A Family Tree of Optical Transients from Narrow-Line Seyfert 1 Galaxies
Sara Frederick, Suvi Gezari, Matthew J. Graham, Jesper Sollerman, Sjoert van Velzen, Daniel A. Perley, Daniel Stern, Charlotte Ward, Erica Hammerstein, Tiara Hung, Lin Yan, Igor Andreoni, Eric C. Bellm, Dmitry A. Duev, Marek Kowalski, Ashish A. Mahabal, Frank J. Masci, Michael Medford, Ben Rusholme, Richard Walters
DDraft version October 20, 2020
Typeset using L A TEX twocolumn style in AASTeX63
A Family Tree of Optical Transients from Narrow-Line Seyfert 1 Galaxies
Sara Frederick , Suvi Gezari ,
1, 2, 3
Matthew J. Graham , Jesper Sollerman , Sjoert van Velzen , Daniel A. Perley, Daniel Stern , Charlotte Ward, Erica Hammerstein , Tiara Hung , Lin Yan , Igor Andreoni , Eric C. Bellm , Dmitry A. Duev , Marek Kowalski,
13, 14, 15
Ashish A. Mahabal ,
11, 16
Frank J. Masci, Michael Medford ,
18, 19
Ben Rusholme, and Richard Walters Department of Astronomy, University of Maryland, College Park, MD 20742, USA Joint Space-Science Institute, University of Maryland, College Park, MD 20742, USA Space Telescope Science Institute, Baltimore, MD 21218, USA Division of Physics, Mathematics, and Astronomy, California Institute of Technology, Pasadena, CA 91125, USA The Oskar Klein Centre & Department of Astronomy, Stockholm University, AlbaNova, SE-106 91 Stockholm, Sweden Leiden Observatory, Leiden University, P.O. Box 9513,. 2300 RA Leiden, The Netherlands Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Mail Stop 169-221, Pasadena, CA 91109, USA Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064, USA Caltech Optical Observatories, California Institute of Technology, Pasadena, CA 91125, USA Division of Physics, Mathematics and Astronomy, California Institute of Technology, Pasadena, CA 91125, USA DIRAC Institute, Department of Astronomy, University of Washington, 3910 15th Avenue NE, Seattle, WA 98195, USA Deutsches Elektronen Synchrotron DESY, Platanenallee 6, 15738 Zeuthen, Germany Institut f¨ur Physik, Humboldt-Universit¨at zu Berlin, D-12489 Berlin, Germany Columbia Astrophysics Laboratory, Columbia University in the City of New York, 550 W 120th St., New York, NY 10027, USA Center for Data Driven Discovery, California Institute of Technology, Pasadena, CA 91125, USA IPAC, California Institute of Technology, 1200 E. California Blvd, Pasadena, CA 91125, USA University of California, Berkeley, Department of Astronomy, Berkeley, CA 94720, USA Lawrence Berkeley National Laboratory, 1 Cyclotron Rd., Berkeley, CA 94720, USA
Submitted to ApJABSTRACTThe Zwicky Transient Facility (ZTF) has discovered five new events belonging to an emerging classof AGN undergoing smooth flares with large amplitudes and rapid rise times. This sample consists ofseveral transients that were initially classified as supernovae with narrow spectral lines. However, uponcloser inspection, all of the host galaxies display resolved Balmer lines characteristic of a narrow-lineSeyfert 1 (NLSy1) galaxy. The transient events are long-lived, over 400 days on average. We reportUV and X-ray follow-up of the flares and observe persistent UV-bright emission, with two of the fivetransients detected with luminous X-ray emission, ruling out a supernova interpretation. We comparethe properties of this sample to previously reported flaring NLSy1 galaxies, and find that they fallinto three spectroscopic categories: Transients with 1) Balmer line profiles and Fe II complexes typicalof NLSy1s, 2) strong He II profiles, and 3) He II profiles including Bowen fluorescence features. Thelatter are members of the growing class of AGN flares attributed to enhanced accretion reported byTrakhtenbrot et al. (2019). We consider physical interpretations in the context of related transientsfrom the literature. For example, two of the sources show high amplitude rebrightening in the optical,ruling out a simple tidal disruption event scenario for those transients. We conclude that three of thesample belong to the Trakhtenbrot et al. (2019) class, and two are TDEs in NLSy1s. We also aim tounderstand why NLSy1s are preferentially the sites of such rapid enhanced flaring activity.
Corresponding author: Sara [email protected] a r X i v : . [ a s t r o - ph . H E ] O c t Frederick et al.
Keywords: accretion, accretion disks — galaxies: active — galaxies: nuclei — quasars: emission lines— relativistic processes — surveys INTRODUCTIONA galaxy center hosting an active galactic nucleus(AGN) is dominated by its continuum emission. There-fore, a flare originating from this nuclear region requiresa distinctly powerful event to be detectable above thisstochastically variable continuum. A small number ofrapid , smoothly evolving flares have been observed tobe associated with AGN (e.g. Drake et al. 2011; Blan-chard et al. 2017), with few known mechanisms that cancause these events to occur.Intrinsic UV/optical flares, such as those due to en-hanced accretion onto the central supermassive blackhole (SMBH) in the form of gaseous material or starspassing too close to the nucleus, have been observed inthe form of: tidal disruption events (e.g. Gezari et al.2012; van Velzen et al. 2020a), UV-bright flaring eventsthat are associated with accretion rate changes (Trakht-enbrot et al. 2019a), transients with double peaked lineprofiles linked to accretion disk emission (e.g. Halpern &Eracleous 1994), or changing-look AGN — the dramaticchange in spectroscopic AGN classification following arise in continuum level, thought to be connected to un-stable changes in accretion state (e.g. LaMassa et al.2015; Runnoe et al. 2016; MacLeod et al. 2016; Ruanet al. 2016; Stern et al. 2018; Ross et al. 2018; Trakht-enbrot et al. 2019b; Frederick et al. 2019; Graham et al.2020).Phenomena extrinsic to the SMBH accretion engine,such as microlensing of a quasar by a foreground Galac-tic source (e.g. Lawrence et al. 2012) or slowly evolvingsuper-luminous supernova (SLSN) explosions, have alsobeen observed to cause smooth large-amplitude flaresfrom galaxies with AGN (Graham et al. 2017). In rarecases these can be astrometrically indistinguishable fromthe galactic nucleus, and therefore it becomes difficult todiscern whether an explosive disruption to the accretionflow has occurred, and to differentiate this from AGNvariability (Terlevich et al. 1992).Multiwavelength approaches are required to disentan-gle this diverse family of observed flaring behaviors fromAGN. In the golden era of time domain astronomy, evenwith many multichromatic instruments trained on thesky, a number of newly-discovered objects continue todefy placement into a clear-cut observational category.In order of discovery, we present a photometric classcomprised of five rapid flares with similar smooth lightcurve shapes occurring in a subclass of AGN observedby the Zwicky Transient Facility (ZTF) survey: We refer to flare timescales as “rapid” when they occur on weekto month timescales. a) ZTF19aailpwl/AT2019brs ( z = 0 . z = 0 . z = 0 . z = 0 . z = 0 . H = 70 kms − Mpc − , Ω Λ = 0.73 and Ω M = 0.27. OBSERVATIONSThe Zwicky Transient Facility Survey (Bellm et al.2019a; Graham et al. 2019) is comprised of the auto-mated Palomar 48-inch Samuel Oschin Telescope (P48)as well as the Palomar 60-inch SED Machine (P60SEDM; Blagorodnova et al. 2018; Rigault et al. 2019),and has surveyed the Northern Sky with g - and r -bandfilters with a 3-night cadence since 2018 (Bellm et al.2019b). At least 15 images meeting good quality criteriawere stacked to build a coadded reference image of eachobserving field and quadrant in each filter band. Sci-ence images are subtracted by their references and pro-cessed each night by the Infrared Processing and Anal-ysis Center (IPAC) pipeline (Masci et al. 2019). Thecandidate transient alert stream (Patterson et al. 2019)is distributed by the University of Washington Kafkasystem, and filtered through the AGN and black holesScience Working Group’s Nuclear Transients parame-ter criteria (outlined in van Velzen et al. 2019, 2020b)by the Ampel broker (Nordin et al. 2019; Soumagnac &Ofek 2018), with the GROWTH Marshal user interfaceutilized for the coordination of follow-up efforts (Kasli-wal et al. 2019).All 5 transients included in the sample presented herewere selected based on the following criteria: large am-plitude, nuclear variability (∆ g > A nuclear transient was defined as that within 0.5” of the ref-erence galaxy center. Over 9000 nuclear transients passed thisfilter and were ranked during ZTF Phase I, of which 27 wereTDEs, over 7% were classified as SN, and over half were AGN orcandidate AGN. amily Tree of NLSy1 Transients (cid:48)(cid:48) of the center ofthe host galaxy in the reference image) with follow-upor pre-flare spectra consistent with an AGN classifica-tion. This selection was not systematic (and thereforenot complete), but rather the result of ongoing intersect-ing and collaborative searches for changing look AGN(Frederick et al. 2019), TDEs (van Velzen et al. 2019,2020b), and superluminous supernovae (Lunnan et al.2019; Yan et al. 2020) relying on partial human vettingfrom the ZTF transient alert stream, from which thissample emerged as more examples were collected. Asystematic search for NLSy1 transients in ZTF will bethe focus of a future study.2.1.
