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A Family Tree of Optical Transients from Narrow-line Seyfert 1 Galaxies

About: This article is published in The Astrophysical Journal.The article was published on 2021-10-01 and is currently open access. It has received 21 citations till now.

Summary (6 min read)

1. Introduction

  • Therefore, a flare originating from this nuclear region requires a distinctly powerful event to be detectable above this stochastically variable continuum.
  • All transients in the sample are referred to by their International Astronomical Union (IAU) recognized transient designations throughout.

2. Observations

  • The ZTF Survey (Graham et al. 2019; Bellm et al. 2019a) comprises the automated Palomar 48 inch Samuel Oschin Telescope (P48) as well as the Palomar 60 inch Spectral Energy Distribution Machine (P60 SEDM; Blagorodnova et al.
  • At least 15 images meeting good-quality criteria were stacked to build a coadded reference image of each observing field and quadrant in each filter band.
  • Science images are subtracted by their references and processed each night by the Infrared Processing and Analysis Center (IPAC) pipeline (Masci et al. 2019).

2.1. Optical Photometry

  • All transients in the sample were detected prepeak using ZTF difference imaging photometry.
  • All magnitude changes are reported in the g band unless otherwise noted.
  • The host galaxy displayed variability at the 2 mag level in V-band CRTS data from 2009 to 2013 (variability which was not observed in ZTF forced photometry prior to the transient).
  • Phase I, of which 27 were TDEs, over 7% were classified as SN, and over half were AGN or candidate AGN.

2.2. Optical Spectroscopy

  • All spectroscopic follow-up observations for the sample are summarized in Table 1.
  • Spectra obtained with the Low-Resolution Imaging Spectrometer (LRIS) on the Keck I 10 m telescope were reduced automatically with the LRIS reduction pipeline Lpipe Perley (2019).
  • The follow-up FLOYDS-N (Arcavi et al. 2019) and LDT (PI: Gezari) spectra showed a steep blue continuum and a strong H II profile with Bowen fluorescence features, indicating it became a flaring SMBH belonging to the observational class established by Trakhtenbrot et al. (2019a).
  • AT2019avd—The spectrum taken with NOT (PI: Sollerman) on 2019 March 15 near the first optical peak showed strong Balmer line emission, no detection of a H II line complex, and evidence for a Fe II complex, characteristic of NLSy1 galaxies.

2.3. UV Photometry

  • The authors triggered target-of-opportunity monitoring observations with the Neil Gehrels Swift Telescope (Gehrels et al. 2004) for all transients in the sample.
  • Using the HEASOFT command uvotsource, the authors extracted UVOT photometry within a 5″ radius circular aperture and using an annular background region centered on the coordinates of the optical transient.
  • The authors compare ZTF g- and r-band difference imaging, Wide-field Infrared Survey Explorer (WISE) difference imaging, Swift X-ray Telescope (XRT) monitoring, and Swift UVOT detections subtracted by the archival Galaxy Evolution Explorer (GALEX; Martin et al. 2005) All-Sky Imaging Survey (AIS; Bianchi et al. 2017) near-UV (NUV; λeff= 2310 Å) host measurements (measured with a 6″ radius aperture).
  • The UV light curves of the sample tend to follow the shape of the optical.

2.4. X-Rays

  • The authors found only two transients in the sample to be detected in the X-rays in follow-up Swift XRT observations: AT2019pev and AT2019avd.
  • AT2019brs was detected only once, and then only at a low level.
  • X-ray follow-up spectra are reported in Table 1.
  • —Similar to the UV light curve, the shape of the X-ray flare of AT2019pev followed the optical, from its fade through its second rise 100 days after peak .

2.6. Summary of Preflare Host Galaxy Properties from Archival Observations

  • Table 2 presents approximate amplification factors of the flares in the optical, UV, IR, and X-ray wavebands with respect to archival observations for each source in the sample described further in this section.
  • AT2019brs—This is the only optical transient in this sample spectroscopically identified as an AGN prior to its onset.
  • Section 2.4 contains more details on the timing and levels of these archival observations.

3.1. Photometry

  • The difference imaging light curves for the sample are shown in terms of absolute magnitudes in Figure 3.
  • The authors show the sample alongside various NLSy1-related events from the literature, which are described in more detail in Section 4.
  • CSS100217 displayed some variability prior to the transient, unlike any of the events in this sample.
  • AT2017bgt was observed only during its fade in difference imaging, so the authors instead show its aperture photometry (from the ASAS-SN Photometry Database;25 Jayasinghe et al. 2019), which also shows the rise of the source.
  • AT2018dyk is by far the least-luminous transient shown.

