AT 2019avd: A novel addition to the diverse population of nuclear transients
A. Malyali, A. Rau, A. Merloni, K. Nandra, J. Buchner, Z. Liu, S. Gezari, J. Sollerman, B. Shappee, B. Trakhtenbrot, I. Arcavi, C. Ricci, S. van Velzen, A. Goobar, S. Frederick, A. Kawka, L. Tartaglia, J. Burke, D. Hiramatsu, M. Schramm, D. van der Boom, G. Anderson, J. C. A. Miller-Jones, E. Bellm, A. Drake, D. Duev, C. Fremling, M. Graham, F. Masci, B. Rusholme, M. Soumagnac, R. Walters
AAstronomy & Astrophysics manuscript no. at2019avd © ESO 2021January 22, 2021
AT 2019avd: A novel addition to the diverse population of nucleartransients
A. Malyali , A. Rau , A. Merloni , K. Nandra , J. Buchner , Z. Liu , S. Gezari , , J. Sollerman , B. Shappee ,B. Trakhtenbrot , I. Arcavi , , C. Ricci , , S. van Velzen , , , A. Goobar , S. Frederick , A. Kawka ,L. Tartaglia , , J. Burke , , D. Hiramatsu , , M. Schramm , D. van der Boom , G. Anderson ,J. C. A. Miller-Jones , E. Bellm , A. Drake , D. Duev , C. Fremling , M. Graham , F. Masci , B. Rusholme ,M. Soumagnac , , and R. Walters Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germanye-mail: [email protected] Department of Astronomy, University of Maryland, College Park, MD 20742, USA Space Telescope Science Institute, Baltimore, MD 21218, USA Department of Astronomy and the Oskar Klein Centre, Stockholm University, AlbaNova, SE 10691 Stockholm, Sweden Institute for Astronomy, University of Hawaii at Manoa, 2680 Woodlawn Dr., Honolulu, HI 96822 The School of Physics and Astronomy, Tel Aviv University, Tel Aviv 69978, Israel CIFAR Azrieli Global Scholars program, CIFAR, Toronto, Canada Núcleo de Astronomía de la Facultad de Ingeniería, Universidad Diego Portales, Av. Ejército Libertador 441, Santiago, Chile Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, China Center for Cosmology and Particle Physics, New York University, NY 10003, USA Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands The Oskar Klein Centre, Department of Physics, AlbaNova, Stockholm University, SE 10691 Stockholm, Sweden International Centre for Radio Astronomy Research - Curtin University, GPO Box U1987, Perth, WA 6845, Australia INAF - Osservatorio Astronomico di Trieste, Via G.B. Tiepolo, 11, I-34143 Trieste, Italy Department of Physics, University of California, Santa Barbara, CA 93106-9530, USA Las Cumbres Observatory, 6740 Cortona Dr, Suite 102, Goleta, CA 93117-5575, USA Graduate School of Science and Engineering, Saitama Univ., 255 Shimo-Okubo, Sakura-ku, Saitama City, Saitama 338-8570,Japan DIRAC Institute, Department of Astronomy, University of Washington, 3910 15th Avenue NE, Seattle, WA 98195, USA Division of Physics, Mathematics, and Astronomy, California Institute of Technology, Pasadena, CA 91125, USA IPAC, California Institute of Technology, 1200 E. California Boulevard, Pasadena, CA 91125, USA Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, USA Department of Particle Physics and Astrophysics, Weizmann Institute of Science, Rehovot 76100, Israel California Institute of Technology, Pasadena, CA 91125, USAReceived XXX; accepted YYY
ABSTRACT
We report on
SRG / eROSITA, ZTF, ASAS-SN, Las Cumbres, NEOWISE-R, and Swift
XRT / UVOT observations of the unique ongoingevent AT 2019avd, located in the nucleus of a previously inactive galaxy at z = . kT ∼
85 eV) that was (cid:38)
90 times brighterin the 0 . − σ upper flux detection limit (with no archival X-ray detection at this position). The ZTFoptical light curve in the ∼
450 days preceding the eROSITA detection is double peaked, and the eROSITA detection coincides withthe rise of the second peak. Follow-up optical spectroscopy shows the emergence of a Bowen fluorescence feature and high-ionisationcoronal lines ([Fe x ] 6375 Å, [Fe xiv ] 5303 Å), along with persistent broad Balmer emission lines (FWHM ∼ − ). Whilstthe X-ray properties make AT 2019avd a promising tidal disruption event (TDE) candidate, the optical properties are atypical foroptically selected TDEs. We discuss potential alternative origins that could explain the observed properties of AT 2019avd, such as astellar binary TDE candidate, or a TDE involving a super massive black hole binary. Key words. keyword 1 – keyword 2 – keyword 3
1. Introduction
Actively accreting supermassive black holes (SMBHs) have longbeen known to exhibit large amplitude flaring behaviour (e.g.Tohline & Osterbrock 1976; Antonucci & Cohen 1983; Pen-ston & Pérez 1984; Shappee et al. 2014; Storchi-Bergmann et al.2017; Frederick et al. 2019), whereby multi-epoch observations of galaxy nuclei, over year-long timescales, have revealed dras-tic changes in their luminosity. The physical mechanisms re-sponsible for producing extreme accretion rate changes are stillunclear, although various models have been suggested, such asstate transitions in the inner disc (Noda & Done 2018; Ross et al.2018), radiation pressure instabilities in the disc ( ´Sniegowska &
Article number, page 1 of 19 a r X i v : . [ a s t r o - ph . H E ] J a n & A proofs: manuscript no. at2019avd
Czerny 2019), or tidal disruption events (TDEs; Merloni et al.2015; Chan et al. 2019).Whilst the sample of ignition events in galactic nuclei waspreviously limited to only a few objects, the advance of wide-field, high-cadence surveys over the last decade has facilitatedthe discovery of an increasing number of extreme state changes.This has resulted in tighter constraints on the timescales of flar-ing events for these systems. For example, Trakhtenbrot et al.(2019b) recently reported a new class of SMBH accretion eventthat sees a large amplitude rise in the optical / UV luminosity overtimescales of months.In addition to triggering drastic changes in the accretion ratein AGNs, TDEs can also cause quiescent black holes to transi-tion into short-lived active phases. In a TDE, a star that passestoo close to a BH is torn apart by strong tidal forces, with afraction of the bound stellar debris then being accreted onto theBH (Hills 1975; Young et al. 1977; Gurzadian & Ozernoi 1981;Lacy et al. 1982; Rees 1988; Phinney 1989). Early TDE candi-dates were first identified through detection of large-amplitude(at least a factor of 20), ultra-soft X-ray flares (black-body tem-peratures between 40 and 100 eV) from quiescent galaxies dur-ing the ROSAT survey (Bade et al. 1996; Komossa & Bade 1999;Komossa & Greiner 1999; Grupe & Leighly 1999; Greiner et al.2000). Since then, the vast majority of TDE candidates havebeen optically selected, such as through the Sloan Digital SkySurvey (SDSS; e.g. van Velzen et al. 2011; Merloni et al. 2015),the Panoramic Survey Telescope and Rapid Response System(Pan-STARRS; e.g. Gezari et al. 2012; Holoien et al. 2019a),the Palomar Transient Factory (PTF; e.g. Arcavi et al. 2014), theIntermediate Palomar Transient Factory (iPTF; e.g. Blagorod-nova et al. 2017; Hung et al. 2017), the All Sky AutomatedSurvey for SuperNovae (ASAS-SN; e.g. Holoien et al. 2014,2016; Wevers et al. 2019; Holoien et al. 2019b), and the ZwickyTransient Facility (ZTF; e.g. van Velzen et al. 2019, 2020). Op-tically selected TDEs are characterised as blue nuclear tran-sients with light curves showing longer / shorter rise and decaytimescales relative to supernovae (SNe) / AGN , and a relativelysmooth power-law decline. Optical spectroscopic follow-up ofthese events post-peak reveals blue continua (blackbody temper-atures ∼ K) with various broad emission lines (full width athalf maximum, FWHM (cid:46) km s − ); a recent characterisationof the di ff erent TDE spectroscopic classes was presented by vanVelzen et al. (2020). Although a number of TDE candidates havealso been found through UV selection ( GALEX , Gezari et al.2008, 2009), and X-ray selection (
XMM-Newton
Slew, Esquejet al. 2007, 2008; Saxton et al. 2012, 2017), most of our under-standing of TDEs is currently biased towards this set of observedproperties of optically-selected TDEs.Whilst most previous TDE searches focused on identify-ing TDEs in quiescent galaxies, an increasing number of can-didates for TDEs in AGNs are being proposed in the literature(Merloni et al. 2015; Blanchard et al. 2017; Trakhtenbrot et al.2019a; Liu et al. 2020; Ricci et al. 2020). In certain cases, thedistinction between TDE and non-TDE-induced SMBH accre-tion state changes is becoming increasingly blurred (see alsoNeustadt et al. 2020). Variants of TDEs have also been proposedto explain more exotic phenomena, such as the recently observedquasi-periodic eruptions (QPEs) in a few galactic nuclei (Mini-utti et al. 2019; Giustini et al. 2020; King 2020), and periodicflaring seen in an AGN (Payne et al. 2020). Other origins forextreme nuclear transients involve SNe in the AGN accretion For large, well-defined AGN flares similar to those seen in Fredericket al. (2019), as opposed to stochastic AGN variability. h m s s s J2000 J AT2019avd
Fig. 1.