Optical Photometry
All transients in the sample were detected pre-peakusing ZTF difference imaging photometry. The smoothlight curve shapes (with scatter ∆ g < . g band unless otherwise noted. An anal-ysis of the rise times to peak are measured and reportedin Section 3.1.1. We report the g -band magnitude-weighted offsets for each transient, calculated usingEquation 3 in van Velzen et al. (2019). ZTF forced pho-tometry for the sample is shown in Figure 10 of theAppendix. ZTF19aailpwl — (RA=14:27:46.41, Dec=+29:30:38.6,J2000.0) was first detected on 2019 Feb 08 as a nucleartransient within 0 . (cid:48)(cid:48)
17 of the host galaxy center. The hostgalaxy displayed some variability at the < ZTF19abvgxrq — (RA=04:29:22.72, Dec=+00:37:07.6,J2000.0), also known as Gaia19eby, was first detectedon 2019 Sept 04 as a nuclear transient within 0 . (cid:48)(cid:48)
15 ofthe host galaxy center. ATLAS, Gaia, and PanSTARRsalso reported observations of this source on the Tran-sient Name Server (TNS) with discovery dates of 2019Sep 04, 2019 Sep 13, and 2019 Sep 26, respectively. Thehost galaxy displayed no variability above the 0.5 maglevel in CRTS.
ZTF19aatubsj — (RA= 17:09:06.86, Dec=+26:51:20.7,J2000.0) was detected on 2019 Apr 27 with a signifi-cant flux increase with respect to the reference imageand with an offset from the nucleus of its host of 0 . (cid:48)(cid:48) ZTF19abvgxrq passed the ZTF TDE working group’s tidal dis-ruption event criteria, and was given the nickname “StannisBaratheon” for ease of discussion. When it was found to beamong a class of AGN-associated objects serendipitously de-tected by ZTF, the other sources in the class were retroactivelygiven the names of other Game of Thrones characters in thesame Great House - and collectively referred to fondly as “TheBaratheons”, whose motto is, appropriately, “Ours is the Fury”. the 2 mag level in V-band CRTS data from 2009 to2013 (variability which was not observed in ZTF forcedphotometry prior to the transient).
ZTF19aaiqmgl — (RA=08:23:36.77, Dec=+04:23:02.5,J2000.0), also known as eRASSt J082337+042303 , wasdetected by ZTF beginning on 2019 Feb 09 within 0 . (cid:48)(cid:48) ZTF18abjjkeo — (RA=11:07:42.91, Dec=+74:38:02.0,J2000.0) was detected beginning on 2020 Apr 05 within0 . (cid:48)(cid:48)
02 of its host galaxy center. The ZTF forced pho-tometry for this source shows no variability above thelevel of the galaxy for >
400 days. The host galaxy ofZTF18abjjkeo was beyond the survey limits of CRTS.2.2.
Optical Spectroscopy
All spectroscopic follow-up observations for the sam-ple are summarized in Table 1, and each epoch is shownin Figure 11 of the Appendix. The phases of the op-tical follow-up spectra with respect to the features inthe ZTF light curves are annotated on Figure 1. Alltransients in this sample have spectral characteristics ofNLSy1 galaxies, i.e. strong Balmer line emission withFWHM < − , along with other spectral fea-tures which are highlighted below and explored in detailin Section 3.2. We reduced Palomar 60” SED Machine(P60/SEDM; Program PIs: Gezari, Sollerman, Kulka-rni) spectra with pysedm (Rigault et al. 2019), and allother spectra with pyraf using standard procedures. ZTF19aailpwl — showed a striking difference to theSDSS spectrum showing it was a NLSy1 as early as 2006(Abolfathi et al. 2018; Rakshit et al. 2017). The follow-up Folded Low Order whYte-pupil Double-dispersedSpectrograph North (FLOYDS-N; Arcavi et al. 2019 andLowell Discovery Telescope (LDT, formerly DCT; PI:Gezari) spectra showed a steep blue continuum and astrong He II profile with Bowen fluorescence features,indicating it became a flaring SMBH belonging to theobservational class established by Trakhtenbrot et al.(2019a).
ZTF19abvgxrq — was spectroscopically identified asa NLSy1 on 2019 Sept 15 with the Liverpool Tele-scope (LT; PI: Perley) SPectrograph for the Rapid Ac-quisition of Transients (SPRAT), based on the widthof the Balmer emission lines and the strength of the[O III] λ λ λ This was the only source in the sample to be detected by theextended ROentgen Survey with an Imaging Telescope Array(eROSITA, part of the Russian-German ”Spectrum-Roentgen-Gamma” (SRG) mission; Cappelluti et al. 2011), and was giventhe name eRASSt J082337+042303. This X-ray detection coinci-dent with the transient’s host galaxy is described in Section 2.4.
Frederick et al. − − − − − − − A b s o l u t e M ag n i t ud e ZTF18abjjkeo + 5 mag ss ZTF19aailpwl - 2.5 mag s s s
ZTF19aatubsj - 2 mag s sssssssss s ss ss ssssssssss s s ss s
ZTF19aaiqmgl + 1.5 magZTF19abvgxrq + 0.5 mag
Figure 1.
Comparison of the ZTF g - and r -band difference imaging light curve shapes and absolute magnitudes of the sample.ZTF19aatubsj decreases before reaching a second plateau stage, and undergoes significant reddening after the first plateauwhile the others never do. ZTF19abvgxrq rises again symmetrically after decreasing to pre-flare levels, as does ZTF19aaiqmgl.The light curves have been shifted in absolute magnitude space for visual purposes, as indicated alongside the object name.Overlap of the g and r light curves reflects true colors such that the initial colors approach g − r = 0 mag for all transients inthe sample. Observations at other wavelengths are shown in Figure 2. Spectroscopic epochs are labeled for each light curvewith an ‘S’ below ZTF19aatubsj and ZTF18abjjkeo and above the rest. (2019a) objects. Near peak it was observed with Keck10-m Low Resolution Imaging Spectrometer (LRIS; PI:Graham) as well as the LDT Deveny Spectrograph (PI:Gezari) and the KAST Double Spectrograph on the Lick3-m Shane Telescope (PI: Foley), which confirmed thestrong blue continuum and clearly defined and persistentBowen fluorescence features. ZTF19aatubsj — was observed 8 days after peak on2019 Jul 03 with the Double Spectrograph (DBSP) onthe Palomar 200-inch Hale Telescope (P200; PI: Yan).We measured a significant “blue horn” component ofH β and marginally detected He II. The transient con-tinuum of ZTF19aatubsj faded to reveal an underlyingFe II complex in the Nordic Optical Telescope (NOT; PI:Sollerman) spectrum taken nearly 368 days after peakon 2020 Apr 30, with no evidence for He II emission. ZTF19aaiqmgl — The spectrum taken with NOT (PI:Sollerman) on 2019 Mar 15 near the first optical peak showed strong Balmer line emission, no detection of aHe II line complex, and evidence for a Fe II complex,characteristic of NLSy1 galaxies. A follow-up FLOYDS-S spectrum taken 444 days after peak and reported tothe Transient Name Server (TNS) by Trakhtenbrot et al.(2020) showed the appearance of He II and Bowen flu-orescence features and a “blue horn” in H β . Again thisevent was classified as a member of the Trakhtenbrotet al. (2019a) observational class of flaring NLSy1s. ZTF18abjjkeo — In the LT (PI: Perley) spectrum ofZTF18abjjkeo taken on 2020 May 18 8 days after peak,the narrow component of the He II profile is significantlyblueshifted. No Fe II line complex was detected in thespectra of this transient.2.3.
UV Photometry
We triggered target-of-opportunity monitoring obser-vations with the Neil Gehrels
Swift
Telescope (Gehrels amily Tree of NLSy1 Transients Table 1.