3.1.1. Light-curve Timescales

  • The authors measured the rise-to-peak timescales of the sample by fitting Gaussians to the light curves shown in Figure 3 using the lmfit package (Newville et al. 2016).
  • The authors observe a potential correlation (with a correlation coefficient of r=−0.56, p = 0.08) between the luminosity (specifically the absolute magnitudes MV and Mg) and rise-to-peak rest-frame timescales of the sample (trise in days) with the following relation: M=−0.04trise− 19.03, shown in Figure 4.
  • Fitting the light curves with quadratic functions resulted in the same correlation within the error estimates.
  • Within this framework, AT2018dyk appears underluminous for how fast it rises.
  • AT2017bgt was observed only during its fading phase in difference imaging, and so was excluded from this portion of the analysis.

3.1.2. Rebrightening

  • It is notable that two sources in the sample, AT2019pev and AT2019avd, each have a dramatic rebrightening episode.
  • Following a flare and an approximately ∼2 mag fade from peak, both return to nearly half their maximum luminosity before seasonal gaps in visibility.
  • The authors explore possible interpretations of this rebrightening in Section 4. 3.1.3. UV/Optical to X-Ray Ratio.

3.2. Spectroscopy

  • From the FWHM of the broad Balmer emission lines, the authors classified all sources in the sample as NLSy1s.
  • The authors compare the host (when available) and transient spectra of this sample to other transients in NLSy1s in Figure 6 (showing the full wavelength range of the observations) and 7 (rest wavelength 3700–5150Å, showing clearly the He II, Fe II, and Hβ line profiles).
  • The presence and strength of Fe II are uncorrelated with other spectroscopic properties of the transients shown.
  • The authors note that although “only” is used in the categorization naming based on the presence of spectral features, all have strong Balmer features.

3.2.2. The Fe II complex

  • Reverberation mapping studies of AGNs show that the line-complex-emitting region is measured farther than the Balmer-line-emitting region (e.g., Barth et al. 2013; Rafter et al. 2013).
  • TDE AT2018fyk also showed low ionization lines including an Fe II (37,38) emission multiplet emerging for 45 days during the TDE and forms a class of Fe-rich TDEs along with ASASSN-15oi and PTF09ge (Wevers et al. 2019).
  • Therefore, this feature may indicate the presence of an AGN but is not always useful in determining the nature of a particular AGN-related flare.
  • For two of the transients in this sample, whether or not the Fe II complex can be seen in optical spectra depends on the phase and the continuum brightness of the transient—for AT2019fdr it was not observed for 368 days, and for AT2019avd it became no longer visible during the second rise 444 days after the initial spectrum was taken.

3.3. X-Rays

  • The Swift XRT data were collected in photon counting mode and processed28 using HEASOFT v6.22 (Evans et al.
  • The authors assessed best-fit models utilizing χ2 statistics and XSPEC version 12.9.1a (Arnaud 1996).
  • The authors note that the soft excess observed in NLSy1s can mimic the blackbody temperatures expected for TDEs (e.g., Boller et al. 1996).
  • The X-ray spectral index of AT2019avd was quite high even with regard to these events, with Γ∼ 4–6.

3.4. Black Hole Masses and Eddington Ratios

  • The authors measured the BH masses of the sample using two different methods, each with important caveats: the virial mass method, which may systematically underestimate BH masses for NLSy1s, and the host galaxy luminosity, which may be contaminated by the presence of an AGN.
  • GWe derive the Eddington ratio for the sample, measuring Lbol at maximum brightness using difference imaging in the optical (g-band or r-band) filter with central wavelength closest to 5100Å in the rest frame and assuming the bolometric correction Lbol= 9λL5100A (Kaspi et al. 2000).the authors.the authors.
  • For each transient in the sample, the authors report a range of Eddington ratios in Table 3 bracketed by the Eddington ratio measured assuming the virial mass estimate for the BH mass and the Eddington ratio measured assuming BH mass derived from the host galaxy luminosity.

