SED.Noteworthy, our last optical spectrum was taken on +475 day.So, at late-epoch, when UV is relatively less dominant, the photometric results become more consistent with spectroscopic results.However, beyond +475 day, we could only compute black body temperature and radius from photometric measurements.Therefore, with the existing data set, it is not possible to compare the results from photometry and spectroscopy beyond +475 day.Important information about the emitter of UVoptical radiation can be drawn from the correlation of temperature with radius shown in the fourth panel of the Figure .In the early evolutionary phases (< 200 days in rest frame), the radius (R) and temperature (T) follow the relation T∝R −0.47 (blue dash line), while in the later phases (> 400 days in rest frame), they follow the relation T∝R −0.66 (red dot-dash line).The black solid line shows the radiustemperature relation for a standard accretion disk (T∝R −3/4 , Shakura & Sunyaev 1973).The transition of the radius-temperature relation from the shallower slope at the early epoch towards the standard accretion disk scenario at the late epoch essentially demonstrates that UV-optical emitting region has an accretion-disk like structure, which was initially evolving dynamically (just after the event), settled down with time, and approached to a steady accretion disk in due course of time.

Origin of X-ray & Radio photons
Thanks to the quasi-simultaneous, high-resolution X-ray and radio observations, it is clear that, the X-ray and radio emissions originate from the same loation.Figure 8 shows the nature of radio emission from the central component shown in Figure 6.The Left panel of Figure 8 shows the post-disruption temporal evolution of the X-band radio luminosity between days ∼ 111 and ∼ 313.It also shows the upper limit of the C-band radio luminosity at 313 day and compares it to the 5 GHz radio luminosities of other transients (adopted from Alexander et al. (2020)) like TDEs, radio-luminous SNe, and the central radio luminosities of Seyfert galaxies and radio-quiet AGNs.Clearly, AT2020ohl is several orders of magnitude fainter than other radio transients.As shown in the Figure, the X-band luminosity of AT2020ohl increased only by 1.3 order in 200 days.This evolutionary rate is much slower than that of the other radio-TDEs shown in the Figure .At +313 days post-disruption, it was 10 times dimmer than the central radio-luminosity of low-luminous Seyfert galaxies and  2 of Alexander et al. (2020).Right Panel shows the comparison of radio spectra of AT2020ohl with the radio-spectra of some radio luminous TDEs at comparable (and late) epochs, viz.SWJ1644+57 (Zauderer et al. 2013), ASASSN-14li, XMMSL1 J0740-85 (Alexander et al. 2016(Alexander et al. , 2017)), and AT2019dsg (Cannizzaro et al. 2021).
low-luminous radio-quiet AGNs.At a similar phase of evolution, its radio luminosity is comparable only with some of the low-luminosity radio SNe.However, it had no other signature that suggests it could be a supernova.Given its spectral properties at other wavelengths, particularly in the optical and NUV, we rule out the possibility of the association of AT2020ohl with any stellar explosion or disruption phenomenon − it is neither a supernova nor a tidal disruption event.Nevertheless, the gradual increase of its high-frequency radio luminosity marks the possible slow emergence of an outflow from the center of NGC6297.
The normalized radio spectra of AT2020ohl are compared with some of the radio-luminous TDEs in the right panel of Figure 8.Here, cyan and blue points are the X-band luminosities of the transient at +111 day and +313 day, respectively.The red-filled circle corresponds to the total integrated flux in C-band, and the downward triangle represents the upper limit of the C-band transient flux (see §4.4).The orange straight line (between the C-band upper limit and the X-band detection) shows the radio SED of the transient at ∼300 days after the disruption.The positive value of the spectral index (assuming  ∝   ) shows it is in the optically-thick phase at this stage of its evolution.The radio SED of AT2020ohl is also compared with the SEDs of the radio luminous TDEs observed at comparable epochs − SWJ164449.3+573451(at +383d, Zauderer et al. 2013), XMMSL1J0740-85 (+592d, Alexander et al. 2017), ASASSN-14li (+330d, Alexander et al. 2016), and AT2019dsg (+150d, Cannizzaro et al. 2021).Unlike AT2020ohl, the other radio luminous TDEs became optically thin at these radio frequencies by 300 days after the peak.This radio characteristic of AT2020ohl distinguishes it from the canonical radio luminous TDEs, and in fact, advocates its non-TDE origin.