Pan-STARRS g -band image centred on the host galaxy ofAT 2019avd. The dark orange star and red circle mark the ZTF positionand eROSITA localisation respectively, where the radius of the circle isset to the 2 (cid:48)(cid:48) uncertainty on the eROSITA source position. disc (Rozyczka et al. 1995), or interaction of SMBH binaries(SMBHB) with an accretion disc (Kim et al. 2018). It is clearthat such di ff erent physical origins may result in a diverse rangeof observed variability behaviours.In this paper, we report on the ongoing extreme eventAT 2019avd, which is a novel addition to the already diversepopulation of nuclear transients. AT 2019avd is associated tothe previously inactive galaxy 2MASX J08233674 + z = .
029 (see Fig. 1), and was first reported as ZTF19aaiqmglat the Transient Name Server (TNS ) following its discovery byZTF on 2019-02-09 UT (Nordin et al. 2019). The transient wasindependently detected more than a year later on 2020-04-28 as anew ultra-soft nuclear X-ray source (Malyali et al. 2020) duringthe first all-sky survey of the eROSITA instrument (Predehl etal., in press) on-board the Russian / German Spectrum-Roentgen-Gamma (SRG) mission.This work presents X-ray (
SRG / eROSITA, Swift / XRT),optical / UV / mid-infrared (MIR) photometric (ZTF, ASAS-SN, NEOWISE-R, Swift / UVOT), and optical spectroscopic(NOT / ALFOSC, Las Cumbres Floyds, ANU / WiFeS) observa-tions of AT 2019avd. In Section 2, we report our X-ray ob-servations and analysis of AT 2019avd, whilst the photomet-ric evolution and host galaxy properties are presented in Sec-tion 3. We then present details of our optical spectroscopicfollow-up campaign in Section 4, before discussing possible ori-gins for AT 2019avd in Section 5, and conclude in Section 6.We adopt a flat Λ CDM cosmology throughout this paper, with H = . − Mpc − , Ω m = .
309 (Planck CollaborationXIII 2016); z = .
029 thus corresponds to a luminosity distanceof 130 Mpc. All magnitudes will be reported in the AB system,unless otherwise stated.
2. X-ray observations
AT 2019avd was discovered in a dedicated search for candidateTDEs in the first eROSITA all-sky survey (eRASS1). Here, theeROSITA source catalogue (version 945 of the source detectionpipeline of the eROSITA Science Analysis Software, eSASS, https://wis-tns.weizmann.ac.il/ all dates in this paper will be reported in UT format.Article number, page 2 of 19. Malyali et al.: AT 2019avd: A novel addition to the diverse population of nuclear transients l o g [ L . k e V / e r g s ] L increase by a factor ROSATXMMeROSITA
Fig. 2.
Long-term X-ray light curve in the 0.2–2 keV energy band ofAT 2019avd up until the first eROSITA observation. Triangles denote3 σ upper limits for ROSAT / PSPC and
XMM-Newton / EPIC-pn, whilstthe black circle marks the
SRG / eROSITA discovery, where AT 2019avdis at least 90 times brighter than the XMM-Newton σ upper limit. Theerror bar on the eROSITA marker encloses the 68% credible region onthe observed luminosity. Brunner et al. in prep.) was systematically examined for new softX-ray sources associated with the nuclei of galaxies that showedno prior indication of being an AGN.The eROSITA data for AT 2019avd are composed offour consecutive scans with gaps of 4 hr each and a mid-time of 2020-04-28. The total on-source exposure amounts to140 s (see Table 1). The source was localised to (RA
J2000 ,Dec
J2000 ) = (08h23m37s, 04 ◦ (cid:48) (cid:48)(cid:48) ), with a 1 σ positional uncer-tainty of 2 (cid:48)(cid:48) , which is consistent with the nucleus of the galaxy2MASX J08233674 + (cid:48)(cid:48) cen-tred on the above position (84 counts were detected within thisregion). Background counts were selected from a circular annu-lus of inner and outer radii 72 (cid:48)(cid:48) and 144 (cid:48)(cid:48) , respectively. Using thebest-fit spectral model (see Section 2.3), we derived a 0 . − ± × − erg cm − s − (1 σ ).No X-ray source has previously been detected at the loca-tion of AT 2019avd. Using both the Upper Limit Server andwebPIMMS , and assuming an absorbed black-body spectralmodel with kT =
80 eV, and Galactic neutral hydrogen columndensity (see also Section 2.3), N H = . × cm − , we inferan 0 . − σ upper limit of 1.7 × − erg cm − s − for aserendipitous 7 ks XMM-Newton pointed observation obtainedon 2015-04-08 . Earlier constraints can be derived from ROSAT observations obtained on 1990-10-14, 1996-11-13, and 1997-04-11 with 3 σ upper limits of 4.2 × − , 4.0 × − , and 1.2 × − erg cm − s − , respectively.eROSITA thus first observed AT 2019avd in a state whereit had brightened by at least a factor of 90 in the 0 . − Triggered by the eROSITA detection, a series of follow-up ob-servations were performed with the
Neil Gehrels Swift Obser- http://xmmuls.esac.esa.int/upperlimitserver/ https://heasarc.gsfc.nasa.gov/cgi-bin/Tools/w3pimms/w3pimms.pl XMM-Newton
OBSID = Table 1.
Log of
SRG / eROSITA and Swift / XRT observations ofAT 2019avd until 2020-09-16. For eROSITA, the mid-date of the cov-erage in eRASS1 is given.
Date MJD Telescope ObsID Exp. [s]2020-04-28 58967.7
SRG / eROSITA - 1402020-05-13 58982.4 Swift / XRT 00013495001 16172020-05-19 58988.3
Swift / XRT 00013495002 19662020-05-25 58994.0
Swift / XRT 00013495003 19822020-06-03 59003.3
Swift / XRT 00013495004 4942020-06-10 59010.6
Swift / XRT 00013495005 17392020-09-16 59108.4
Swift / XRT 00013495006 2967
Table 2.
Swift
UV photometry (corrected for Galactic extinction usingthe UVOT correction factors in Table 5 of Kataoka et al. 2008). Themodel magnitudes (for the host galaxy) were obtained by convolving thebest-fit SED model (Section 3.3) with the UVOT transmission curves.A hyphen denotes that the given filter was not used on that observationdate.
Date UVW1 UVM2 UVW2Model 18.88 19.16 19.262020-05-13 18 . ± .
04 - -2020-05-19 18 . ± .
15 18 . ± .
11 18 . ± . . ± .
07 18 . ± .
07 18 . ± . . ± .
04 - -2020-06-10 17 . ± .
04 - -2020-09-16 17 . ± .
05 18 . ± .
06 18 . ± . vatory (P.I.s: A. Malyali & B. Trakhtenbrot). Observations wereobtained roughly every 7 days, until the source was no longervisible due to Sun angle constraints; a further Swift observa-tion was then obtained ∼ xrtpipeline task included in version 6.25 of the heasoft package. Spectra for each of the five epochs were extracted us-ing the xrtproducts task. Source counts were extracted from acircular aperture of radius 47 (cid:48)(cid:48) and background counts extractedfrom a circular annulus of inner and outer radii 70 (cid:48)(cid:48) and 250 (cid:48)(cid:48) ,respectively .Observations with the Ultraviolet and Optical Telescope(UVOT; Roming et al. 2005) were obtained simultaneously withthe XRT observations. Imaging was performed at three epochs(00013495001, ..004, ..005) using the UVW1 filter with expo-sures of 1.36, 1.95, and 1.93 ks, respectively. The remainingthree observations utilised all six UVOT filters (UVW2, UVM2,UVW1, U, B, V) with accordingly shorter exposure times.The UVOT flux was extracted with the uvotsource taskusing a 9 (cid:48)(cid:48) radius aperture centred on the optical position ofAT 2019avd, whilst a nearby circular region with 15 (cid:48)(cid:48) radius wasused for background subtraction. The photometry was extractedfrom each unique Swift observation ID, and is presented in Ta-ble 2 (we note that this photometry includes both AGN and hostgalaxy emission in order to be consistent with the SED fittingin Section 3.3). Relative to UV photometry obtained prior to theinitial optical outburst (see Section 3.3 and Fig. 7), AT 2019avdhas brightened by ∼ ∼ . − . Swift obser-vations between 2020-05-13 and 2020-09-16. eROSITA and XRT have di ff erent PSFs and instrument backgrounds,thus the radii of the extraction regions were chosen based on each in-strument and di ff er here. Article number, page 3 of 19 & A proofs: manuscript no. at2019avd C oun t s / s / c m kT eV Fig. 3.