4. Discussion

  • The authors rule out possible physical scenarios for each outburst, beginning with core-collapse SNe IIn.
  • The authors review why the SN interpretation was quickly ruled out in favor of an SMBH accretion scenario and discuss how many of the characteristics of the objects are consistent with both NLSy1s and TDEs.
  • The authors compare the available evidence with other scenarios including TDEs, extreme AGN variability, and binary SMBHs in detail.

4.1. “IIn or Not IIn?”: Preliminary Observational

  • Classification of the Flare Sample Identification of the sample presented here occurred with a slew of conflicting preliminary classifications at early times, which the authors describe below.
  • The narrow emission lines in the spectra of some SLSN (Type IIn) are a result of the highly luminous interaction of SN ejecta from a massive progenitor with dense circumstellar medium.
  • The shapes of the light curves of the transients in this sample looked rather like those of such SNe, in the absence of additional observations.
  • Therefore, the narrow Balmer features in the spectra 30 With rise times on the order of days to weeks.
  • They could have been either Type IIn SNe or NLSy1 AGN, while those with persistent strong H II λ4686 features in their spectra looked similar to that of TDEs.

4.2. A Preponderance of Rapid Optical Transients in Narrowline Seyfert 1 Host Galaxies

  • In the Analysis section (Section 3), the authors compared their sample to data from nuclear transients in the literature that happened to be hosted in NLSy1 galaxies.
  • Here the authors describe a number of distinct events, including the Trakhtenbrot et al. (2019a) observational class of optical flares, the “on” state of AT2018dyk, and the host of PS16dtm and CSS100217:102913+404220, which were all consistent with NLSy1-related activity.
  • The slow UV and spectral emission line evolution over a period of ∼450 days ruled out a TDE, and these were instead interpreted as enhanced accretion onto the SMBH of a preexisting AGN.
  • They presented characteristics of these events indicative of shock interaction, possibly from a new variant of TDE.

4.4.1. Association of the Transients with AGNs

  • There is evidence that all sources in the sample are associated with AGNs rather than distinct explosive events occurring in a normal galaxy.
  • Strong H II profiles, although somewhat rare in association with normal stochastic AGN variability (Neustadt et al. 2020), have been observed before and interpreted as the signature of a sudden enhancement of accretion (e.g., Frederick et al. 2019; Trakhtenbrot et al. 2019a).
  • Persistent X-rays are a likely signature of accretion onto an SMBH rather than an SN.
  • It is typically accompanied by a hard X-ray continuum component (not present in either X-ray-detected transient in this sample) and not nearly as ultrasoft as the X-rays seen in AT2019avd (4 Γ 6), which are slopes more frequently observed in the X-ray spectra of TDEs.
  • Transients with fast-rise/slow-decay (such as those in this sample), along with slow-rise/fast-decay and symmetric light-curve shapes, were well represented in a sample of 51 AGN flares discovered in CRTS (Graham et al. 2017).

4.4.2. The SN Scenario

  • It is highly improbable that these flares are the result of normal SN explosions.
  • Fe II lines in late spectra, such as AT2019fdr did.
  • Therefore, based on this evidence the authors rule out the SN Type IIn scenario.

4.4.3. The TDE Scenario

  • None of the events in this sample are consistent with the set of characteristics typically displayed by an isolated tidal disruption occurring in a dormant SMBH system.
  • This maximum mass to tidally disrupt a solar-type star just outside the event horizon is 108Me.
  • Rebrightening with high amplitudes returning nearly to preflare levels such as that seen in AT2019pev and AT2019avd has neither been observed32 nor predicted (e.g., Chan et al. 2019, 2020) from a TDE.
  • Based on the combination of properties shown in Figure 10, the authors conclude that two of the flares, AT2020hle and AT2019fdr, are better explained as TDEs in NLSy1s than AGN flares, although the interpretation is not clear cut.

4.4.4. The Extreme AGN Variability Scenario

  • Extreme AGN variability, defined empirically as an optical magnitude difference of greater than 1 mag, has been seen in various optically selected samples of quasars, e.g., Lawrence (2016), Rumbaugh et al. (2018), and MacLeod et al. (2019).
  • Graham et al. (2017) presented a sample of quasars displaying extreme variability in CRTS.
  • Some had similar profiles and amplitudes (rising by 2–2.5 mag) but longer timescales (500–1000 days) compared to the flares presented here.
  • Trakhtenbrot et al. (2019b) stated that the fade timescale of AT2017bgt was longer than expected for a TDE.