The combined picture
From multi-wavelength analysis, it was well established that AT2020ohl was a slowly evolving transient.To understand the characteristic of its emission, the SED of the object has been modeled using multi-wavelength broadband data.In this regard, We used the X-band and C-band radio data of the transient observed on +318 day, along with the X-ray data from Swift/XRT, and host-subtracted NUV-optical data from Swift/UVOT observed on the +322 day (observations are quasi-simultaneous).The redshift corrected observed broadband SED is shown in Figure 9 (see the upper-right panel).We modelled the broadband SED using the 'Jets SED modeler and fitting Tool' (JetSeT, Massaro et al. 2006;Tramacere et al. 2009Tramacere et al. , 2011;;Tramacere 2020).It is important to note that the host of AT2020ohl is an S0 galaxy although had a mild pre-transient X-ray emission, showing mild AGN activity and no star-forming lines in its spectrum ( §3).The log of the ratio of its axes (i.e., major axis/minor axis) is logr2526 = 0.16±0.05(Makarov et al. 2014), showing that it is a nearly face-on galaxy.Since the host is a mildly-active galaxy, we consider that there is neither a torus nor a narrow/broad-line regions surrounding the SMBH.This is also consistent with the spectra of the host.
The observed SED is modelled assuming a very simple geometry of the system.During the transient activity, we assume a temporary accretion disk is produced (or there was already a disk-like structure present in this system).This is the origin of the black body radiation, which peaks in UV wavelength and maintaining a constant disk temperature.The overall geometry of the system is Blazar-like, Parameters, which are fixed, have only the 'Start values' (2nd column).The parameters being varied have a start value (2nd column) with minimum and maximum limits (3rd and 4th columns, respectively).The best-fit values of these parameters are given in the 5th column.
without the presence of a torus and narrow & broad line-forming regions.The non-thermal radiation is produced by a spherical homogeneous blob of leptonic plasma (e ± ) entangled with magnetic fields, and it is moving along the jet, which is perpendicular to the plane of the disk.In the JetSeT environment, the system has a nonrotating SMBH of mass M  at its center.Therefore, as described above, the components of the emitted broad-band spectrum are − (1) a black body component produced by a Keplerian disk with an inner radius R  and outer radius R  respectively having a disk luminosity L  , and accretion efficiency     ; and (2) a non-thermal component, which is produced by the spherical blob having a radius R and at a distance R  from the SMBH.The emitter density within the blob is n, the magnetic field is B, and the bulk Lorentz factor of its material is Γ.The motion of the individual emission regions is random and entangled with the magnetic field, causing a spread in the Lorentz-factor between a minimum (  ) and maximum (  ).
The energy distribution of the particles in the blob is assumed to be a power-law (i.e., n(E) ∝E −  ), and the jet points along a small angle  with respect to the line of sight.Table 1 summarizes the initial and final fitted values of the parameters.Since the redshift is well constrained its value is fixed.Given the limited number of constraints, we have also frozen several other parameters in this fitting process.We explored the number density of the emitter (n), the power-law index () of the synchrotron emitting plasma, the accretion efficiency (    ) of the disk, the mass of the black hole (M  ), and the flux density (B) of the magnetic field.The transient was more luminous in the NUV than the optical, indicating that the black body temperature is high and peaking at a lower wavelength.We fixed the L  at ∼ 10 43 erg s −1 (which is consistent with the bolometric luminosity at comparable epoch) so that the black body emission from the disk peaks in the UV.We further assumed that a mildly relativistic outflow produces the non-thermal emission (Γ ∼ 2) and the  ∼ 5 • .The values of all the other parameters (viz.  ,   , R  /R ℎ (where R ℎ is the "Schwarzschild radius" of the SMBH), R  /R ℎ , R, and R  ) are assumed to be consistent with their typical values for a blazer system.The priors (initial-starting and fixed values) are tabulated in the first column of Table 1, and the best-fit parameters (posteriors) and their errors are tabulated in column 5.The model SED is shown in Figure 9, along with the posterior distribution of the parameters.
The non-thermal emission produced by the mild-relativistic outflow is synchrotron self-absorption (SSA) dominated in the radio frequencies and is optically thick.However, it is optically thin at higher frequencies (e.g., X-ray).We have also searched for the Gammaray counterpart from the    archive and nothing was detected, indicating the absence of inverse Comptonized photons.Thus the SED is consistent with a disk-like accretion along with a mildrelativistic outflow.