BXA fit to the eROSITA eRASS1 spectrum. Black markers arethe binned observed data, whilst the red represents the fitted convolvedmodel for tbabs*blackbody (darker and light red bands enclose the68 % and 95 % posterior uncertainty on the model at each energy). Boththe black-body and power-law fits to the (low count) eRASS1 spectrumsuggest that the source is ultra-soft (see Table 4).
X-ray spectra were analysed using the Bayesian X-ray Analy-sis software (BXA, Buchner et al. 2014), which connects thenested sampling algorithm MultiNest (Feroz & Hobson 2008)with the fitting environment CIAO / Sherpa (Freeman et al. 2001)and XSPEC (Arnaud 1996). The spectra were fitted unbinnedusing the C-statistic (Cash 1976), and the eROSITA and XRTbackgrounds were both modelled using the principal compo-nent analysis (PCA) technique described in Simmonds et al.(2018). For each set of eROSITA and XRT spectra, a joint fiton both the source and background spectra was run. Two dif-ferent models for the source spectra were used: (i) an absorbedblack body ( tbabs*blackbody ), and (ii) an absorbed power law( tbabs*powerlaw ). The equivalent Galactic neutral hydrogencolumn density, N H , was allowed to vary by 20% from its tabu-lated value in the HI4PI survey of 2 . × cm − (HI4PI Col-laboration et al. 2016) during fitting. The complete set of priorsadopted under each model is listed in Table 3, whilst an exampleof the BXA fit to the eROSITA spectrum is shown in Fig. 3, andspectral fit results are presented in Table 4.Over the course of the six weeks following the initialeROSITA detection, there was no major variability in the 0 . − . − Swift epochincreased by a factor of about six relative to the previous obser-vation.AT 2019avd remained in an ultra-soft state during the
Swift monitoring campaign, although there is variability in the in-ferred black-body temperatures ( kT ranges between minimumand maximum values of 72 ± ±
10 eV, respectively).The inferred black-body temperatures are similar to those mea-sured in the X-ray emission of previously observed thermalTDEs (45 (cid:46) kT (cid:46)
130 eV, e.g. van Velzen et al. 2020), and arealso consistent with the temperatures of the soft excess shown inAGN (e.g. Table A1 in Gliozzi & Williams 2020).
3. Photometric evolution and host galaxy properties
The region around the position of AT 2019avd has been mon-itored by ZTF (Bellm et al. 2019; Graham et al. 2019) in the r and g bands from 2019-01-12 until the time of writing. On F X [ e r g s c m ] k T [ e V ] Fig. 4.
X-ray evolution of AT 2019avd. The empty and filled blackmarkers represent the eROSITA and XRT observations respectively; er-ror bars enclose 95% of the posterior. (cid:48)(cid:48) . , and r -band magnitude17 . ± .
07 (reference subtracted, Fig. 1).For MJD < > ff erence images. We also obtained additional pho-tometric observations with the Spectral Energy Distribution Ma-chine (SEDM; Blagorodnova et al. 2018) on the Palomar 60-inchtelescope. The SEDM photometry was host-subtracted usingSDSS reference images, as described in Fremling et al. (2016).These two light curves, and the host-subtracted SEDM photom-etry, were then combined for subsequent analysis, and are shownin Fig. 5.After the initial detection on 2019-02-09, AT 2019avd con-tinued to brighten until reaching its maximum observed bright-ness of r ∼ . g -band magnitude of the host nucleus decayednearly monotonically from 17.13 ± ± ± r and g -band magnitudes of ∼ . ∼ . V -band by ASAS-SN (Shappee et al. 2014; Kochanek et al.2017) from February 2012 to November 2018, and in the g -bandfrom October 2017 to September 2020 (the time of writing). Nomajor optical outbursts were seen in the ASAS-SN light curveprior to the ZTF detection (Fig. B.1); given the joint ASAS-SNand ZTF light curves, it is likely that the system ‘ignited’ aroundMJD = https://lasair.roe.ac.uk/object/ZTF19aaiqmgl/ Article number, page 4 of 19. Malyali et al.: AT 2019avd: A novel addition to the diverse population of nuclear transients
Table 3.
Summary of priors adopted in the BXA analysis of the eROSITA and XRT spectra. For each fit, a log-uniform prior on N H between(0 . N H , . N H ) was defined, where N H = . × cm − (see Section 2.3). Γ denotes the slope of a power law, kT the black-body temperature, A the normalisation. The prior over A is in units 1.05 × − erg cm − s − . Model Priors tbabs*bbody log[ kT / keV] ∼ U ( − , A ] ∼ U ( − , tbabs*powerlaw Γ ∼ U (0 , A ] ∼ U ( − , Table 4.
X-ray spectral fit results from applying BXA to the extracted eROSITA and XRT spectra, with uncertainties enclosing 68% of the posteriorfor each parameter. F . − is the inferred observed (unabsorbed) flux under each model. OBSID tbabs*blackbody tbabs*powerlaw N H kT F . − N H Γ F . − [ × cm − ] [eV] [ × − erg cm − s − ] [ × cm − ] [ × − erg cm − s − ]eRASS1 2 . + . − . + − . + . − . . + . − . . + . − . . + . − . . + . − . + − . + . − . . + . − . . + . − . . + . − . . + . − . + − . + . − . . + . − . . + . − . . + . − . . + . − . + − . + . − . . + . − . . + . − . . + . − . . + . − . + − . + . − . . + . − . . + . − . . + . − . . + . − . + − . + . − . . + . − . . + . − . . + . − . . + . − . + − . + . − . . + . − . . + . − . . + . − . l o g [ L X / e r g s ] V ega M agn i t ude W1W2 19.018.518.017.517.016.516.015.515.0 A b s o l u t e M agn i t ude AB M agn i t ude ZTF g ZTF r SEDM g SEDM r
Fig. 5.
NEOWISE-R (non-host subtracted, top) and ZTF / SEDM (middle) light curves of AT 2019avd, with the immediate 0 . − Swift observation from 2020-09-16 are the empty and filled blackmarkers, respectively. The solid grey vertical line marks the MJD of the eRASS1 observation, whilst grey dashed lines mark the times of the NOTand the first FLOYDS spectrum (Table 5). No significant variability before the initial 2019 outburst is observed in the host nucleus of AT 2019avdwith archival NEOWISE-R and ASAS-SN observations (Fig. B.1). The NEOWISE-R observations pre-outburst are observed with mean W1, W2marked out in the top panel by the cream and orange dashed lines respectively. For plotting clarity, we omit the high-cadence ZTF Partnershipobservations obtained between MJD 58820 and 58860, and we rebin the ∼ & A proofs: manuscript no. at2019avd
In the following, we fit the light-curve model presented in equa-tion 1 of van Velzen et al. (2019), which models the rise with ahalf-Gaussian function, and an exponential function for the de-cay, to the first and second peaks of the ZTF light curve, usingUltraNest (Buchner 2016, 2019) as our sampler. Whilst such amodel is not physically motivated, it enables a comparison ofthe timescales involved in the light curve of AT 2019avd withthose of the population of ZTF nuclear transients presented invan Velzen et al. (2019).While fitting the first peak, we first filter out observationsoutside of the MJD period between 58450 and 58650, and wethen run a joint fit of the g and r band observations in flux space.Our model has seven free parameters, defined following the no-tation of van Velzen et al. (2019): σ r and σ g , the rise timescaleof the light curve in the r and g bands respectively; τ r and τ g ,the decay timescale of the light curve in r and g bands; F peak , r and F peak , g , the peak flux in r and g bands; t peak , the time of thepeak of the light curve (to enable a comparison with van Velzenet al. 2019, we assume that the light-curve model peaks at thesame time in both of these bands). For the second peak, we fil-ter out observations outside of the MJD period 58840 and 59115(the late-time SEDM datapoints are used in the fitting), and be-cause we do not sample the decay of this peak, we only modelthe rise here. The model for the second peak has five free pa-rameters, with τ r and τ g now being omitted. We list our priors inTable A.1, and present the fits in Fig. 6.From the posterior means, we infer σ r = . ± . σ g = . ± . τ r = . ± . τ g = . ± . σ , the decaytimescales in each filter significantly di ff er. With τ r > τ g , the firstpeak shows a potential cooling signature during its decay phase,although we are unable to constrain the temperature evolutionduring this because of a lack of contemporaneous observationsin other wavelength bands. Relative to the population of nucleartransients in van Velzen et al. (2019), one sees that these are shortrise and decay timescales relative to those of AGN flares, and arethus more similar to those in the van Velzen et al. (2019) sampleof TDEs and SNe (Fig. 6). As expected from Fig. 5, the inferredrise times for the second peak are longer and more AGN-like,with τ r ∼
88 days and τ g ∼
93 days.