4.4.5. The Gravitational Microlensing Scenario

  • Flares due to microlensing are expected to be observable in difference imaging surveys with the combined baseline of iPTF and ZTF.
  • The rise portions of the light-curve shapes of all the transients measured in Section 3.1.1 being well fit by quadratics is consistent with a lensing event; however, all but AT2020hle have a longer decay with respect to the initial rise.
  • Microlensing by multiple foreground sources can give rise to a symmetric (with respect to the fade) double peak with a dip in the middle of the optical light curve such as that seen in AT2019pev (Hawkins 1998, 2004; Schmidt & Wambsganss 2010).
  • To test this scenario in AT2020hle would require continuing to observe for an additional flare.
  • The microlensing scenario, however, would not account for the strong transient Bowen fluorescence features that appear only at late times in AT2019avd and only at early times in AT2019pev .

4.4.6. The SMBH Binary Scenario

  • Variability on the timescales of years due to a binary SMBHB system would require a subparsec separation (e.g., Graham et al. 2015).
  • In such a system, two SMBHs induce tidal torques carving out a cavity in the circumbinary accretion disk and may be surrounded by their own mini disks at sufficient separations.
  • The interaction of accretion streams with the cavity could cause an outburst on the approximate timescales seen in this sample, which is dependent on the properties of the system.
  • The authors see evidence of offset narrow Balmer emission lines in the spectra of AT2019fdr and AT2019avd, which may indicate a significant separate physical component, although it is unclear what is contributing to those blueshifted velocities.

5. Conclusions

  • The authors report five nuclear flaring events associated with NLSy1s, all serendipitously33 discovered in ZTF.
  • The next step will be to perform a systematic study of the variability of NLSy1s detected in ZTF to assess the completeness and rate of this sample of transients with smoothly flaring light curves and compare to a sample of broad-line AGNs.
  • This research has made use of data obtained through the High Energy Astrophysics Science Archive Research Center Online Service, provided by the NASA/Goddard Space Flight Center.

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Content maybe subject to copyright    Report