Summary of spectroscopic follow-up observations of the sample.Name Obs UT Instrument Exposure (s) ReferenceZTF19aailpwl 2006 Jul 01 SDSS 3000 Abolfathi et al. (2018)2019 Mar 15 FLOYDS-N 3600 Arcavi et al. (2019)2019 Jun 22 LDT Deveny 900 This workZTF19aatubsj 2019 May 25 Palomar 60” SEDM 2250 This work2019 Jun 17 LT SPRAT 900 This work2019 Jun 22 LDT Deveny 900 This work2019 Jul 03 Palomar 200” Hale 600 This work2020 Apr 30 NOT ALFOSC 1750 This workZTF19abvgxrq 2019 Sep 08 Palomar 60” SEDM 2250 This work2019 Sep 15 LT SPRAT 500 This work2019 Sep 22 Palomar 60” SEDM 2250 This work2019 Sep 24 LDT Deveny 600 This work2019 Sep 25 Keck LRIS 300 This work2019 Sep 25 NICER 2000 Kara et al. (2019)2019 Oct 01 Chandra LETG 45400 Miller et al. 20192019 Oct 05 Lick 3-m KAST 1500 This work2019 Oct 12 LT SPRAT 500 This work2019 Oct 15 Chandra LETG 91000 Mathur et al. 20192019 Oct 23 LDT Deveny 900 This work2019 Nov 01 Palomar 60” SEDM 2250 This work2019 Dec 03 LDT Deveny 2400 This work2020 Feb 26 LDT Deveny 2600 This work2020 Jan 30
Swift XRT et al. 2004) for all transients in the sample. Using theHEASOFT command uvotsource we extracted
UVOT photometry within a 5 (cid:48)(cid:48) -radius circular aperture and us-ing an annular background region centered on the coor-dinates of the optical transient.Figure 2 shows the νL ν light curves of all flares inthe sample. We compare ZTF g and r band differenceimaging, WISE difference imaging, Swift XRT moni-toring, and
Swift UVOT detections subtracted by thearchival
Galaxy Evolution Explorer ( GALEX ; Bianchiet al. 2017) All-Sky Imaging Survey (AIS) near-UV(NUV, λ eff = 2310 ˚A) host measurements (measuredwith a 6- (cid:48)(cid:48) radius aperture).We found all transients in the sample to be UV-bright, but with varying UV colors. The UV color ofZTF19aaiqmgl ( U V W − g = − . U V W − g = − . U V W − g = − . − . U V W − g = 0 . X-rays
We found only two transients in the sample tobe X-ray bright in follow-up
Swift XRT observations:ZTF19abvgxrq and ZTF19aaiqmgl. ZTF19aailpwl wasdetected only once, and then only at a low level. Wemeasured an
XRT upper limit of 0.004 counts s − forZTF19aatubsj. X-ray follow-up spectra are reported inTable 1. Swift photometry compared to WISE W1- and
Frederick et al.
W2-band and ZTF g and r -band photometry is shownin Figure 2. The X-ray bright flares in this sample tendto vary in lockstep with the slow UV/optical flares. ZTF19aailpwl — was detected only once in 11 ob-servations during a 16-month monitoring campaign be-tween 2019 Mar 21 and 2020 Jul 7. We measured a3- σ detection of 0.003 counts s − on 18 Apr 2019, justbrighter than the limiting flux. ZTF19abvgxrq — Similar to the UV light curve, theshape of the X-ray flare of ZTF19abvgxrq followed theoptical, from its fade through its second rise (See Fig-ure 2 and Section 3). The unabsorbed 0 . −
10 keV fluxfrom the stacked
XRT spectrum of ZTF19abvgxrq was7 . ± . × − erg cm − s − . ZTF19abvgxrq was pre-viously detected in ROSAT, and NICER observationsshow an increase in flux from this by 100 times (1 × − erg cm − s − ), variable from 11 to 14 counts s − in 3hours (Kara et al. 2019). A 50 ks Chandra LETG grat-ing observation taken just 8 days after peak and reportedby Miller et al. (2019) found a flux consistent with this,with the spectral shape a good fit to a kT = 0 .
24 keVblackbody, and the source variable at the 25% level on2 − . − . . × − erg cm − s − ;their 91 ks Chandra LETG observation was a good fit toa consistent blackbody model and a power law compo-nent typical of AGN with spectral index Γ = 1 .
8, withno intrinsic absorption required.
ZTF19aaiqmgl — was observed only during the sec-ond optical flare (on 2020 Apr 28, 350 days after thefirst ZTF detection), and was the only X-ray brighttransient in the sample with much fainter X-ray νL ν than that of the optical (shown in Figure 2). LikeZTF19abvgxrq, the shape of the X-ray rise followed thatof the second rise. It was detected by eROSITA aseRASSt J082337+042303, a soft X-ray transient con-sistent with the galaxy 2MASX J08233674+0423027(Malyali et al. 2020, Malyali et al. 2020, in prep.) Priorto this, the XMM Slew Survey reported a non-detectionat the location of the host galaxy, with an upper limit of < . × − erg − s − cm − assuming kT bb = 100 eVand N H = 3 × cm − . The SRG flux of 1 . × − erg − s − cm − was 90 times brighter than this upperlimit. No hard X-ray component was detected above2.3 keV. No strong short-term variability on hours-longtimescales was detected, and no strong variability wasdetected between SRG and the 3 Swift XRT monitoringobservations taken afterward with a week-long cadence.Swift and NICER observations over the next 5 monthsshowed an additional increase in X-ray flux by a factorof 10 (Pasham et al. 2020). A careful study of the X-rayproperties of this transient is forthcoming (Malyali et al.2020, in prep.) 2.5. IR Malyali et al. (2020) reported that the WISE colorof ZTF19aaiqmgl was atypically low ( W − W (cid:39) .
07 mag) compared to typical AGN values ( W − W . − . W − W .
45 mag) are also inconsistent with an AGN, thoughnot quite as low as that of ZTF19aaiqmgl. OnlyZTF19aailpwl truly appeared as an AGN in IR, with W − W .
98 mag. The WISE AGN classificationof the sample is summarized in Table 3 in Section 4.A flare in the IR was detected in NeoWISE at the lo-cation of ZTF19aaiqmgl and concurrent with the opticaland X-ray transient. Though the IR flare began muchsooner in 2009, Figure 2 shows that the peak of the flarewas delayed with respect to the first optical peak. Priorto this flare, WISE photometry detected no variabilityat the location of ZTF19aaiqmgl for nearly 5 years. ANALYSIS3.1.
Photometry
The difference imaging light curves for the sample areshown in terms of absolute magnitudes in Figure 3.We show the sample alongside various NLSy1-relatedevents from the literature, which are described in moredetail in Section 4. CSS100217 displayed some variabil-ity prior to the transient, unlike any of the events in thissample. AT2017bgt was observed only during its fade indifference imaging, so we instead show its aperture pho-tometry (from the ASAS-SN Photometry Database ;Jayasinghe et al. 2019) which also shows the rise of thesource. ZTF18aajupnt is by far the least luminous tran-sient shown. We note the similarity of the shapes ofthe light curves of ZTF19aatubsj and PS16dtm, whichis discussed further in Section 4.3.1.1. Light Curve Timescales
We measured the rise-to-peak timescales of the sam-ple by fitting Gaussians to the light curves shown inFigure 3 using the lmfit package. We observe a corre-lation between the luminosity (specifically the absolutemagnitudes M V and M g ) and rise-to-peak timescalesof the sample ( t rise ) with the following relation: M = − . t rise − .
59, shown in Figure 4. Fitting thelight curves with quadratic functions resulted in thesame correlation within the error estimates. Interest-ingly, TDEs also show a positive correlation between risetime to peak and luminosity (van Velzen et al. 2020b).ZTF18aajupnt appears under-luminous for how fast itrises. AT2017bgt was observed only during its fadingphase in difference imaging, and so was excluded fromthis portion of the analysis.3.1.2.
Rebrightening https://asas-sn.osu.edu/photometry amily Tree of NLSy1 Transients . . . . . . l og ν L ν [ e r g s − ] ZTF19aaiqmgl
Swift UVW1Swift UVM2Swift UVW2Swift XRT eROSITA SRGW1W2ZTF r ZTF gZTF iP60 rP60 g 0 50 100 150 200 250 300 350Days Since Rise to Peak42 . . . . . . . . . l og ν L ν [ e r g s − ] ZTF19abvgxrq . . . . . . . l og ν L ν [ e r g s − ] ZTF19aailpwl . . . . . . . . l og ν L ν [ e r g s − ] ZTF19aatubsj . . . . . l og ν L ν [ e r g s − ] ZTF18abjjkeo
Figure 2.