The location of AT 2019avd was observed in the W µ m)and W µ m) bands by the Wide-Field Infrared Survey Ex-plorer mission (WISE, Wright et al. 2010) in 2010, Near-EarthObject WISE (NEOWISE; Mainzer et al. 2011) in late 2010 and2011, and from December 2013 until now, twice per year as partof the NEOWISE reactivation mission (NEOWISE-R; Mainzeret al. 2014). The NEOWISE-R light curve was obtained fromthe NASA / IPAC Infrared Science Archive by compiling allsource detections within 5 (cid:48)(cid:48) of the ZTF transient position. Indi-vidual flux measurements were rebinned to one data point perNEOWISE-R all-sky scan (using a weighted mean) and con-verted into magnitudes. The resulting light curve is shown inFig 5.The MIR light curve was observed to be flat prior to theinitial ZTF outburst, but showed significant brightening in the https://github.com/JohannesBuchner/UltraNest https://irsa.ipac.caltech.edu/frontpage/ Rise e-folding time [days]10 F ade e - f o l d i ng t i m e [ da ys ] AGNSN otherSN IaTDE
Fig. 6.
AT 2019avd variability compared with previously classified ZTFnuclear transients (non-AT 2019avd data presented originally in vanVelzen et al. 2020), with red and green stars computed from the fittedmodel components for each respective filter. The red and green verticallines mark the e-folding rise time of the second optical peak in the r and g bands, respectively. We also plot the rise and decay e-fold timescalesinferred from the ASAS-SN V -band light curve of the nuclear transientAT 2017bgt (Trakhtenbrot et al. 2019b; see also Section 5.1) with ablack marker. Not only is the double-peaked light curve of AT 2019avdclearly distinct from the other light curves of sources in the AT 2017bgtnuclear transient class, but the first peak of AT 2019avd decays muchfaster than the AT 2017bgt flare, whilst the second peak rises muchslower than the AT 2017bgt flare. first NEOWISE-R epoch obtained thereafter. Observations ob-tained ∼ ∼ W − W ∼ .
08 mag in AllWISE, to a moreAGN-like W − W ∼ . W − W W − W (cid:38) . ff ective at lowerAGN luminosities; see discussion in Padovani et al. 2017). The spectral energy distribution (SED) of the host galaxy ofAT 2019avd was compiled from archival UV to MIR pho-tometry from
GALEX (FUV, NUV), SDSS DR12 ( g , r , i , z ),UKIDSS ( y , J , H , K ), and AllWISE (W1, W2). The SED wasmodelled using CIGALE (Burgarella et al. 2005; Boquien et al.2019), which allows the estimation of the physical parametersof a galaxy by fitting composite stellar populations combinedwith recipes describing the star formation history and attenu-ation. The best-fitting model (see Fig. 7) is that of a galaxywith a stellar mass of (1 . ± . × M (cid:12) , a star forma-tion rate (SFR) of 0 . ± . M (cid:12) yr − , and little attenuation,E(B − V) = . ± .
02 mag, which experienced a burst of starformation 3 . ± . ‘Archival’ is defined here by photometry taken prior to the initial ZTFoptical outburst.Article number, page 6 of 19. Malyali et al.: AT 2019avd: A novel addition to the diverse population of nuclear transients The SED fit suggests that the host galaxy did not show strongsigns of nuclear activity prior to the detection of AT 2019avd.This is further supported by the absence of a radio counter-part in the FIRST catalogue (Becker et al. 1995) within 30 (cid:48)(cid:48) ofAT 2019avd, with a catalogue upper detection limit at this posi-tion of 0.96 mJy / beam .
4. Optical spectral analysis
On 2019-03-15, ∼
33 days after the first observed peak in theZTF light curve, an optical spectrum of AT 2019avd was ob-tained by Gezari et al. (2020) with the Alhambra Faint ObjectSpectrograph and Camera (ALFOSC) on the 2.56 m NordicOptical Telescope (NOT). The spectrum was obtained with a 1 (cid:48)(cid:48) . iraf based software, in-cluding bias corrections, flat fielding, wavelength calibration us-ing HeNe arc lamps imaged immediately after the target and fluxcalibrations using observations of a spectrophotometric standardstar.No further spectra were taken until after eROSITA had de-tected the large-amplitude soft-X-ray flare from AT 2019avd inlate April 2020, which triggered a further five epochs of spec-troscopy (dates listed in Table 5) using the FLOYDS spectro-graphs (Brown et al. 2013) mounted on the Las Cumbres Obser-vatory 2m telescopes at Haleakala, Hawaii, and Siding Spring,Australia. Each spectrum was taken with a 3.6ks exposure, us-ing the ‘red / blu’ grism and a slit width of 2 (cid:48)(cid:48) . The spectra werereduced using PyRAF tasks as described in Valenti et al. (2014).FLOYDS covers the entire 3500-10000 Å range in a single expo-sure by capturing two spectral orders (one red and one blue) si-multaneously, yielding R ∼ ff erent orders are usuallymerged into a single spectrum using the region between 4900and 5700 Å, which is present in both the red and blue orders.However, in this case, in order to avoid erroneous wavelengthshifts at the blue edge of the red order (where there are fewer ar-clines), all FLOYDS spectra were merged using a reduced stitch-ing region of 5400 to 5500 Å . This stitching was done manu-ally in Python, by replacing fluxes in that wavelength range withan average of the linear interpolations of the two orders.In addition, a higher resolution spectrum (R ∼ apall which allowed for background subtraction.A comparison of the NOT and WiFeS spectra is presentedin Fig. 8, and the spectral evolution in the FLOYDS spectrais shown in Fig. 9. A log of the spectroscopic observations of http://sundog.stsci.edu/cgi-bin/searchfirst . The most extreme arcline used to calibrate each order is at ∼ Table 5.
Spectroscopic observations of AT 2019avd.
UT Date Tel. Instrument Exp. [ks] Airmass2019-03-15 NOT ALFOSC 1.8 1.52020-05-10 FTS FLOYDS-S 3.6 1.42020-05-12 FTS FLOYDS-S 3.6 1.62020-05-18 FTN FLOYDS-N 3.6 1.62020-05-29 ANU WiFeS 1.8 1.52020-05-31 FTS FLOYDS-S 3.6 1.72020-06-06 FTS FLOYDS-S 3.6 1.9AT 2019avd is presented in Table 5. We note that we have notfound any archival optical spectra of the host galaxy that wereobtained prior to the initial 2019 outburst discovered by ZTF.
The NOT spectrum from 2019-03-15 appears similar to broadline AGN spectra, showing a relatively flat continuum (in termsof F λ ) and broad Balmer emission lines (H α , H β , H γ , H δ ;Fig. 8). However, the strong Fe ii complex that is frequently seenin some AGNs is not present. The H α profile is asymmetric dueto the blending of unresolved H α and narrow [N ii ] 6549, 6583 Ålines, whilst the asymmetry of the H γ line is likely due to blend-ing of H γ and [O iii ] 4363 Å emission. The other notable featuresare the [S ii ] doublet at 6717 and 6731 Å (again blended, but laterresolved in the WiFeS spectrum), and the weak He I emission at5876 Å. As no archival spectrum of the host galaxy is available,we are unable to judge whether or not the main observed emis-sion features appeared at the onset of the extreme optical vari-ability. The WiFeS spectrum from 2020-05-29 (Fig. 8) showsthe same emission features as the NOT spectrum, with the ad-dition of a broad emission feature around 4680 Å and an appar-ent increase in intensity of a set of high-ionisation coronal lines([Fe xiv ] 5303 Å and [Fe x ] 6375 Å, with ionisation potentials of392 and 262eV respectively). We assume that the [Fe x ] is notblended with the [O i ] 6364 Å emission feature, because the lat-ter is expected to be a third of the intensity of the [O i ] 6300 Åemission (e.g. Pelat et al. 1987), which is not detected.The FLOYDS spectra (Fig. 9) show no major evolution inthe Balmer emission line profiles, and show the broad emissionfeature around 4680 Å from 2020-05-10 (for epochs with su ffi -ciently high S / N ratios in the blue wavelength range), which wasreported to the TNS (and first identified) in Trakhtenbrot et al.(2020).
For the two higher resolution spectra (NOT and WiFeS), theregion around the main observed emission lines is fitted sepa-rately (H γ, < λ < ii , 4500Å < λ < β, < λ < α, < λ < ii ] dou-blet, 6650Å < λ < ± iii ] 5007 Å, [Fe x ] 6375 Å). Each emission line complex ismodelled with multiple Gaussians (an overview of these is pre-sented in Table A.2), and each complex is fitted independentlyof the others. For all spectral fits, we assume a flat continuumcomponent during the fitting process, and run our model fittingusing the region slice sampler option within UltraNest. Spectralfits for the NOT and WiFeS spectra are shown in Figs. 10 and11, whilst the spectral fit results are listed in Tables 6, 7, and 8. Article number, page 7 of 19 & A proofs: manuscript no. at2019avd Observed wavelength [nm]10 F [ m Jy ] Model spectrumArchival observed fluxesUVOT: 2020-05-19UVOT: 2020-05-25UVOT: 2020-09-16
Fig. 7.