A Family Tree of Optical Transients from Narrow-line Seyfert 1 Galaxies
Sara Frederick
1
, Suvi Gezari
1,2,3
, Matthew J. Graham
4
, Jesper Sollerman
5
, Sjoert van Velzen
6
, Daniel A. Perley
7
,
Daniel Stern
8
, Charlotte Ward
1
, Erica Hammerstein
1
, Tiara Hung
9
, Lin Yan
10
, Igor Andreoni
4
, Eric C. Bellm
11
,
Dmitry A. Duev
4
, Marek Kowalski
12,13,14
, Ashish A. Mahabal
4,15
, Frank J. Masci
16
, Michael Medford
17,18
,
Ben Rusholme
16
, Roger Smith
10
, and Richard Walters
10
1
Department of Astronomy, University of Maryland, College Park, MD 20742, USA; sfrederick@astro.umd.edu
2
Joint Space-Science Institute, University of Maryland, College Park, MD 20742, USA
3
Space Telescope Science Institute, Baltimore, MD 21218, USA
4
Division of Physics, Mathematics and Astronomy, California Institute of Technology, Pasadena, CA 91125, USA
5
The Oskar Klein Centre & Department of Astronomy, Stockholm University, AlbaNova, SE-106 91 Stockholm, Sweden
6
Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA Leiden, The Netherlands
7
Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK
8
Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Mail Stop 169-221, Pasadena, CA 91109, USA
9
Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064, USA
10
Caltech Optical Observatories, California Institute of Technology, Pasadena, CA 91125, USA
11
DIRAC Institute, Department of Astronomy, University of Washington, 3910 15th Avenue NE, Seattle, WA 98195, USA
12
Deutsches Elektronen Synchrotron DESY, Platanenallee 6, 15738 Zeuthen, Germany
13
Institut für Physik, Humboldt-Universität zu Berlin, D-12489 Berlin, Germany
14
Columbia Astrophysics Laboratory, Columbia University in the City of New York, 550 W 120th St., New York, NY 10027, USA
15
Center for Data Driven Discovery, California Institute of Technology, Pasadena, CA 91125, USA
16
IPAC, California Institute of Technology, 1200 E. California Boulevard, Pasadena, CA 91125, USA
17
University of California, Berkeley, Department of Astronomy, Berkeley, CA 94720, USA
18
Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, USA
Received 2020 October 16; revised 2021 June 30; accepted 2021 July 1; published 2021 October 12
Abstract
The Zwicky Transient Facility (ZTF) has discovered ve events (0.01 < z < 0.4) belonging to an emerging class of
active galactic nuclei (AGNs) undergoing smooth, large-amplitude, and rapidly rising ares. This sample consists
of several transients initially classied as supernovae with narrow spectral lines. However, upon closer inspection,
all of the host galaxies display Balmer lines with FWHM(Hβ) 9001400 km s
1
, characteristic of a narrow-line
Seyfert 1 (NLSy1) galaxy. The transient events are long lived, over 400 days on average in the observed frame. We
report UV and X-ray follow-up of the ares and observe persistent UV emission, with two of the ve transients
detected with luminous X-ray emission, ruling out a supernova interpretation. We compare the properties of this
sample to previously reported aring NLSy1 galaxies and nd that they fall into three spectroscopic categories: 1)
Balmer line proles and Fe
II complexes typical of NLSy1s, 2) strong He II proles, and 3) He II proles including
Bowen uorescence features. The latter are members of the growing class of AGN ares attributed to enhanced
accretion reported by Trakhtenbrot et al. We consider physical interpretations in the context of related transients
from 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 the sample belong to the
Trakhtenbrot et al. class and two are tidal disruption events in NLSy1s. We also hypothesize as to why NLSy1s are
preferentially the sites of such rapid enhanced aring activity.
Unied Astronomy Thesaurus concepts: Accretion (14); Active galaxies (17); AGN host galaxies (2017); Active
galactic nuclei (16); Seyfert galaxies (1447); High energy astrophysics (739)
1. Introduction
A galaxy center hosting an unobscured, or Type 1, active
galactic nucleus (AGN) is dominated by its continuum
emission. Therefore, a are originating from this nuclear
region requires a distinctly powerful event to be detectable
above this stochastically variable continuum. A small number
of rapid,
19
smoothly evolving ares have been observed to be
associated with AGNs (e.g., Drake et al. 2011; Blanchard et al.
2017), with few known mechanisms that can cause these events
to occur.
Intrinsic UV/optical ares, such as those due to enhanced
accretion onto the central supermassive black hole (SMBH) in
the form of gaseous material or stars passing too close to the
nucleus, have been observed in the form of tidal disruption
events (TDEs; see the recent review by van Velzen et al. 2021
and references therein), UV-bright aring events that are
associated with accretion rate changes (Trakhtenbrot et al.
2019a), transients with double-peaked line proles linked to
accretion disk emission (e.g., Halpern & Eracleous 1994),or
changing-look AGNsthe dramatic change in spectroscopic
AGN classication following a rise in continuum level, thought
to be connected to unstable changes in accretion state (e.g.,
LaMassa et al. 2015; MacLeod et al. 2016; Ruan et al. 2016;
Runnoe et al. 2016; Ross et al. 2018; Stern et al. 2018;
Frederick et al. 2019; Trakhtenbrot et al. 2019b; Graham et al.
2020 and references therein). There also exists the intriguing
possibility of links between these classes, such as TDEs
occurring in preexisting AGN disks (e.g., Merloni et al. 2015;
Chan et al. 2019, 2020).
The Astrophysical Journal, 920:56 (23pp), 2021 October 10 https://doi.org/10.3847/1538-4357/ac110f
© 2021. The American Astronomical Society. All rights reserved.
19
We refer to are timescales as rapid when they occur on a week to month
timescales.
1