We track the colors of the transients in the sample with a νL ν light curve, comparing the ZTF and WISE data toconcurrent high cadence Swift UVOT and
XRT monitoring observations. The X-ray rise and fade of ZTF19abvgxrq tracks theoptical/UV with no significant delay. We subtracted the host galaxy light as estimated by
GALEX
NUV measurements fromthe
Swift UVOT observations. Times are given in days since first ZTF detection. The X-ray errorbars are comparable to thesize of the data points. See Figure 10 for pre-outburst forced photometry.
Frederick et al.
It is noteable that two sources in the sample,ZTF19abvgxrq and ZTF19aaiqmgl, each have a dra-matic rebrightening episode. Following a flare and anapproximately ∼ Weexplore possible interpretations of this rebrightening inSection 4. 3.1.3.
UV/Optical to X-ray Ratio
We derive the simultaneous UV/optical-to-X-rayspectral slope ratio ( α OX ) from the Swift UVOT and
XRT observations of the sample, (as well as upper lim-its assuming Γ X = 2, when applicable). We com-pute unabsorbed X-ray flux densities at 2 keV us-ing the PIMMS v4.10 web tool . Following Eq. 4of Tananbaum et al. (1979) and Eq. 11 of Grupeet al. (2010), the definition of this ratio is α OX =0 . L /L ). Of the transients detectedin X-rays, the α OX of ZTF19abvgxrq evolves over 150days between 1.1 and 1.4, and ZTF19aailpwl is observedin X-rays during only one epoch with α OX = 1 .
7, equiva-lent to that of the late time detections of ZTF19aaiqmgl.The range of α OX measured for the sample is consistentwith that of NLSy1s (0 . < α OX < .
8; Gallo 2006).3.2.
Spectroscopy
From the FWHM of the broad Balmer emission lines,we classified all sources in the sample as NLSy1s. We fitthe H α and H β line profiles of the host (when available)and transient spectra of the sample with the non-linearleast-squares minimization and curve-fitting routine inthe lmfit Python package. The results of these fitsare shown in Figure 5. Using a Lorentzian profile forthe broad H α component fit provided an improvementof the fit over that of a Gaussian profile, as would beexpected based on studies of NLSy1s (e.g. Niko(cid:32)lajuket al. 2009).We compare the host (when available) and transientspectra of this sample to other transients in NLSy1s inFigures 6 (showing the full wavelength range of the ob-servations) and 7 (rest wavelength 3700 − β line profiles). In Fig-ure 7 we color-code the sample (as well as these knownNLSy1-related transients in the literature) based on theobservational classification scheme we establish in Sec-tion 4.3, named after the features discussed in the fol- We note that ASASSN-15lh showed a large amplitude “double-humped” structure in its UV light curve. https://cxc.harvard.edu/toolkit/pimms.jsp lowing sections: “He II only”, “He II+N III”, and “Fe IIonly” .When compared to the newly discovered flaring eventsto those in the literature, it is clear that AT2017bgt(Trakhtenbrot et al. 2019a) has a much strongerHe II+N III Bowen fluorescence profile, CSS100217(Drake et al. 2009) has stronger narrow emission linesoverall, and ZTF18aajupnt (Frederick et al. 2019) hasa weaker blue continuum. The presence and strengthof Fe II is uncorrelated with other spectroscopic prop-erties of the transients shown. Of the ZTF sam-ple, the transient spectrum of ZTF19aatubsj shows thestrongest Fe II complex. However, ZTF19aatubsj showsno strong He II + Bowen fluorescence features whilethe others in the ZTF sample do. ZTF19aatubsj andZTF19aaiqmgl both show offset blue components of H β .3.2.1. Strong He II profiles in AGN?
In the discovery paper for transient ASASSN-18jd,Neustadt et al. (2020) emphasized the relatively rare na-ture of strong He II emission in AGN in general, notingthe exceptions in the Trakhtenbrot et al. (2019a) obser-vational class of flares as well as the rapid changing-lookAGN event ZTF18aajupnt (Frederick et al. 2019). Astrong He II line profile is common (but not ubiquitous)in the spectra of TDEs, and they are typically accompa-nied by Bowen fluorescence features (e.g. Blagorodnovaet al. 2019; van Velzen et al. 2020b). ZTF19aaiqmgl,ZTF19abvgxrq, ZTF19aailpwl look the most similar toAT2017bgt spectroscopically. They are spectroscopi-cally classified as “He II+N III”-type flares in Figure 7.3.2.2.
The Fe II complex
A strong Fe II line complex (blueward and redwardof H β +[O III] in optical spectra, between 4434 ˚A and5450 ˚A) is a distinguishing feature of NLSy1 galaxies.Reverberation mapping studies of AGN show that theline complex emitting region is measured farther thanthe Balmer line emitting region (e.g. Barth et al. 2013;Rafter et al. 2013). The Fe II complex seen in PS16dtmwas interpreted as evidence of the system being a NLSy1prior to the onset of the flare. CSS100217 also displayeda strong Fe II complex and was interpreted as a SNin a NLSy1 (Drake et al. 2011). TDE AT2018fyk alsoshowed low ionization lines including an Fe II (37,38)emission multiplet emerging for 45 days during the tidaldisruption event, and forms a class of Fe-rich TDEsalong with ASASSN–15oi and PTF–09ge (Wevers et al.2019). Therefore, this feature may indicate the presenceof an AGN, but is not always useful in determining thenature of a particular AGN-related flare. For two ofthe transients in this sample, whether or not the Fe II We note that although “only” is used in the categorization nam-ing based on the presence of spectral features, all have strongBalmer features. amily Tree of NLSy1 Transients
200 0 200 400 600 800 100026242220181614 A b s o l u t e M agn i t ude ZTF19aailpwl ( g ) - 2.5 magZTF19aatubsj ( g ) - 1 magZTF19abvgxrq ( g ) + 0.5 magZTF19aaiqmgl ( g ) + 1.5 magZTF18abjjkeo ( g ) + 5 mag
200 0 200 400 600 800 1000Days since rise to peak24232221201918 A b s o l u t e M agn i t ude AT2017bgt ( V ) + 1.2 magZTF18aajupnt ( g ) - 2.5 magCSS100217 ( V )PS16dtm ( V ) Figure 3.
The difference imaging light curves of the ZTF sample (upper panel) compared to the published light curves ofNLSy1-related events from the literature (lower panel): changing-look LINER ZTF18aajupnt(Frederick et al. 2019), TDE in aNLSy1 PS16dtm (Blanchard et al. 2017), SN in a NLSy1 CSS100217 (Drake et al. 2011), and the aperture photometry of flaringNLSy1 AT2017bgt (Trakhtenbrot et al. 2019a). We show only g − band observations for the ZTF sample (upper panel), andomit errorbars for visual purposes. Note the differences in optical filters shown ( g in green, V in blue), the differences in colorsand markers used to represent the same filters for visual clarity, as well as the difference in y-axis scale between the panels.CRTS data for CSS100217 >
200 days prior to the transient is not shown on this scale, but showed no significant activity for > Frederick et al.
20 40 60 80 100 120Rise to Peak (days)262422201816 P ea k A b s o l u t e M agn i t ude ilpwltubsjbvgxrqiqmgl bjjkeoZTF18aajupntPS16dtmCSS100217 M = -0.04 t rise Figure 4.
Correlation of the rise times of the sample light curves with the maximum absolute magnitude. Fits to light curvesare described in Section 3.1.1. The same color scheme and markers are used as in Figure 3. complex can be seen in optical spectra depends on thephase and the continuum brightness of the transient —for ZTF19aatubsj it was not observed for 368 days, andfor ZTF19aaiqmgl it became no longer visible duringthe second rise 444 days after the initial spectrum wastaken. 3.3.
X-rays
There are only two significantly X-ray detected tran-sients in the sample: ZTF19abvgxrq and ZTF19aaiqmgl.We show their X-ray spectra in Figure 8 fit to power lawmodels. The third, ZTF19aailpwl, was only detected inone epoch and not at a level that allowed for the signal-to-noise necessary for a spectrum.The X-ray spectrum of ZTF19abvgxrq is measured by
Swift XRT with a power law index of Γ = 2 . ± . . ± .