Spectral energy distribution of the host galaxy of AT 2019avd compiled from archival GALEX, SDSS, UKIDSS, and ALLWISE photometry,with the best-fit model shown as a red solid line. The three epochs of
Swift
UVOT photometry where all filters were used are also plotted.AT 2019avd shows a ∼ Table 6.
Emission line ratios relative to [O iii ] 5007Å, where the inferred [O iii ] 5007Å flux in each spectrum is 1 . ± . × − erg cm − s − and 4 . ± . × − erg cm − s − . The two spectra were obtained with di ff erent slit widths and orientations, and have not been calibrated withindependent photometric measurements, hence the line ratios relative to [O iii ] 5007Å reported here. A dashed entry indicates that a given emissionline was not clearly detected in the optical spectral fitting. Date N iii ii β H α [N ii ] (6549 + ii ] (6716 + + − + − + − . + . − . + − + − + − + − + − . + . − . Table 7.
Emission line ratios from the WiFeS spectrum, where the nar-row components were resolved. The superscript ‘b’ and ‘n’ denote thebroad and narrow components, respectively.
Line 1, Line 2 F(Line 1) / F(Line 2)H α n , H β n . ± . α b , H β b . ± . ii β b . ± . iii β b . ± . x ], [O iii ] 5007 2 . ± . xiv ], [O iii ] 5007 3 . ± . From the best-fitting spectral models, we infer a broad Balmerdecrement, F (H α b ) / F (H β b ), of 3.4 in the WiFeS spectrum (weuse superscripts ‘b’ and ‘n’ to refer to the broad and narrow com-ponents of a given emission line when such are clearly detected).Such a decrement is consistent with what is observed in AGNs(e.g. Dong et al. 2005, 2007; Baron et al. 2016), and is slightlyhigher than the predicted value of around 2.74-2.86 for caseB recombination (Baker & Menzel 1938) and thus a photoioni-sation origin. Whilst it was originally thought that the observed The predicted value is dependent on the assumed gas density andtemperature.
Table 8.
Line widths inferred from the WiFeS spectrum.
Line FWHM [km s − ]N iii ± ii ± β n ± β b ± iii ] 5007 384 ± xiv ] 5303 1558 ± x ] 6375 768 ± α n ± α b ± ii ] 6549 319 ± ± E ( B − V ) ∼ .
17 and 0 .
65 mag from the
Article number, page 8 of 19. Malyali et al.: AT 2019avd: A novel addition to the diverse population of nuclear transients Å ]0.00.51.01.52.02.53.0 N o r m a li s ed F HH [ O III]
H H e II N III [ O III] [ O III] [ F e X I V ] Å ]02468 N o r m a li s ed F H e I [ N II] H [ N II] [ F e X ] [ S II] [ S II]
Fig. 8.
Comparison of NOT and WiFeS spectra (black and blue respectively). The top panel shows the wavelength range 3800-5450 Å, while thebottom panel shows the 5600-6800 Å range. The most notable changes are (a) the emergence of the broad emission feature around rest-framewavelength 4686 Å and (b) an increase in intensity of the high-ionisation coronal Fe lines ( ∼ ii ] doublet at 6716 and6731 Å. Neither are shown corrected for Galactic extinction. The NOT spectrum was normalised by its continuum flux in the 5100-5200 Å range(rest frame), whilst the blue and red arms of the WiFeS spectra were normalised in the 5100-5200 Å and 6400-6450 Å ranges respectively (restframe). broad and narrow Balmer emission lines respectively (using theCalzetti et al. 2000 extinction law) . We note that the E(B-V)inferred from the Balmer decrement is larger than that inferredfrom SED fitting, which was performed on photometry that in-cluded light emitted from a larger region in the host galaxy thanthat probed by the Balmer decrement analysis. Both the FLOYDS and the WiFeS spectra show the emergenceof a broad emission feature around 4680 Å, which is likely ablend of He ii iii ii emission in AGNs, we disfavour an Fe ii originhere on the basis of no strong Fe ii bump being observed from the Alternatively, the inferred E ( B − V ) values are 0.10 and 0.59 if weassume that the intrinsic Balmer decrement is 3.06 as in Dong et al.(2007). strongest Fe ii transitions in the 4500-4600 Å or ∼ ∼ − , the He ii emission in AT 2019avdis much stronger relative to the Balmer emission in the AGNcomposite.The N iii ii Ly α photons at 303.783 Å areproduced after recombination of He ++ , and can then eitherescape, ionise neutral H or He, or, because of the wavelengthcoincidence of O iii iii . If the latter happens, then the later decay of the excitedO iii can produce a cascade of emission lines escaping the region(e.g. 3047, 3133, 3312, 3341, 3444, and 3760 Å ), and eventu- The He ii ionisation potential is 54.4 eV. Unfortunately, our spectra do not cover the 3000-4000 Å range todetect the other O iii
Bowen lines. Article number, page 9 of 19 & A proofs: manuscript no. at2019avd Å ]0.51.01.52.0 N o r m a li s ed F + c on s t an t Å ]0246 N o r m a li s ed F + c on s t an t Fig. 9.
Evolution of the Bowen + H β (top) and H α (bottom) Balmeremission lines observed through the five epochs of FLOYDS spec-troscopy. Grey dashed lines match those in Fig. 8. Epochs 2020-05-31and 2020-06-06 were of low S / N in the blue wavelength range, and thusare omitted from the plot here. The minor evolution of the H α peak po-sition over the FLOYDS spectra was deemed to be most likely due toaperture-related e ff ects during observations. ally a FUV O iii iii , which further triggers a cascadeof emission lines (N iii / soft-X-ray photons in order toproduce the He ii Ly α photons.We measure relative line intensities of F (He ii ) / F (H β b ) ∼ . F (N iii / F (He ii ) ∼ .
65 and F (N iii / F (H β b ) ∼ .
37. Netzer et al. (1985) predicted the relative Bowen line in-tensities in AGNs under a range of di ff erent metal gas den-sities and abundances, where they found that to produce thehigh F (He ii ) / F (H β b ) ratios seen in AT 2019avd as well asthe high observed F (N iii / F (H β b ) ratio, the gas produc-ing the Bowen fluorescence must have very high density ( n H > . cm − ) and high N and O abundances relative to cosmicabundances. From the line fitting seen on the WiFeS spectrum in Fig. 11, weinfer the luminosities of the [Fe x ] 6375 Å and [Fe xiv ] 5303 Åemission lines to be ∼ × and ∼ × erg s − . We alsoinfer relative intensities of F ([Fe x ] 6375) / F ([O iii ] 5007) ∼ . F ([Fe xiv ] 6375) / F ([O iii ] 5007) ∼
3. Based on the coronalline ratio definitions proposed in Wang et al. (2012), AT 2019avdis classified as an extreme coronal line emitter (ECLE), whereextreme is defined relative to the line ratios seen in coronalline AGNs (e.g. Nagao et al. 2000 report a maximum line ra-tio for F ([Fe X] 6375) / F ([O iii ] 5007) of 0.24 over a sample of 124 Seyferts). Also, given the non-detected set of [Fe vii ] emis-sion lines in AT 2019avd which are seen in some ECLEs, andrelatively weak [O iii ] 5007 Å emission, AT 2019avd belongs tothe subset of ECLEs that were designated as TDEs in Wang et al.(2012).The Fe coronal lines are narrower relative to the He ii andN iii xiv ] 5303 Å and [Fe x ] 6375 Å of 1560 ±
140 and 770 ±
40 km s − respectively. Under the assumption that the line widthsare set by the virial motion of the gas, this suggests that thecoronal lines are produced further away from the BH than theBowen lines, and also with the higher ionisation coronal linesbeing produced closer to the BH than the lower ionisation lines.The width of [Fe xiv ] 5303 Å is comparable to the observedBalmer emission. We also note the di ff ering line profiles of the[Fe xiv ] 5303 Å and [Fe x ] 6375 Å emission, with the lattershowing a stronger blue asymmetry (Fig. 11).As discussed in Wang et al. (2012), the weakness of [Fe vii ]emission relative to [Fe x ] and [Fe xiv ] may be explained throughthe coronal line gas either being overionised under a high X-rayflux, or due to collisional de-excitation of [Fe vii ], because it hasa lower critical density ( ∼ cm − ) compared with the higherionisation lines ( ∼ cm − , Korista & Ferland 1989). We assume that the gas that produces the broad H β emission isvirialised around the SMBH at the centre of the galaxy, and usethe ‘single epoch’ mass-estimation technique (e.g. Vestergaard& Peterson 2006) to infer the black hole mass using the follow-ing scaling relation from Assef et al. (2011):log (cid:32) M BH M (cid:12) (cid:33) = A + B log (cid:32) λ L λ erg s − (cid:33) + C log (cid:32) FWHMkm s − (cid:33) , (1)with A = . B = .