Phenomena extrinsic to the SMBH accretion engine, such as
microlensing of a quasar by a foreground Galactic source (e.g.,
Lawrence et al. 2012) or slowly evolving superluminous
supernova (SLSN) explosions, have also been observed to
cause smooth large-amplitude ares from galaxies with AGNs
(Graham et al. 2017). In rare cases these can be astrometrically
indistinguishable from the galactic nucleus, and therefore, it
becomes difcult to discern whether an explosive disruption to
the accretion ow has occurred and to differentiate this from
AGN variability (Terlevich et al. 1992).
Multiwavelength approaches are required to disentangle this
diverse family of observed aring behaviors from AGNs. In the
golden era of time domain astronomy, even with many
multichromatic instruments trained on the sky, a number of
newly discovered objects continue to defy placement into a
clear-cut observational category.
In order of discovery, we present a photometric class
composed of ve rapid ares with similar smooth light-curve
shapes occurring in a subclass of AGNs observed by the
Zwicky Transient Facility (ZTF) survey:
(a) ZTF19aailpwl/ AT2019brs (z = 0.37362)
(b) ZTF19abvgxrq/AT2019pev (z = 0.097)
(c) ZTF19aatubsj /AT2019fdr (z = 0.2666)
(d) ZTF19aaiqmgl/AT2019avd (z = 0.0296)
(e) ZTF18abjjkeo/
AT2020hle (z = 0.103)
In Section 2 we present the follow-up of these ares. In
Section 3 we compare the results of their respective multi-
wavelength follow-up campaigns to observations of a variety of
related objects found in recent years, and in Section 4 we
attempt to place them into a classication scheme based
on observational properties, summarized in Section 5. All
transients in the sample are referred to by their International
Astronomical Union (IAU) recognized transient designations
throughout. All magnitudes are reported in the AB system and
light curves are shown in the observed frame unless otherwise
stated. We have adopted the following cosmology: H
0
=
70 km s
1
Mpc
1
, Ω
Λ
= 0.73, and Ω
M
= 0.27.
2. Observations
The ZTF Survey (Graham et al. 2019; Bellm et al. 2019a)
comprises the automated Palomar 48 inch Samuel Oschin
Telescope (P48) as well as the Palomar 60 inch Spectral Energy
Distribution Machine (P60 SEDM; Blagorodnova et al. 2018;
Rigault et al. 2019) integral eld unit spectrograph and has
surveyed the Northern Sky with g- and r-band lters with a
three-night cadence since 2018 (Bellm et al. 2019b). At least 15
images meeting good-quality criteria were stacked to build a
coadded reference image of each observing eld and quadrant
in each lter band. Science images are subtracted by their
references and processed each night by the Infrared Processing
and Analysis Center (IPAC) pipeline (Masci et al. 2019 ). The
candidate transient alert stream (Patterson et al. 2019 ) is
distributed by the University of Washington Kafka system and
ltered through the AGN and BH Science Working Groups
Nuclear Transients
20
parameter criteria (outlined in van Velzen
et al. 2019, 2021) by the Ampel broker (Soumagnac &
Ofek 2018; Nordin et al. 2019), with the GROWTH Marshal
user interface utilized for the coordination of follow-up efforts
(Kasliwal et al. 2019).
All 5 transients included in the sample presented here were
selected based on the following criteria: large-amplitude,
nuclear variability (Δg > 1 mag in difference imaging photo-
metry, and within 0
5 of the center of the host galaxy in the
reference image) with follow-up or preare spectra consistent
with an AGN classication. This selection was not systematic
(and therefore not complete), but rather the result of ongoing
intersecting and collaborative searches for changing-look AGN
(Frederick et al. 2019), TDEs (van Velzen et al. 2019, 2021),
and superluminous supernovae (SLSNe; Lunnan et al. 2020;
Yan et al. 2020) relying on partial human vetting from the ZTF
transient alert stream, from which this sample emerged as more
examples were collected. A systematic search for NLSy1
transients in ZTF will be the focus of a future study.
2.1. Optical Photometry
All transients in the sample were detected prepeak using ZTF
difference imaging photometry. The smooth light-curve shapes
(with scatter Δg < 0.1 mag) of the sample are shown in
Figure 1. All magnitude changes are reported in the g band
unless otherwise noted. An analysis of the rise times to peak are
measured and reported in Section 3.1.1. We report the g-band
magnitude-weighted offsets for each transient, calculated
using Equation (3) in van Velzen et al. (2019). ZTF forced
photometry for the sample is shown in Figure 11 of the
Appendix.
AT2019brs(R.A. = 14:27:46.41, decl. =+29:30:38.6,
J2000.0), also known as ZTF19aailpwl, it was rs t detec ted
on 2019 February 08 as a nuclear transient within 0
17 of the
host galaxy center.
AT2019pev(R.A. = 04:29:22.72, decl. =+00:37:07.6,
J2000.0) also known as ZTF19abvgxrq and Gaia19eby, it
was r st detected on 2019 Se pt em ber 4 as a nuclear transient
within 0
15 of the host galaxy center. ATLAS, Gaia, and
PanSTARRs also reported observations of this source on the
Transient Name Server ( TNS ) with discovery d ates of 2019
September 4, 2 019 September 13, and 2019 September 26,
respective ly. The ho st galaxy displayed no variability above
the 0.5 mag level in CRTS.
AT2019fdr(R.A. = 17:09:06.86, decl. =+26:51:20.7,
J2000.0) also known as ZTF19aa tub sj, it was det ecte d on
2019 April 27 wit h a signicant ux increase with respect to
the reference image and wi th an offset from the nucleus of its
host of 0
13. During a coverage gap in the rst 40 days of the
rise,ATLASreportedanintriguingbump feature (Smartt
et al. 2019). The host galaxy di sp laye d variability at the 2 mag
level in V-band CRTS data from 2009 to 2013 (variability
which was not observed in ZTF forced photometry prior to the
transient).
AT2019avd(R.A. = 08:23:36.77, decl. =+04:23:02.5,
J2000.0) also known as eRASSt J082337+042303
21
and
ZTF19aaiqmgl, it was detected by ZTF beginning on 2019
February 9 within 0
06 of its host galaxy. The host showed no
variability in CRTS for 15 yr prior to its rapid rise to peak.
20
A nuclear transient was dened as that within 0 5 of the reference galaxy
center. Over 9000 nuclear transients passed this lter and were ranked during
ZTF Phase I, of which 27 were TDEs, over 7% were classied as SN, and over
half were AGN or candidate AGN.
21
This was the only source in the sample to be detected by the extended
ROentgen Survey with an Imaging Telescope Array (eROSITA, part of the
Russian-German Spectrum-Roentgen-Gamma (SRG) mission; Predehl et al.
2021) and was given the name eRASSt J082337+042303. This X-ray detection
coincident with the transients host galaxy is described in Section 2.4 .
2
The Astrophysical Journal, 920:56 (23pp), 2021 October 10 Frederick et al.