9; Boller et al. 1996; Forster& Halpern 1996; Molthagen et al. 1998; Rakshit et al.2017). The spectrum of ZTF19abvgxrq could also beexplained by a 150 eV blackbody with a Γ = 2 powerlaw component and no intrinsic absorption (Kara et al.2019). We note that the soft excess observed in NLSy1scan mimick the blackbody temperatures expected forTDEs (e.g. Boller et al. 1996).The spectral index of ZTF19abvgxrq (Γ ∼
3) was sim-ilar to that of AT2018fyk, interpreted as a TDE withlate-time disk formation (Wevers et al. 2019), as well asZTF18aajupnt, interpreted as a changing-look LINER“turning-on” into a NLSy1 (Frederick et al. 2019). TheX-ray spectral index of ZTF19aaiqmgl was quite higheven with regard to these events, with Γ ∼ − Black Hole Masses
We measured the black hole masses of the sample us-ing two different methods, each with important caveats:The virial mass method, which may systematically un-derestimate BH masses for NLSy1s, and the host galaxyluminosity, which may be contaminated by the pres-ence of an AGN. The M BH calculated from the hostgalaxy luminosity is M BH , M r = − . M r, host − .
96 fol-lowing McLure & Dunlop (2002), and the standard virialmethod (e.g. Shen et al. 2011) was employed to ob-tain the virial black hole masses from FWHM H β re-ported in Table 2. The transient Eddington ratio es-timates depend on the BH masses ( M BH ) as L Edd =1 . × ( M BH /M (cid:12) ) erg s − , For each transient inthe sample, we report a range of Eddington ratios inTable 2 bracketed by the Eddington ratio measured as-suming the virial mass estimate for the BH mass, andthe Eddington ratio measured assuming BH mass de-rived from the host galaxy luminosity. The range in BHmasses, and therefore Eddington ratios, shown in Ta-ble 2 is quite large. We estimate statistical and system-atic uncertainties of 0 . − . M BH = 3 . × M (cid:12) from the observed Chandra X-rayluminosity (this observation is described in more detailin Section 2.4). This is closer to, but not consistentwith, the virial mass estimate, meaning that the tran-sient may not have been accreting near the Eddingtonlimit at the time of the X-ray observation. amily Tree of NLSy1 Transients Table 2.
Black hole mass measurements of the sample from optical spectra and host galaxy properties. NLSy1s are typicallythought to be lower mass, highly accreting systems, but we show here that the uncertainty in the mass estimates generatessignificant uncertainty in the estimates of the Eddington ratios (described in Section 3.4). M r, host is the r -band de Vaucouleursand exponential disk profile model fit magnitude from the SDSS DR14 photometric catalog. The host of ZTF18abjjkeo is notin the SDSS footprint, and so we instead use the Pan-STARRS1 r -band Kron magnitude of this source (Chambers et al. 2016).Name M r, host λL A FWHM H β log M BH , M r log M BH , vir L/L
Edd (mag) (10 erg s − ) (km s − ) [ M (cid:12) ] [ M (cid:12) ]ZTF19abvgxrq -21.36 5.00 ± ±
49 7.7 6.4 0.066-1.5ZTF18abjjkeo -20.94 2.24 ± ±
270 7.5 6.4 0.048-0.62ZTF19aailpwl -22.38 42.6 ± ± a ± ±
35 7.2 6.1 0.023-0.29ZTF19aatubsj -21.51 21.9 ± ±
57 7.8 7.1 0.24-1.2a. The FWHM(H β ) for ZTF19aailpwl agrees with the measurement in Rakshit et al. (2017) within the error estimates.4. DISCUSSIONIn this section, we rule out possible physical scenariosfor each outburst, beginning with core collapse super-novae IIn. We review why the supernova interpreta-tion was quickly ruled out in favor of a supermassiveblack hole accretion scenario, and discuss how manyof the characteristics of the objects are consistent withboth NLSy1s and TDEs. We compare the available ev-idence with other scenarios including TDEs, extremeAGN variability, and binary SMBHs in detail. We alsodiscuss NLSy1 galaxies as the preferential hosts for theseand other similar events, and outline a scheme for clas-sifying future events based on the presence of spectralfeatures.4.1. “IIn or not IIn?”: Preliminary ObservationalClassification of the Flare Sample
Identification of the sample presented here occurredwith a slew of conflicting preliminary classifications atearly times, which we describe below.The narrow emission lines in the spectra of some SLSN(Type IIn) are a result of the highly luminous interac-tion of supernova ejecta from a massive progenitor withdense circumstellar medium. Therefore, under specialcircumstances, nuclear SNe can look spectroscopicallyvery similar to rapid flares from NLSy1s in the optical(e.g. Moriya et al. 2018). The shapes of the light curvesof the transients in this sample looked rather like thoseof such supernovae, in the absence of additional obser-vations. The smoothness of the flares in particular wasunique with respect to typical stochastic AGN variabil-ity, and made these transients noteworthy for allocationof follow-up resources. Therefore, the narrow Balmerfeatures in the spectra of these transients, coupled withtheir light curve shapes, left uncertainty in their early With rise times on the order of days to weeks. classifications. They could have been either Type IInsupernovae or NLSy1 AGN, while those with persistentstrong He II λ A Preponderance of Rapid Optical Transients inNarrow-line Seyfert 1 Host Galaxies
In the Analysis section ( § < log( M BH [ M (cid:12) ]) <
8; e.g. Mathur et al. 2001),and/or higher observed accretion rates (Pounds et al.1995; Wang et al. 1996; Marconi et al. 2008; Grupe et al.2010; Xu et al. 2012). The virial masses derived fromspectral measurements of the population may also beexplained with geometrical effects, when interpreted asthe classic broad-line AGN seen along a lower inclina-tion angle between the broad-line emitting region andthe line of sight (Decarli et al. 2008; Baldi et al. 2016;Rakshit et al. 2017).Studies of NLSy1s typically find them to be highlyphotometrically variable only in the X-rays. At opticalwavelengths, however, Klimek et al. (2004) found thatrapid, high amplitude variability was rare in a sample of172 observations of NLSy1s across 33 nights. Ai et al.(2010) also found that NLSy1s had systematically loweroptical variability amplitudes ( (cid:46) . Frederick et al. F ( e r g / c m / s / Å )
1e 15 ZTF19abvgxrqH FWHM: 745 ± 47 km s ) (a) H α ZTF19abvgxrq 0d F ( e r g / c m / s / Å )
1e 16 ZTF19abvgxrqH FWHM: 878 ± 49 km s ) (b) H β ZTF19abvgxrq 0d F ( e r g / c m / s / Å )
1e 15 ZTF19aatubsjH FWHM: 448 ± 61 km s ) (c) H α ZTF19aatubsj +8d F ( e r g / c m / s / Å )
1e 16 ZTF19aatubsjH lFWHM: 1208 ± 57 km s ) (d) H β ZTF19aatubsj +8d F ( e r g / c m / s / Å ) ZTF19aailpwlH FWHM: 1603 ± 142 km s ) (e) H α ZTF19aailpwl − F ( e r g / c m / s / Å ) ZTF19aailpwlH FWHM: 1050 ± 77 km s ) (f) H β ZTF19aailpwl − F ( e r g / c m / s / Å )
1e 15 ZTF19aaiqmglH FWHM: 1187 ± 24 km s ) (g) H α ZTF19aaiqmgl +84d F ( e r g / c m / s / Å )
1e 16 ZTF19aaiqmglH FWHM: 1433 ± 35 km s ) (h) H β ZTF19aaiqmgl +84d F ( e r g / c m / s / Å ) ZTF18abjjkeoH FWHM: 1289 ± 111 km s ) (i) H α ZTF18abjjkeo +8d F ( e r g / c m / s / Å ) ZTF18abjjkeoH FWHM: 1199 ± 270 km s ) (j) H β ZTF18abjjkeo +8d
Figure 5.
Gaussian fits to the H α +[N II] and H β line profiles of all transients in the sample show that their Balmer lines havea FWHM consistent with (and Lorentzian Balmer profiles characteristic of) that of narrow-line Seyfert 1s. The offset blue peakin the H β profile of ZTF19aatubsj is marked by a vertical line. amily Tree of NLSy1 Transients Rest Wavelength (˚A)05101520 N o r m a li ze d F λ + C o n s t a n t ZTF18aajupnt (Frederick+2019)CSS100217 (Abazajian+2009)CSS100217 (Drake+2011)PS16dtm (Blanchard+2017)AT 2017bgt (Trakhtenbrot+2019)ZTF18abjjkeo (+8d)ZTF19aaiqmgl (+444d)ZTF19aatubsj (+368d)ZTF19aailpwl (-13y)ZTF19aailpwl (+34d)ZTF19abvgxrq (+0d)Fe II H e II H e II Mg II [OIII] [NeV] [O II] H δ H γ H β [O III] H α + [N II] LL L
Figure 6.