Probable origin of AT2020ohl
We rule out the possibility of associating stellar explosion or disruption phenomena with the transient event AT2020ohl/ASASSN-20hx from the multi-wavelength analysis.The absence of any regular supernova line in the spectra immediately dismisses a nuclear-SN origin of this event.The same argument is also primarily applicable to rejecting the possibility of a TDE.
The rate of the rise of AT2020ohl and its host's mass are not similar to those of canonical TDEs (Hinkle et al. 2022).On the other hand, as discussed in §4.1, between +120 and +400 day, the temporal decay index of the UV-optical lightcurve was −0.41±0.02.A similar decay rate was also observed in the X-ray light curve, which is much flatter than the t −5/3 law.These properties show that AT2020ohl is most likely not associated with a stellar disruption event under the gravity of SMBH.
Nevertheless, the combined disc-wind models of TDEs (Strubbe & Quataert 2009;Lodato & Rossi 2011) can explain the UV-optical lightcurves to some extent.According to these models, the bound matters of the star form disk while the unbound debris form wind-like outflow, causing a relatively flatter post-maximum decay in NUVoptical luminosity, maintaining a nature of t −5/12 .Although this decay index is consistent with the observations of AT2020ohl, there are large differences between the observed and predicted X-ray luminosities and X-ray decay rates.While models predict a higher luminosity and roughly t −5/3 decay law over a period of ∼ 1000 days after disruption, the observed X-ray luminosity of AT2020ohl was much less than the NUV luminosity, and the corresponding X-ray decay rate was flatter than t −5/3 law.Moreover, the origin of the X-ray is thermal by the Lodato & Rossi (2011) model, whereas, in §5.3, we argued that it is completely non-thermal.
From the UV-optical bolometric lightcurve, the migration of the radius-temperature relation toward a steady disk condition ( §5.1), and the multi-band SED modelling ( §5.3), it is now established that an optically thick thermally-emitting accretion disk must be present in this system.The variation in the UV-optical lightcurves and the non-standard behaviour of the radius-temperature relation at the early phases may be a manifestation of a dynamically evolving accretion disk.Since the base of the jet is supposed to be connected with the inner accretion disk, a variation in the X-ray light curve is also expected, if it originates due to the reprocessing of accretion disk photons.However, as discussed in §5.3, the X-ray and NUV-optical photons may have different origin and a strong correlation between light curves in these wavebands is unexpected.
A deeper investigation, like high-resolution multi-wavelength imaging, is required to probe the center of NGC6297 and to understand the origin of such ambiguous nuclear transients.It is noticeable that NGC6297 is an S0 galaxy which may be a post-merger remnant.The presence of an extended galaxy exponential disk profile in this system ( §3) strengthens the post-merging scenario.The plausible existence of an LLAGN at the center of the host further strengthens the existence of a mild-accretion disk around the SMBH, which is otherwise not luminous.It is possible that in long periodical intervals, this disk either gets an additional matter supply from its host or the disk is intervened by some other compact object (like a bare-SMBH of a merger), causing a sudden accretion and flaring activity of the disk in the UV-optical bands as well as an outflow, which is the origin of the non-thermal X-ray and radio emission.A similar possibility was studied in the BL-Lac object OJ287 based on its long-term optical lightcurve (e.g., Sillanpaa et al. 1988), radio VLBA observations (e.g., Agudo et al. 2012;Gómez et al. 2022), and observations in other wavebands.Recently, high-spatial resolution (∼0.′′ 1) observations of UGC4211 − a post-merger galaxy have revealed the existence of binary SMBH with separation of ∼ 230pc (Koss et al. 2023).Noteworthy, VLBI observations have confirmed the existence of binary SMBHs, where the separation between two compact objects is as less as 7.3 pc (e.g., Kharb et al. 2017 and references therein).Therefore, the existence of binary SMBH at the center of NGC6297 can not be ruled out, and certainly, high-spatial resolution observations are necessary to confirm this possibility.
In the JVLA B-array X-band radio image (Figure 6), the knot observed in the north-east direction from the center of the galaxy is at a projected angular separation of 1. ′′ 5, which corresponds to a distance of ∼ 265pc (≡ 850 lightyears), which implies that this knot is not causally related to AT2020ohl.Unfortunately, we could not obtain JVLA observations at other frequencies (preferably in the C-band) with comparable spatial resolution so that the spectral index of the knot and, therefore, its age could be calculated.However, given that the X-band flux of the knot is comparable to that of the central emission ( §4.4) if we assume that the knot was indeed associated with the central activity due to an event similar to AT2020ohl, it must happened more than 850 years ago.
The transition between active and inactive galaxies and vice versa is not completely understood.Merging of galaxies may be a key process for transition from inactive to active state.Since S0-galaxies have gone through the merging process, the study of their central regions is important to probe the post-merger scenario.It will also shed light on the gas accumulation process at the centers of these systems.The discovery of AT2020ohl/ASASSN-20hx is an example in this regard.Although in this study we ruled out the association of stellar explosion or disruption phenomena, we could not precisely point out whether the sudden supply of matter to the pre-existing accretion disk triggered the event or it is due to a close interaction with another SMBH which was already a part of a binary SMBH system at the center of the host.Multi-wavelength high-resolution imaging has been planned to probe this system in the near future. The hardness ratio (HR) is defined as the ratio of 2-10 keV and 0.3-2 keV flux.