52 and C =
2. From the measuredFWHM of the broad H β component 1420 km s − and L = λ L λ (5000Å) ∼ × erg s − from the WiFeS spectrum , wethen infer log[ M BH / M (cid:12) ] ∼ .
3, albeit with a large uncertainty of ∼ From the fitting of the WiFeS spectrum, we infer line flux ra-tios of log[[N II] 6583 / H α n ] = − . + . − . and log[[O iii ]5007 / H β n ] = . + . − . . According to a Baldwin, Phillips, andTerlevich (BPT) line diagnostic test (Baldwin et al. 1981), suchline ratios suggest that a blend of star formation and AGN ac-tivity is responsible for producing the narrow line emission inthe host galaxy of AT 2019avd (Kau ff mann et al. 2003; Kewleyet al. 2006). Without an archival spectrum though, it is unclearwhether the [O iii ] 5007 Å and [N ii ] 6583 Å lines have increasedin intensity since the initial ZTF outburst, or an AGN-like ionis-ing source has always been present. L λ (5100Å) is computed from the mean of L λ between 5095 and5105 ÅArticle number, page 10 of 19. Malyali et al.: AT 2019avd: A novel addition to the diverse population of nuclear transients N o r m a li s ed F F Å ]0.000.250.500.751.00 N o r m a li s ed F F Å ] Fig. 10.
Zoomed-in plots of the main emission lines observed in both the NOT and WiFeS spectra (top and bottom panels respectively). The blackline is the observed flux density, and the grey error bars are the associated uncertainties. We plot our fitted spectral model to the data for each regionin red (including background component), whilst the blue and orange lines along the bottom represent the contribution of each source componentto the fit (further described in Table A.2). The lower plots in each panel show the residuals in the spectral fitting, where δ F λ is the di ff erencebetween the observed F λ and the model F λ , normalised by the model F λ . We note that the double peaked appearance of the He ii emission line inthe WiFeS spectrum is most likely non-physical and due to the noisy optical spectrum, as no other broad lines show such similar line profiles. Assuming that each observed emission line is broadened due toits virial motion around the central BH, we can use the measuredFWHMs to obtain rough estimates of the distances from the cen-tral ionising source at which each line is produced (Fig. 12).Similar to previous work (e.g. Korista et al. 1995; Kollatschny2003; Bentz et al. 2010), we also find evidence for a stratifiedBLR, whereby the higher ionisation lines are produced in re-gions closer to the BH.
5. Discussion
Based purely on its X-ray luminosity evolution, AT 2019avdmost likely involves an accreting SMBH at the centre of a galaxy.Whilst the large amplitude X-ray flaring (factor of (cid:38) . − × erg s − (using spec-troscopic z = . If AT 2019avd is related to AGN activity that was not inducedby a TDE (herein referred to simply as AGN ‘activity’ or ‘vari- ability’ ), then the combination of its X-ray and optical lightcurves make it one of the most extreme cases of AGN variabilityobserved to date.It is clear that the X-ray spectrum of AT 2019avd (sec-tion 2.3) is far softer than what is commonly seen in Seyfert 1s;for example, the power-law slope for Swift
OBSID 00013495001was 5 . + . − . , whilst Nandra & Pounds (1994) model the observedpower-law slope distribution with a Gaussian distribution ofmean 1.95 and standard deviation 0.15. However, based on themeasured FWHMs of the broad Balmer emission lines in theoptical spectrum, it would be classified as a NLSy1, and softerspectral indices have also been observed in the NLSy1 popula-tion; a systematic ROSAT study of this by Boller et al. (1996)found power-law slopes of up to ∼
5. NLSy1s are also knownto exhibit rapid, large-amplitude X-ray variability (e.g. Bolleret al. 1996). As the X-ray variability of NLSy1s over longertimescales has not been extensively monitored before, how com-mon AT 2019avd-like X-ray flares are within this population iscurrently unclear. For this reason, the X-ray properties alone can-not be used to state that the observed variability in AT 2019avdwas induced by a TDE.However, AT 2019avd shows a number of features in itsoptical spectrum that are infrequently seen in NLSy1s. First,NLSy1s commonly show strong Fe ii emission (e.g. Rakshitet al. 2017), whereas this is not seen in the WiFeS spectrum,and only a weak Fe ii complex is seen in the NOT spectrumin AT 2019avd. Instead, the most prominent Fe emission weobserve are the transient, ECLE-like higher ionisation coronal As a TDE may transform a quiescent BH into an AGN, the variabilityin BHs induced by TDEs is also just a subset of AGN variability.Article number, page 11 of 19 & A proofs: manuscript no. at2019avd N o r m a li s ed F [Fe XIV]2000 1000 0 1000 2000Velocity [km s ]0.50.60.70.80.91.0 N o r m a li s ed F [Fe X] Fig. 11.
Best-fit single Gaussians (red) to the transient [Fe xiv ] 5303 Å(top) and [Fe x ] 6375 Å (bottom) coronal lines observed in the WiFeSspectrum. The lower ionisation line of the pair, [Fe x ] 6375 Å, is moreasymmetric, its broad base appears slightly blueshifted, and can also befitted by a pair of Gaussians of FWHMs 330 ±
40 km s − and 900 ±
100 km s − (blue line), with F ([Fe x ] 6375) / F ([O iii ] 5007) ∼ . lines of [Fe xiv ] 5303 Å and [Fe x ] 6375 Å in the WiFeS spec-trum. During our spectroscopic follow-up campaign, we also ob-serve the appearance of He ii iii iii ] 5007Å emission line. A likely reason forthis is that the host galaxies of the other flares had persistent,higher luminosity AGNs in them prior to the optical outburst,relative to AT 2019avd. In addition, AT 2019avd’s large ampli-tude, ultra-soft X-ray flare, and its optical light-curve evolutionmake it unique amongst the AT 2017bgt flare class.Finally, we stress that the double-peaked optical variabilityshown by AT 2019avd is unprecedented for a NLSy1, whichwhen combined with its X-ray properties, make AT 2019avdclearly unique relative to all previous examples of AGN vari-ability. Further examples of NLSy1 variability seen during theZTF survey will be presented in a separate publication (Freder-ick et al. 2020). r [pc] I S C O P e r i c en t r e r ad i u s C i r c u l a r i s a t i on r ad i u s B L R r ad i u s I nne r t o r u s r ad i u s r [cm] r [10 pc] H b H b H e II Å [ F e X ] Å [ F e X I V ] Å Fig. 12.
Estimated radii from the BH where di ff erent observed opti-cal emission lines are produced in AT 2019avd, compared with var-ious key physical length scales predicted in the literature (assuminglog[ M BH / M (cid:12) ] = . As AT 2019avd shows a very-large-amplitude, soft-X-ray flarefrom the nucleus of a galaxy that shows no strong signs of priorAGN activity, it appears similar to the predicted observationalsignatures for TDEs (e.g. Rees 1988) and most of the previ-ous X-ray-selected thermal TDE candidates (Bade et al. 1996;Komossa & Bade 1999; Komossa & Greiner 1999; Grupe &Leighly 1999; Greiner et al. 2000; Saxton et al. 2019). On theother hand, its optical spectrum shows a far weaker blue contin-uum component relative to that seen in optically selected TDEs,as well as narrower Balmer emission lines (for TDEs where theseare detected); based on these two pieces of evidence, it would bestraightforward to declare that AT 2019avd is not a TDE can-didate, according to criteria for optical TDE selection in vanVelzen et al. (2020).The observed broad Balmer emission lines in AT 2019avd in-stead appear more like those commonly seen in the broad emis-sion lines of Seyfert 1s. With such similarity, a mechanism anal-ogous to the broad line emission in AGNs is likely operating
Article number, page 12 of 19. Malyali et al.: AT 2019avd: A novel addition to the diverse population of nuclear transients Å ]012345678 N o r m a li s ed F + c on s t an t AT2019avdAT2017bgtOGLE17aajF01004-2237
Fig. 13.