Comparison of the ZTF sample of flares (in blue), as well as discovery spectra for the NLSy1-related events fromthe literature (in black): changing-look LINER ZTF18aajupnt (Frederick et al. 2019), TDE in a NLSy1 PS16dtm (Blanchardet al. 2017), SN in a NLSy1 CSS100217 (Drake et al. 2011), and Bowen fluorescent flare AT2017bgt (Trakhtenbrot et al. 2019a),and their pre-event spectra when available (in grey). For ZTF19aatubsj and ZTF19aaiqmgl here and in Figure 7, we plot thespectra after continuum fading rather than the discovery spectra, to display the features used in the spectroscopic classificationscheme discussed in Section 4.3. ZTF19aatubsj and ZTF19aaiqmgl show offset blue peaks in H β , and the peak of He II is offsetfrom 4686 ˚A in ZTF18abjjkeo. line Seyfert 1s in a sample of 275 AGN at 0 . < z < . Frederick et al.
Rest Wavelength (˚A)0 . . . . . . . . N o r m a li ze d F λ + C o n s t a n t AT 2017bgt (Trakhtenbrot+2019)ZTF19aaiqmgl (+444d)ZTF19abvgxrq (+0d)ZTF19aailpwl (+34d)ZTF18abjjkeo (+8d)PS16dtm (Blanchard+2017)ZTF18aajupnt (Frederick+2019)ZTF19aatubsj (+368d)CSS100217 (Drake+2011)
Fe II
Fe II onlyHe II onlyHe II+Bowen H e II O III N III [O II] H δ /[N III] H γ /[O III] H β [OIII] Figure 7.
Zoom-in on the 4000-5000 ˚A region of Figure 6 showing the comparison of the strength of H β , Fe II, and [O III] ofthe sample with NLSy1-related events in the literature. We color-code the sample and establish categories based on the presenceand absence of key emission line features as described in Section 4.3. Blue spectra indicate the presence of He II, and blackspectra indicate transients which displayed Fe II only, (though we note that PS16dtm showed both features). Those in greendisplay Bowen fluorescence features in addition to He II, and are most spectroscopically similar to AT2017bgt and the otherAGN flares comprising the sample in Trakhtenbrot et al. (2019a). amily Tree of NLSy1 Transients − − − − n o r m a li ze d c o un t ss − k e V − c m − power law (Γ = 2 . ± . Energy (keV)012 R a t i o [ D a t a/ M o d e l] (a) ZTF19abvgxrq . − − − − n o r m a li ze d c o un t ss − k e V − c m − power law (Γ = 5 . ± . . R a t i o [ D a t a/ M o d e l] (b) ZTF19aaiqmgl Figure 8.
Left panel: An absorbed power law fit and ratio residuals to the ∼
100 ks stacked
Swift XRT spectrum ofZTF19abvgxrq (spectral index Γ = 2 . ± . ∼ . ± . NLSy1 galaxies by their Balmer FWHMs. Their opti-cal spectra were unusual for NLSy1s in that they showedstrong “double-peaked” He II profiles with contributionsfrom the N III λ ∼
450 days ruledout a TDE, and these were instead interpreted as en-hanced accretion onto the SMBH of a pre-existing AGN.AT2017bgt was presented as the prototype of these dra-matic SMBH UV/optical flares irradiating the BLR. Itshowed a very slow decrease in optical flux over severalmonths following a relatively shallow ( ∼ ∼
80 days from a previous non-variable state.During the transient, the X-rays increased by a factor of2 − g − r color change as it fadedslowly over 1 year. This was the only AGN with Balmerlines consistent with a NLSy1 among a new class of“changing-look LINERs”, including SDSS 1115+0544(Yan et al. 2019).PS16dtm (iPTF16ezh/SN 2016ezh) was a near-Eddington but X-ray-quiet nuclear transient with strongFe II emission and T BB ∼ . × K. It rose over ∼ L bol [ergs s − ] > ∼
50 and ∼
100 dayswhile maintaining a constant blackbody temperature.The event was interpreted as a TDE exciting the BLR ina well studied, spectroscopically-confirmed NLSy1 with M BH ∼ M (cid:12) (Blanchard et al. 2017). X-ray upperlimits showed dimming by at least an order of magni-tude compared to archival observations, but Blanchardet al. 2017 predicted the X-rays would reappear after theobscuring debris (oriented perpendicularly to the accre-tion disk) had dissipated. We show the V -band ASASSNphotometry for PS16dtm in Figure 4 which appears sim-ilar in shape and absolute magnitude to ZTF19aatubsj,though longer in duration.CSS100217:102913+404220 displayed a high state( M V = − . α and was interpreted either as a Type IIn SN(Drake et al. 2011) or a TDE (Saxton et al. 2018) nearthe nucleus ( ∼
150 pc) of a NLSy1 in a star forminggalaxy. It eventually faded back to slightly below itsoriginal level after one year, which was interpreted asinteracting with and subsequently flushing a portion ofthe accretion disk.Similar events are not unheard of in broad-line AGNsystems, though they may be comparatively more rare.6
Frederick et al. N o r m a li ze d F λ + C o n s t a n t TDE AT2019dsg (van Velzen+2020)SN IIn 2005bx -15d (Kiewe+2012)NLS1 Mrk 618ZTF19abvgxrqZTF19aailpwlZTF19aatubsjZTF19aaiqmglZTF18abjjkeo H e II H e II [OIII][NeV][O II] H δ H γ H β [O III] H α + [N II] Figure 9.
We compare the spectra of the transient sample (in black) to archetypal NLSy1 Mrk 618, as well as a normal TypeIIn supernova, SN 2005bx (Kiewe et al. 2012), and AT2019dsg, a normal TDE in a star forming galaxy with Bowen fluorescencefeatures and a coincident neutrino detection (van Velzen et al. 2020b; Stein et al. 2020).
Neustadt et al. (2020) reported a candidate for sucha rapidly flaring event with quasar-like properties,ASASSN 18jd, although continued observations of thistransient will be critical for a better understanding ofthe properties of the host.4.3.
Observational Classification: The “Family Tree”of NLSy1-associated Transients
In Table 3, we use this sample to motivate a frame-work for quickly classifying similarly ambiguous flaringevents. We investigate the following: • AGN/NLSy1 characteristics (an empirical W − W β <
3; Rakshit et al. 2017), • TDE characteristics (host black hole mass belowthe Hills mass ( ∼ M (cid:12) ), and a lack of coolingor significant rebrightening), • X-ray properties (the presence of which can occurin both AGN and TDEs, but are less likely in theSN scenario).We apply these criteria in Table 3 and color code themas blue or green based on whether they favor the TDE orAGN scenario, respectively (as the SN scenario has beenruled out in Section 4.1). The spectroscopic class, basedon the presence of N III Bowen fluorecence features, FeII, and/or He II λ amily Tree of NLSy1 Transients >
5) andno intrinsic absorption are most likely associated withTDEs, and those with strong Bowen fluorescence profilesand slow UV and spectral evolution are likely associatedwith enhanced accretion onto supermassive black holesfrom a pre-existing accretion disk. The timing of a midinfrared flare may also help to distinguish between anAGN and a TDE — if it precedes the optical, it is likelyassociated with AGN variability, but if it follows as anecho, it may be associated with a TDE (van Velzen et al.2016).van Velzen et al. (2020b) established a spectroscopicclassification scheme for the sample of TDEs discoveredduring the first half of the ZTF survey, distinguishingthose with and without He II in a single epoch. Abouthalf of the TDEs in that sample were “H-only”, andonly one was “He-only”. They found that higher densityconditions were likely for the rest of the TDEs which hadH and He lines, as well as Bowen features.For the flaring NLSy1 sample presented here, we es-tablish the following spectroscopic classes to describeeach of the transients based on the presence or absenceemission features crucial to their physical interpreta-tions:1. “He II only”,2. “He II+N III”, and3. “Fe II only”,and we propose the following naming convention forthese classes: “NLSy1-HeII”, “NLSy1-HeII+NIII”, and“NLSy1-FeII”. Physical Interpretation of the Transient Flares
In the following section we consolidate all that isknown about the relevant properties of each object inthe sample, and compare them with the related tran-sients in NLSy1s in the literature, to explore each of thefollowing scenarios: A PS16dtm-like TDE in a NLSy1,A Sharov-21-like microlensing event, a CSS100217-likeSN in a NLSy1, and a binary SMBH scenario.4.4.1.