Figure 1 .
Figure 1.NGC6297 − the host of AT2020ohl.Left Panel: Surface brightness of the galaxy as a function of radius (blue points).The black curve shows the resulting model fit for the SDSS -band data from the combination of Sersic (blue dash-dot line), inner exponential-disk (green dash line), and outer exponential-disk (red dotted line) profiles.The The dotted-dashed purple line is the background, while the inset shows a ∼ 1.5 ′ × 1.5 ′ SDSS colour composite image of NGC6297.Right Panel: The UV-optical spectrum of the center of NGC6297.The blue spectrum shows the original flux calibrated SDSS observation (using an optical fiber of radius 1. ′′ 5).The green spectrum is obtained after re-scaling the SDSS spectrum with respect to the central 'g' and 'r' band fluxes of the host computed within an aperture of radius 3. ′′ 5 (red points).These photometric measurements have been done on the pre-flare SDSS images.The magenta curve is the low-order polynomial fit over the available pre-flare UV data.The inset shows the region around the Na i D5890, 5896 doublet in the calibrated observed spectrum.The cyan shaded region shows the contribution due to the Milky Way, while the red region marks that due to the host galaxy.

Figure 2 .
Figure 2. The X-ray, UV, and optical lightcurves of AT2020ohl.The Lower Panel shows the UV-optical lightcurve of the transient.All the observed magnitudes of the transient have been obtained by subtracting the host central flux from the measured fluxes of the central region at different epochs as detailed in §3.Variations in magnitudes in small and large timescales are noticeable.Upper Panel shows the corresponding variations in the X-ray fluxes.The red and blue vertical lines respectively mark the last non-detection and first detection reported by ASASSN.Note that the last non-detection was noticed ∼2 days before the epoch when the first rise was observed by TESS ( §2) The black dashed vertical lines mark the epochs when HCT spectroscopic observations were performed.

Figure 3 .
Figure 3.The X-ray, UV, and optical emission from AT2020ohl.Left Panel shows the lightcurves of AT2020ohl in ATLAS o and c-bands.Right Panel shows only the early part (post-maximum) of the lightcurve observed from Swift.Significant variations in X-ray, and UV-optical light curves of the transient are noticeable.The solid and dashed lines over-plotted with the data highlight the variation of flux (qualitatively) at the early and late phases after the maximum respectively.

Figure 4 .
Figure 4.The Left panel shows the variation of hadness ratio (HR) and spectral index () of the X-ray emission from the source during its evolution.The Right panel shows the average X-ray spectrum of the transient during its evolution in first 100 days.See text for the detail.

Figure 5 .
Figure 5. Spectroscopic evolution of AT2020ohl.The Upper Panel shows the spectra of the transient observed at +68d, +76d, +97d, +113d, +160d, +273d,  and +475d.The SDSS spectrum of the host galaxy has also been shown.Over the timescale of ∼500 days after the onset of the event, no strong line due to a TDE-like event has been observed.Narrow vertical grey regions mark the positions of teluric lines.Middle and Bottom Panels demonstrate the fitting of the Blackbody spectrum at two different epochs (+68d & +160d).The addition of the Blackbody spectrum (characterized by a single temperature 'T') emitted by a spherical region of radius 'R', with the host spectrum, can reproduce the transient spectrum at that epoch.For the +68d optical spectrum, the temperature and radius are, respectively T 68 = 8966 ± 50K & R 68 ∼ 3.1 × 10 15 cm, whereas the +160d spectrum can be reproduced with a blackbody surface at T 160 = 9100 ± 115K & R 160 ∼ 1.9 × 10 15 cm.These show there is not much evolution of the transient between these two epochs.The implication of this analysis has been discussed in §5.