Comparison of the optical spectrum of AT 2019avd with thoseof the three nuclear transients recently identified as a new class of flaresfrom accreting SMBHs in Trakhtenbrot et al. (2019b). The two dashedgrey lines mark the positions of N iii ii F (He ii / F (H β ), and at least one Bowen emis-sion line (N iii in AT 2019avd, whereby the line widths of hydrogen recombi-nation lines are set by the gas kinematics (whereas some TDEsmay have line widths set by repeated non-coherent electron scat-tering; e.g. Roth & Kasen 2018), and the high densities in theBLR result in the line intensity responding e ff ectively instanta-neously to changes in the continuum flux. In the limit of a weakTDE-like reprocessing layer , the optical spectrum of a TDEmay appear similar to that of an AGN, as has been previouslysuggested (e.g. Gaskell & Rojas Lobos 2014). The timescalesfor the evolution of the spectral features in such systems maybe di ff erent from those observed in optically selected TDEs, asthey originate from a region further away from the BH than thereprocessing layer.The optical emission mechanism in TDEs is currently notwell understood, although it is thought to arise either fromshocks produced from stellar debris stream self-intersections(Shiokawa et al. 2015; Piran et al. 2015), or from debris repro-cessing the emission from an accretion disc (e.g. Loeb & Ulmer1997; Ulmer et al. 1998; Roth et al. 2016; Roth & Kasen 2018).However, it is unclear how luminous the shocks are from streamself-intersections, whilst for the reprocessing scenario we still donot understand where the reprocessor is situated, where it forms,how large its covering angle would be from the BH, how e ffi -ciently it converts disc emission into the optical wavebands, orhow all of these aspects are a ff ected by the properties of the BHand those of the disrupted star. There is currently not a large And likely a lack of optically-selected observed TDE features. enough sample of TDEs selected through both
X-ray and opticalsurveys to test these various models of optical emission, and toproperly assess the various complex underlying selection e ff ectslikely present in the existing TDE candidate population. A keyexample of these e ff ects is the fact that only a small fraction ofoptically selected TDEs show transient X-ray emission ( ∼ , we cannot rule out aTDE-related origin for AT 2019avd simply on the basis of a lackof optically selected TDE features in the optical spectrum. How-ever, we do disfavour the canonical TDE interpretation (seen inoptically selected TDEs) for this flare on the basis of the double-peaked optical light curve, which has not been observed in any ofthe TDEs identified by ZTF so far. Secondary maxima have pre-viously been seen in the light curves of some TDE candidates (acompilation is presented in Fig. 8 of Wevers et al. 2019), thoughnot at optical wavelengths and of far smaller amplitude increasecompared with AT 2019avd (with the exception of the TDE inan AGN candidate in Merloni et al. 2015). A large fraction of stars may exist in binary systems (e.g. Lada2006). Mandel & Levin (2015) studied the various outcomes ofa binary star passing close to a SMBH from a nearly radial orbit.In ∼
20% of such approaches, a double tidal disruption event(DTDE) is produced, whereby both stars in the binary are dis-rupted in succession. These latter authors estimated that ∼
10% ofall stellar tidal disruptions could be associated with DTDEs, withsuch events expected to produce double-peaked light curves.We can use the inferred rise-to-peak timescales from the ZTFlight curves to test the feasibility of whether AT 2019avd mayhave been triggered by a DTDE, specifically for the case whereeach peak is associated with the rise to peak mass fallback ofeach successive disruption. Guillochon & Ramirez-Ruiz (2013)present the time taken for a single TDE to reach peak mass fall-back rate (in their equation A2): t peak = B γ (cid:32) M BH M (cid:12) (cid:33) / (cid:32) M (cid:63) M (cid:12) (cid:33) − (cid:32) R (cid:63) R (cid:12) (cid:33) / years , (2)where B γ is a function of β , the ratio of the tidal radius of the BHto the pericentre of the orbit of the star, γ is the polytropic indexof the star , M BH is the black hole mass, and M (cid:63) and R (cid:63) are themass and radius of the star being disrupted.Similarly to Merloni et al. (2015), we then generate a grid of M (cid:63) and β , log-uniformly between (0.1 M (cid:12) , 100 M (cid:12) ) and (0.5, 4),respectively, and compute R (cid:63) for each M (cid:63) using the mass–radiusrelationship for zero-age main sequence stars presented in Toutet al. (1996). For each possible combination of M (cid:63) and β , and fora black hole with log[ M BH / M (cid:12) ] ∼ .
3, we check whether it canproduce t peak (using equation 2) within 20% of the observed peaktimescales in the ZTF light curves ( ∼
24 days and ∼
260 daysfor the first and second peak respectively). We also enforce the Although the 4 X-ray bright TDEs in van Velzen et al. (2020) weremonitored at a high cadence with ZTF and
Swift
UVOT, these wereoptically-selected TDEs. We use γ = / . M (cid:12) < M (cid:63) < M (cid:12) , and γ = / M (cid:63) outside this range, as in Mockler et al. 2019.Article number, page 13 of 19 & A proofs: manuscript no. at2019avd M [ M ] Fig. 14.
Constraints on the M (cid:63) , β parameter space, obtained for ex-plaining the origin of AT 2019avd as a DTDE on SMBH. Red markersrepresent a permitted M (cid:63) , β configuration, whilst a region that containsgrey hashing represents a configuration that is not able to reproducethe observed timescales for the given peak. Results were obtained fora black hole with log[ M BH / M (cid:12) ] ∼ .
3. Since there are no red markerson the second optical peak plot, there is no permitted M (cid:63) , β pairing thatcan reproduce the observed peak timescale for the second optical peak.The black dashed lines bound 0 . M (cid:12) < M (cid:63) < M (cid:12) , where we adopt γ = / constraint that its tidal radius lies outside of the Schwarzschildradius for the system, so that it can produce a TDE with the starbeing swallowed whole by the black hole.We plot the permitted regions of the M (cid:63) , β parameter spacein red in Fig. 14, where we see that no main sequence binary starconfiguration can reproduce the observed rise times for both thefirst and second peaks. It would also be possible to obtain fur-ther constraints on the feasibility of this scenario based on theobserved peak luminosities (similar to Merloni et al. 2015) andtheir ratio, as well as from the inferred properties of the binaryitself, such as from the time between the two observed peaks(which could be used to constrain the semi-major axis) and theinferred mass ratio. However, the constraints provided from t peak are perhaps the simplest to implement and are su ffi cient to high-light the caveats of a simple DTDE interpretation.Bonnerot & Rossi (2019) recently suggested that followingthe disruption of a stellar binary, the two separate debris streamsmay collide prior to their fallback onto the black hole. These col-lisions then shock-heat the gas, and were predicted to producean optical flare prior to the main flare of the disruption event.Such a model for a binary TDE could potentially explain theobserved double-peak light curve, and the observed emergenceof the Bowen feature after the second peak (the soft X-rays canonly be emitted once the accretion disc has formed). However,a caveat to this interpretation is that both a strong ionising fluxand high gas densities are required for Bowen fluorescence tobe produced, and we cannot confidently state here that the rea-son for not observing Bowen lines in the NOT spectrum is theabsence of an X-ray-emitting accretion disc during that observa-tion, because the absence of Bowen lines may also be due to in-su ffi ciently high gas densities (not all TDEs that are X-ray brighthave displayed Bowen emission lines). We do not rule out thismore complex DTDE scenario for AT 2019avd here, but do notperform a detailed comparison between the simulations in Bon-nerot & Rossi (2019) and AT 2019avd in the present paper. An-other alternative could be that AT 2019avd involved some type ofTDE about a SMBH binary (e.g. Liu et al. 2009; Coughlin et al.2017), where in such systems, the presence of the secondary BH can perturb the accretion flow onto the primary, leading to inter-mittent light curves. The spectra of Type IIn SNe can appear similar to those of AGNs(e.g. Filippenko 1989), as they can show broad and narrow emis-sion lines, an absence of P-Cygni profiles, and higher luminosi-ties and slower decay timescales relative to normal Type II SNe(Nyholm et al. 2020). Type IIn SNe typically also show the high-est X-ray luminosities amongst all SNe. However, AT 2019avdhas a L . − that is about an order of magnitude higher thanwhat is seen in most X-ray-luminous Type IIn SNe, when con-sidering the sample of IIn shown in Fig. 3 of Dwarkadas &Gruszko (2012). Furthermore, the X-ray emission from Type IInSNe is predicted to be hard (e.g. Ofek et al. 2013), whilst thatof AT 2019avd is ultra-soft. Based on the X-ray emission alone,we disfavour the idea that both optical peaks in AT 2019avd arerelated to a single Type IIn supernova.Given the observed peak and decay timescales (Fig. 6), thepeak absolute magnitude of the optical light curve ( ∼ − . ii ,Bowen, and coronal lines are not seen in the NOT spectrum,and only in the spectra taken after the second peak. However,the probability of observing both a Type IIn SN and an AGN‘turn on’ event within just over a year of each other is extremelysmall given the apparent rarity of extreme ‘turn-on’ events inAGNs (especially those showing an AT 2019avd-like X-ray out-burst) and the expected detection rates for Type IIn SNe (e.g.Feindt et al. 2019), and we therefore disfavour a scenario whereAT 2019avd is the chance coincidence of a Type IIn SN and ex-treme AGN ignition event within roughly one year of each other.