Association of the Transients with AGN
There is evidence that all sources in the sample are as-sociated with AGN rather than distinct explosive eventsoccurring in a normal galaxy. Although these outburstsmay not necessarily be the result of an intrinsic enhance-ment in AGN accretion activity, transients with fast-rise/slow-decay (such as those in this sample, along with We note that although hydrogen features are not explicitly namedin this feature classification scheme, all spectra of the transientsshow resolved narrow (1000 < FHWM < − ) Balmerfeatures (see Section 3.2). slow-rise/fast-decay, and symmetric light curve shapes)were well-represented in a sample of 51 AGN flares dis-covered in CRTS (Graham et al. 2017).Rakshit et al. (2017) spectroscopically classified theSDSS spectrum of the host galaxy of ZTF19aailpwl asharboring an AGN NLSy1 >
12 years prior to the onsetof the smoothly flaring transient reported here.As evident in Figure 9, the strengths of the Balmerlines in the transient spectra are most consistent withthat of a NLSy1. Ne V λ (cid:46) Γ (cid:46) The SN Scenario
It is highly improbable that these flares are the resultof normal SN explosions. We observe long-lived U -bandemission in ZTF19aatubsj, persistent UV emission in alltransients in the sample, and strong transient X-ray de-tections in ZTF19abvgxrq and ZTF19aaiqmgl. There isalso only a small likelihood of a SN in the host galaxyalong the line of sight unassociated with the AGN. Thestrongest evidence against the normal supernova sce-nario is the persistence of the He II emission features ∼ −
100 days after the onset of the flare — such flashionization signatures are only visible in supernova spec-tra at very early times (e.g. Khazov et al. 2016; Bruchet al. 2020).At least one of these transients (ZTF19aatubsj) sharesa number of properties with CSS100217, which displayedsoft X-rays and was interpreted as a SN IIn explosion inan AGN disk. The SN interpretation of CSS100217 waslargely based on light curve energetics, which are simi-lar to those of this sample. The g − r color change, andthe peak magnitude of − (cid:46) M V (cid:46) −
22 are very simi-lar in particular between CSS100217 and ZTF19aatubsj.Type IIn supernovae can exhibit strong Fe II lines in latespectra, such as ZTF19aatubsj did.However, in contrast, the light curve evolutiondiffers in that CSS100217 fades at least twice asquickly as ZTF19aatubsj. Also, the Fe II complex ofCSS100217 was always visible throughout the flare, andDrake et al. (2011) observed a broad ∼ − com-ponent in H α which got broader with time in subsequentfollow-up spectra of CSS100217. Strong P Cygni profiles8 Frederick et al.
Table 3.
Comparison of the properties of individual objects in the sample (upper table) and NLSy1-related transients inthe literature (lower table). ” (cid:8) ” means that property is observed, and ” × ” indicates that characteristic was not observed.“UV-bright” refers to the persistence of UV-brightness, and “Rebrighten” refers to a significant recovery of at least half thepeak luminosity of the source. Following the convention of Figure 7, blue (green) indicates a property associated with the TDE(flaring AGN) scenario.Name log M BH < β< β < g − r UV-bright X-ray Γ W1-W2 Re- Spec. class Interp.[ M (cid:12) ] km s − [flux ratio] ∼ > a brightenZTF19abvgxrq (cid:8) (cid:8) × (cid:8) (cid:8) (cid:8) × (cid:8) HeII+NIII AGNZTF19aailpwl × (cid:8) (cid:8) (cid:8) (cid:8) (cid:8) (cid:8) b (cid:8) × HeII+NIII AGNZTF19aatubsj (cid:8) (cid:8) (cid:8) (cid:8) × (cid:8) × × × FeII TDEZTF19aaiqmgl (cid:8) (cid:8) (cid:8) (cid:8) × (cid:8) × (cid:8) HeII+NIII AGNZTF18abjjkeo (cid:8) (cid:8) × (cid:8) (cid:8) - - × × HeII TDECSS100217 (cid:8) (cid:8) (cid:8) (cid:8) × (cid:8) (cid:8) × FeII AGNPS16dtm (cid:8) (cid:8) (cid:8) (cid:8) (cid:8) (cid:8) c × × HeII+FeII TDEAT2017bgt (cid:8) (cid:8) (cid:8) (cid:8) (cid:8) (cid:8) × (cid:8) HeII+NIII AGNZTF18aajupnt (cid:8) (cid:8) × (cid:8) × (cid:8) × × HeII AGNa. We select the less conservative color cut presented in Stern et al. (2012).b. The single low level
XRT detection of ZTF19aailpwl occurred only once throughout the follow-up campaign and was notenough to take a reliable spectral measurement.c. The host of PS16dtm displayed X-rays only prior to and following the fading of, but not for the duration of, the transient. are observed in the optical spectra of SN, and from suchprofiles we would expect an absence of absorption on theblue end of the Balmer line profiles, rather than emissionas in the spectra of ZTF19aaiqmgl and ZTF19aatubsj.Therefore, based on this evidence we rule out the SNType IIn scenario.4.4.3.
The TDE Scenario
The Hills mass is the mass for which the tidal Rocheradius is equivalent to the gravitational Schwarzschildradius of the black hole, beyond which a star (that wouldotherwise be tidally pulled apart) is instead left intact asit passes the event horizon (Hills 1975). This maximummass to tidally disrupt a solar-type star just outside theevent horizon is 10 M (cid:12) . Therefore a SMBH mass signif-icantly above this limit would likely rule out a TDE. Ofthe supermassive black hole masses derived for the hostgalaxies, only that of ZTF19aailpwl is inconsistent witha TDE scenario, (although we note that it is consistentwithin the typical uncertainty for such mass measure-ments). The range of absolute magnitudes of the flaresin this sample ( − < M r < −
19 mag) also tend tobe intrinsically brighter at peak than all but one of theZTF TDEs ( M r > −
20 mag) reported in van Velzenet al. (2020a), AT2018iih ( M r = − . nor predicted (e.g. Chan et al. 2019,2020) from a TDE. In these cases with rebrightening, aTDE is strongly ruled out.ZTF19aaiqmgl and ZTF19aatubsj, like AT2018fyk,only showed Fe II at certain times during the flare.ZTF19aaiqmgl only displayed Fe II during its first peak,and in ZTF19aatubsj, the Fe II complex got morevisible as the transient faded. ZTF19aatubsj is theonly transient in the sample with a lack of He II fea-tures in its spectra. Within the van Velzen et al.(2020b) spectral classification scheme for optical TDEs,ZTF19aatubsj would be a H-only TDE, with the Fe IIcomplex attributed to the NLSy1 host. It is importantto note that the transients with blue horn features inH β , ZTF19aaiqmgl and ZTF19aatubsj, may be signa-tures of wind ejecta with a velocity distinct from theAGN.Enhanced N III lines such as that seen in theNLSy1-HeII+NIII spectroscopic class (ZTF19abvgxrq, Except in the case of the periodicity of ASASSN 14ko, whichwas interpreted as a possible repeating partial TDE (Payne et al.2020). amily Tree of NLSy1 Transients . − .