Figure 6 .
Figure 6.This figure shows the JVLA radio and CHANDRA X-ray counterparts of AT2020ohl, superposed on the archival optical image of the host (the bright big galaxy at the centre) taken from the PanSTARR survey (North is up and East is in the left direction).The JVLA X-band (B-array) contours (black) show the compact radio component distinguishing the centre (O), nearby knot (K) in the north-east, and the nearby compact radio source 'G'.The central region has been magnified in the inset where CHANDRA X-ray counterpart (in red) has also been shown.The green dot-dashed contours show the JVLA X-band (D-array) observations, while the magenta dashed contours show the JVLA C-band (D-array) observations.The X-band B array contours have been plotted at 3 [1, 1.5, 2, 2.5], where the rms noise is  = 3 Jy beam −1 .For the X-band D array also, the contours have been plotted at 3 [1, 2, 4], where the rms noise is  = 6Jy beam −1 .For the C-band D array contours have been plotted at 3 [1, 2, 4], where the rms noise is  = 5 Jy beam −1 .

Figure 7 .
Figure 7.The UVOIR bolometric lightcurve of AT2020ohl.From top, the first, second and third panel respectively describes the temporal evolution of bolometric luminsity (L   ), Radius of the emitting surface, and its Temperature.The lower most panel shows the observed variation of tempearture with radius.The color-bar indicates the phase in rest frame.The blue dash line is fit to the data having phase value < 200 days in rest frame showing T∝R −0.47 .The red dot-dash line is fit to the data having phase value > 400 days in rest frame showing T∝R −0.66 .The black solid line shows the classical temterature-radius relation for an accretion disk (T∝R −3/4 ).

Figure 8 .
Figure 8. Evolution of AT2020ohl in radio-band.Left Panel shows the light curve and its comparison with other energetic sources, viz.radio counterparts of SNe (region marked with dotted line), emission from the central part of Syefert galaxy (region marked with dot-dashed line), and central radio luminosity of Radio-Quiet AGNs (region marked with dashed line), and radio-TDEs (other points).The figure has been adopted from Alexander et al. (2020) and further modified.The downward gray triangles are showing upper limits of the fluxes of several TDEs which have been labeled with numbers as per first column of Table2ofAlexander et al. (2020).Right Panel shows the comparison of radio spectra of AT2020ohl with the radio-spectra of some radio luminous TDEs at comparable (and late) epochs, viz.SWJ1644+57(Zauderer et al. 2013), ASASSN-14li, XMMSL1 J0740-85(Alexander et al. 2016(Alexander et al. , 2017)), and AT2019dsg(Cannizzaro et al. 2021).

Figure 9 .
Figure 9.The broad-band SED modelling of AT2020ohl.X-ray and Radio are produced from the synchrotron process, considering the single powerlaw model.However, the NUV-optical emission is mainly from the Blackbody radiation from the disk.The upper right inset shows the observed data points and different components of the model, along with their sum (in red) and residual.The corner plot shows the posterior distribution of 5 variables in the MCMC analysis.

Table 1 .
Parameters of JetSeT model Parameter (unit) Start value Min value Max valueBest-fit value (error)

Table A1 .
Journal of spectroscopic observations of AT2020ohl  With reference to the explosion epoch JD 2459023.3 At 0.6 m. This epoch marks the pre-transient spectroscopic observation of the host by the SDSS survey.

Table A2 .
Log of radio observation of AT2020ohl  from JVLA in C (4-8 GHz) & X (8-12 GHz).Here, only the fluxes of the central emission from NGC6297 have been tabulated  With reference to the explosion epoch JD 2459023.3 The upper limit has no error bars

Table A3 .
/UVOT photometry of AT2020ohl The magnitude of the transient at different UVOT bands have been determined after subtracting the host flux at the transient location (see §3).  With reference to the explosion epoch JD 2459023.3

Table A3 -
continued Continuation of TableA3