6. Conclusions
This paper presents an overview of a set of multi-wavelengthobservations of an exceptional nuclear transient, AT 2019avd,whose main observed features are as follows:1. eROSITA detected an ultra-soft ( kT ∼
85 eV) X-ray bright-ening ( (cid:38)
90 times brighter than a previous 3 σ upper fluxlimit) from a previously X-ray-inactive galaxy (Section 2).2. AT 2019avd was initially observed on a weekly basis with Swift
XRT / UVOT for 6 weeks following the eROSITA detec-tion. The host had brightened in all UVOT bands by ∼ GALEX observations, and was observedwith 0 . − Swift observation ∼ . − . − at least
600 relative to the 3 σ upper detection limit derived from an XMM-Newton pointing in 2015.3. In the 450 days prior to the eROSITA detection, ZTF ob-served a double-peaked light curve (Section 3). The first op-tical peak shows rise and decay timescales akin to TDEs andSNe, whilst the rise time of the second peak is more similarto those seen in AGNs. No optical outbursts were detected
Article number, page 14 of 19. Malyali et al.: AT 2019avd: A novel addition to the diverse population of nuclear transients during ASAS-SN observations over the seven years preced-ing the initial outburst seen by ZTF.4. Optical spectroscopic follow-up finds transient He ii emis-sion, Bowen fluorescence lines, and high-ionisation coronallines ([Fe x ] 6375 Å, [Fe xiv ] 5303 Å) in the spectra takenafter the second optical peak, but not in the spectrum taken30 days after the first peak. The presence of such a set oflines requires an intense source of soft X-ray emission andextremely high densities. Broad Balmer emission lines weredetected in spectra 30 days after the first peak in the ZTFlight curve, as well as in all spectra taken in the weeks afterthe eROSITA detection with FWHM ∼ − (Sec-tion 4).AT 2019avd thus shows a set of observed features whichhave never been observed together in the same nuclear transientbefore, and further complicates the non-trivial task of distin-guishing the physical origin of large-amplitude variability seenin galactic nuclei. Whilst a discussion on the potential originsof this transient is presented in Section 5, it is still unclear whathas triggered such exotic behaviour. Detailed simulations wouldbe welcome to distinguish between the various possible scenar-ios. These will be well complimented with future planned obser-vations ( Swift , NICER , XMM-Newton ) monitoring the late-timeevolution of AT 2019avd. Finally, we note that during its eightsuccessive all-sky surveys in the following years, eROSITA willsystematically monitor the X-ray variability of AGNs and mapout the population of nuclear transients. With this information,we will be able to better understand the extent of the X-ray vari-ability shown by AT 2019avd, and make a more informed judge-ment on the origin of this transient.
Acknowledgements.
We thank the anonymous referee, and the journal editor,Sergio Campana, for constructive comments which helped improve this paper.AM thanks the Yukawa Institute for Theoretical Physics at Kyoto University,where discussions held during the YITP workshop YITP-T-19-07 on Interna-tional Molecule-type Workshop "Tidal Disruption Events: General RelativisticTransients” were useful to complete this work. AM thanks Mariuz Gromadzki,Giorgos Leloudas and Clive Tadhunter for sharing optical spectra. A.M. ac-knowledges support from and participation in the International Max-Planck Re-search School (IMPRS) on Astrophysics at the Ludwig-Maximilians Universityof Munich (LMU). BJS is supported by NSF grant AST-1907570. BJS is alsosupported by NASA grant 80NSSC19K1717 and NSF grants AST-1920392 andAST-1911074. BT acknowledges support from the Israel Science Foundation(grant number 1849 /
19) IA is a CIFAR Azrieli Global Scholar in the Gravity andthe Extreme Universe Program and acknowledges support from that program,from the European Research Council (ERC) under the European Union’s Hori-zon 2020 research and innovation program (grant agreement number 852097),from the Israel Science Foundation (grant numbers 2108 /
18 and 2752 / / CaliforniaInstitute of Technology and the University of Arizona. NEOWISE is funded bythe National Aeronautics and Space Administration. We thank the Las CumbresObservatory and its sta ff for its continuing support of the ASAS-SN project.ASAS-SN is supported by the Gordon and Betty Moore Foundation throughgrant GBMF5490 to the Ohio State University, and NSF grants AST-1515927and AST-1908570. Development of ASAS-SN has been supported by NSF grantAST-0908816, the Mt. Cuba Astronomical Foundation, the Center for Cosmol-ogy and AstroParticle Physics at the Ohio State University, the Chinese Academyof Sciences South America Center for Astronomy (CAS- SACA), and the Vil-lum Foundation. The Pan-STARRS1 Surveys (PS1) have been made possiblethrough contributions of the Institute for Astronomy, the University of Hawaii,the Pan-STARRS Project O ffi ce, the Max-Planck Society and its participating in-stitutes, the Max Planck Institute for Astronomy, Heidelberg and the Max PlanckInstitute for Extraterrestrial Physics, Garching, The Johns Hopkins University,Durham University, the University of Edinburgh, Queen’s University Belfast,the Harvard-Smithsonian Center for Astrophysics, the Las Cumbres Observa-tory Global Telescope Network Incorporated, the National Central Universityof Taiwan, the Space Telescope Science Institute, the National Aeronautics andSpace Administration under Grant No. NNX08AR22G issued through the Plan-etary Science Division of the NASA Science Mission Directorate, the NationalScience Foundation under Grant No. AST-1238877, the University of Maryland,and Eotvos Lorand University (ELTE). This work was partially based on obser-vations made with the Nordic Optical Telescope, operated at the Observatorio delRoque de los Muchachos, La Palma, Spain, of the Instituto de Astrofisica de Ca-narias. Some of the data presented here were obtained with ALFOSC, which isprovided by the Instituto de Astrofisica de Andalucia (IAA) under a joint agree-ment with the University of Copenhagen and NOTSA. References
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Table A.1.
Priors adopted in the fitting of the ZTF light curves. The riseand decay timescales are in units of days, whilst t peak is in MJD. F max refers to the maximum observed flux within the given peak. PriorsPeak 1 log[ τ r , g ] ∼ U (0 , log[300]), log[ σ r , g ] ∼ U (0 , log[300])log[ F peak , r ] ∼ U (log[0 . F max , r ] , log[10 F max , r ])log[ F peak , g ] ∼ U (log[0 . F max , g ] , log[10 F max , g ]) t peak ∼ U (58450 , τ r , g ] ∼ U (0 , log[300])log[ F peak , r ] ∼ U (log[0 . F max , r ] , log[10 F max , r ])log[ F peak , g ] ∼ U (log[0 . F max , g ] , log[10 F max , g ]) t peak ∼ U (59000 , Table A.2.
Overview of the varying set of Gaussians used for modellingthe emission lines in the NOT and WiFeS spectra.
Region ComponentsH γ Single Gaussian for each of H γ and [O iii ]4363 Å.He ii Single Gaussian component for each ofHe ii iii ] 4640 Å.H β Broad and narrow Gaussian component.H α Broad and narrow Gaussian component forH α , single Gaussian for each of [N ii ] 6549and 6583 Å.[S ii ] doublet Single Gaussian for each of [S ii ] 6716 and6731 Å.[O iii ]5007 Å,[Fe xiv ]5303 Å,[Fe x ]6375 Å Single Gaussian for each. Appendix A: Optical spectrum and light-curvefitting
In Table A.1, we list the priors adopted in the fitting of the ZTF / SEDM light curves, whilst in Table A.2, we list the priors usedin our fitting of the NOT and WiFeS optical spectra.
Appendix B: Long-term light curve of AT 2019avd
In Fig. B.1, we plot the long-term light curve of AT 2019avd,including the ASAS-SN data. ASAS-SN (Shappee et al. 2014)observed the location of AT 2019avd in V -band from February2012 to November 2018 and in g -band from October 2017 toSeptember 2020 (the time of writing). The V - and g -band ob-servations were reduced using a fully automated pipeline de-tailed in Kochanek et al. (2017) based on the ISIS image sub-traction package (Alard & Lupton 1998; Alard 2000). Duringeach visit, ASAS-SN observed three 90-second dithered imagesthat are then subtracted from a reference image. For the g -bandwe modified the standard pipeline and rebuilt the reference im-age without any images with JD ≥ ff ected by two factors.First, there is a bright nearby star that is not resolved from thehost galaxy in ASAS-SN data and added noise to the subtrac-tions. Second, the location of AT 2019avd is right on the edge of two ASAS-SN fields. To help alleviate these issues and increasethe ASAS-SN limiting magnitude we stacked the subtractionswithin a maximum of 10 days. We then used the IRAF pack-age apphot to perform aperture photometry with a two-pixel, orapproximately 16 . (cid:48)(cid:48)
0, radius aperture on each subtracted image,generating a di ff erential light curve. The photometry was cali-brated using the AAVSO Photometric All-Sky Survey (Hendenet al. 2015). Article number, page 18 of 19. Malyali et al.: AT 2019avd: A novel addition to the diverse population of nuclear transients . . . V e ga M ag n i t ud e W1W256500 57000 57500 58000 58500MJD1718192021 A B M ag n i t ud e ASAS-SN V ASAS-SN g ZTF g ZTF r
Fig. B.1.
Long-term NEOWISE-R, ASAS-SN, and ZTF light curves of AT 2019avd. The early and late black dashed lines mark the 2015