98 mag (Table 3). The IRflare associated with ZTF19aaiqmgl could be interpretedas a dust echo, similar to those seen in a number ofTDEs (van Velzen et al. 2016). A host-subtracted SEDfit to the
Swift photometry of ZTF19aaiqmgl gives ablackbody temperature consistent with that of knownTDEs, 10 . K.The X-ray variability of TDEs can vary erraticallyduring a flare (e.g. Wevers et al. 2019; van Velzen et al.2020b). Although soft X-ray excesses with Γ ∼ The Extreme AGN Variability Scenario
Graham et al. (2017) presented a sample of quasarsdisplaying extreme variability in CRTS. Some had sim-ilar profiles and amplitudes (rising by 2 − ∼ The Gravitational Microlensing Scenario
Flares due to microlensing are expected to be observ-able in difference imaging surveys with the combinedbaseline of iPTF and ZTF. The rise portions of thelight curve shapes of all the transients measured in Sec-tion 3.1.1 being well-fit by quadratics is consistent witha lensing event, however, all but ZTF18abjjkeo havea longer decay with respect to the initial rise. Mi-crolensing by multiple foreground sources can give riseto a symmetric (with respect to the fade) double peakwith a dip in the middle of the optical light curve suchas that seen in ZTF19abvgxrq (Hawkins 1998, 2004;Schmidt & Wambsganss 2010). The cuspy shape ofthe first peak is also characteristic of microlensing lightcurves. ZTF19aaiqmgl also showed a second peak inits light curve, but the first peak was a lot more rapidand luminous than the second. To test this scenario inZTF18abjjkeo would require continuing to observe foran additional flare.The microlensing scenario, however, would not ac-count for the strong transient Bowen fluorescence fea-tures that appear only at late times in ZTF19aaiqmgl,and only at early times in ZTF19abvgxrq (Figure 11).Meusinger et al. (2010) explained a similar event as abackground quasar with a UV flare in J004457+4123,also known as Sharov 21, being microlensed by a fore-ground star in M31.Microlensing is characteristically achromatic, andtherefore would be ruled out by the clear evidence for g − r color change observed in ZTF19aatubsj.4.4.6. The SMBH Binary Scenario
Variability on the timescales of years due to a bi-nary SMBHB system would require a subparsec sepa-ration (e.g. Graham et al. 2015). In such a system, twoSMBHs induce tidal torques carving out a cavity in thecircumbinary accretion disk, and may be surrounded bytheir own minidisks at sufficient separations. The inter-action of accretion streams with the cavity could causean outburst on the approximate timescales seen in thissample, which is dependent on the properties of the sys-tem. This phenomenon is seen in simulations of SMBHbinaries (e.g. Ryan & MacFadyen 2017; Gold 2019).We see evidence of offset narrow Balmer emissionlines in the spectra of ZTF19aatubsj and ZTF19aaiqmgl,which may indicate a significant separate physical com-ponent, although it is unclear what is contributing tothose blueshifted velocities.0
Frederick et al. CONCLUSIONSWe report five nuclear flaring events associated withNLSy1s, all serendipitously discovered in ZTF. Wemeasured their photometric characteristics (such as lightcurve shape, g − r color, and rise to peak luminos-ity, finding a correlation between rise time and absolutemagnitude), and spectroscopic properties. We then es-tablished groupings of the objects in the sample basedon analyses of the months-long follow-up campaigns ofthese objects. Based on observed groupings of the sam-ple, we propose the following naming scheme of spec-troscopic classes of such transients for use in future op-tical surveys: “NLSy1-HeII”, “NLSy1-HeII+NIII”, and“NLSy1-FeII”. We ruled out the possibility that theseare Type IIn supernovae occurring in NLSy1 systems.Despite the heterogeneity of the sample’s properties,two of the flares presented in this work have multiwave-length characteristics which could be consistent withTDEs in NLSy1s (ZTF19aatubsj and ZTF18abjjkeo),with spectral classes of NLSy1-FeII and NLSy1-HeII,respectively. This is a high TDE rate relative to qui-escent galaxies, which are more abundant than NLSy1s.The prevalence of TDE candidates in the NLSy1 AGNclass could be a natural result of their hosting smallerblack holes compared to typical broad-line AGN, andtherefore satisfying the Hills mass criterion for an ob-servable TDE. However, without pre-event spectra andX-ray imaging to isolate the contribution of the putativeTDE to the composite NLSy1+TDE emission, flaringdue to extreme AGN variability cannot be definitivelyruled out. For two in the sample (ZTF19abvgxrq andZTF19aaiqmgl), we can rule out the simple TDE sce-nario from rebrightening in their light curves, and wedetermine that they, along with ZTF19aailpwl (whichhad a pre-flare NLSy1 spectral classification and a blackhole mass estimate too large to host a canonical TDE),are likely outbursts related to enhanced accretion in ex-cess of typical AGN variability, and with spectral fea-tures we classify as “NLSy1-HeII+NIII”, and membersof the Trakhtenbrot et al. (2019a) class of AGN flares.Given this sample, together with the growing numberof interesting rapid optical transients associated withNLSy1s we reviewed in the literature, we posed the ques-tion of why such environments are observed to preferen-tially host these outbursts. Given the relative fractionof NLSy1s found with respect to other AGN classes inspectroscopic surveys such as SDSS ( ∼ As the Trakhtenbrot et al. (2019a) observational class was estab-lished midway through the ZTF survey, we had not been system-atically filtering such events when the population became appar-ent in the nuclear transients alert stream search.
1. A selection bias due to shorter timescales for lowermass BH systems (like NLSy1s), which are there-fore more likely to be captured within the baselineof wide field optical surveys,2. A systematic disregard of smooth flares in broadline AGN during transient searches, or3. A true intrinsic rate enhancement due to instabil-ities causing rapid changes in the observable envi-ronments or accretion efficiencies of these systems.Follow-up strategies of optical transients in AGN thatare similarly ambiguous at early times may stand to ben-efit from the framework we offer here. We hope thisclassification scheme will guide real-time predictions forpotential future behavior of large amplitude flares inNLSy1s, which are clearly an interesting population forfuture study. The next step will be to perform a sys-tematic study of the variability of NLSy1s detected inZTF, to assess the completeness and rate of this sam-ple of transients with smoothly flaring light curves, andcompare to a sample of broad-line AGN. Expanding onthe small number of unusual transients associated withNLSy1s not only sheds light on the parameter space inwhich they reside, but also provides the framework fora decision tree for understanding such outbursts whenthey are inevitably captured at higher rates in upcomingwide field surveys. This will be imperative to establish inadvance of larger and deeper surveys such as ZTF PhaseII and the Vera C. Rubin Observatory (formerly knownas LSST; Ivezi´c et al. 2019), to which the timescales ofthese flares are well-suited. Continued multiwavelengthmonitoring of the entire sample will be important to de-termine the host properties for those with sparse dataprior to the transient, and for understanding the evolu-tion and nature of these flares. amily Tree of NLSy1 Transients
Swift dataarchive.
Facilities:
PO:1.2m, PO:1.5m, Hale, Swift(XRT andUVOT), DCT, NOT, Shane, Liverpool:2m
Software:
Pyraf,Lmfit,HEAsoft,PIMMS2
Frederick et al.
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200 0Days since discovery171819202122 F o r ce d P h o t o m e tr y M ag n i t ud e (a) ZTF19abvgxrq − −
200 0 200 400Days since discovery182022 F o r ce d P h o t o m e tr y M ag n i t ud e (b) ZTF19aatubsj F o r ce d P h o t o m e tr y M ag n i t ud e (c) ZTF19aailpwl − −
100 0 100 200 300 400Days since discovery182022 F o r ce d P h o t o m e tr y M ag n i t ud e (d) ZTF19aaiqmgl −
200 0 200 400Days since discovery1819202122 F o r ce d P h o t o m e tr y M ag n i t ud e (e) ZTF18abjjkeo Figure 10.
Forced photometry of the sample from ZTF Data Release 3. Colors correspond to r -, g -, and i -band 3- σ detections,and triangles correspond to 5- σ upper limits. An ‘X’ marks the rise to peak in the difference imaging light curve of ZTF18abjjkeo,which was discovered in data too recent to be included in the ZTF DR3, and therefore only shows the flux level of the hostgalaxy. The data points in the light curves beyond 2020 will be released in the final ZTF photometry data release. amily Tree of NLSy1 Transients Rest Wavelength (˚A)0510152025 N o r m a li ze d F λ + C o n s t a n t ZTF19abvgxrq LDTZTF19abvgxrq Keck1ZTF19abvgxrq +10d LickZTF19abvgxrq +17d LTZTF19abvgxrq +28d LDTZTF19abvgxrq +69d LDTZTF19abvgxrq +154d LDTZTF19abvgxrq +354d LDT
Fe II H e II O III N III [O II] H δ /[N III] H γ /[O III] H β [OIII] 3800 4000 4200 4400 4600 4800 5000 Rest Wavelength (˚A)012345 N o r m a li ze d F λ + C o n s t a n t ZTF19aailpwl –4605d SDSSZTF19aailpwl +34d FLOYDS-NZTF19aailpwl +133d LDT
Fe II H e II O III N III [O II] H δ /[N III] H γ /[O III] H β [OIII]3800 4000 4200 4400 4600 4800 5000 Rest Wavelength (˚A)0 . . . . . . . . N o r m a li ze d F λ + C o n s t a n t ZTF19aatubsj +45d P60ZTF19aatubsj +50d LTZTF19aatubsj +55d LDTZTF19aatubsj +66d P200ZTF19aatubsj +104d P60ZTF19aatubsj +368d NOTZTF19aatubsj +403d P60ZTF19aatubsj +408d LDTZTF19aatubsj +506d LDT
Fe II H e II O III N III [O II] H δ /[N III] H γ /[O III] H β [OIII] 3800 4000 4200 4400 4600 4800 5000 Rest Wavelength (˚A)024681012 N o r m a li ze d F λ + C o n s t a n t ZTF19aaiqmgl +22d NOTZTF19aaiqmgl +443d P60ZTF19aaiqmgl +575d P60ZTF19aaiqmgl +576d P60ZTF19aaiqmgl +579d P60ZTF19aaiqmgl +444d FLOYDS-S
Fe II H e II O III N III [O II] H δ /[N III] H γ /[O III] H β [OIII]3800 4000 4200 4400 4600 4800 5000 Rest Wavelength (˚A)012345 N o r m a li ze d F λ + C o n s t a n t ZTF18abjjkeo +6d P60ZTF18abjjkeo +8d LT
Fe II H e II O III N III [O II] H δ /[N III] H γ /[O III] H β [OIII] Figure 11.
Spectroscopic follow-up of the sample summarized in Table 1, showing the evolution of the He II, H ββ