AT2020hle(R.A. = 11:07:42.91, decl. =+74:38:02.0,
J2000.0 ) also known as ZTF18abjjkeo, it was detected
beginning on 2020 April 05 within 0
02 of its host galaxy
center. The ZTF forced photometry for this source shows no
variability above the level of the galaxy for >400 days. The host
galaxy of AT2020hle was beyond the survey limits of CRTS.
2.2. Optical Spectroscopy
All spectroscopic follow-up observations for the sample are
summarized in Table 1, and each epoch is shown in Figure 12
of the Appendix. The spectroscopic data of the sample shown
in Figures 7 and 6 are archived online at the Weizmann
Interactive Supernova data REPository (WISeREP; Yaron &
Gal-Yam 2012).
22
The phases of the optical follow-up spectra
with respect to the features in the ZTF light curves are
annotated in Figure 1. All transients in this sample have
spectral characteristics of NLSy1 galaxies, i.e., strong Balmer
line emission with FWHM < 2000 km s
1
, along with other
spectral features that are highlighted below and explored in
detail in Section 3.2.
Spectra taken with the Lowell Discovery Telescope (LDT;
formerly DCT) Deveny spectrograph (PI: Gezari), the Alham-
bra Faint Object Spectrograph and Camera (ALFOSC) on the
2.56 m Nordic Optical Telescope (NOT; PI: Sollerman), and
the KAST Double Spectrograph on the Lick 3 m Shane
Telescope (PI: Foley) were reduced using standard IRAF
procedures including wavelength calibration using arc lamps
and ux calibration using a standard star. Spectra obtained with
the Low-Resolution Imaging Spectrometer (LRIS) on the Keck
I 10 m telescope were reduced automatically with the LRIS
reduction pipeline Lpipe Perley (2019). Spectroscopy
obtained with the Double Beam Spectrograph (DBSP) on the
Palomar 200 inch Hale Telescope (P200; PI: Yan) were
reduced using the pyraf-dbsp pipeline (Science Software
Branch at STScI 2012; Bellm & Sesar 2016). Folded Low
Order whYte-pupil Double-dispersed Spectrograph North
(FLOYDS-N) spectra were obtained from the TNS (Arcavi
et al. 2019). Data taken by the robotic 2 m Liverpool Telescope
(LT) SPectrograph for the Rapid Acquisition of Transients
(SPRAT; PI: Perley) were reduced by the standard pipeline
provided by the Observatorio del Roque de los Muchachos.
Lower-resolution (R 100) spectra taken by the Palomar 60
inch SEDM (Program PIs: Gezari, Sollerman, Kulkarni) were
reduced automatically with
pysedm (Rigault et al. 2019).
AT2019brsshowed dramatic changes relative to the 2006
SDSS spectrum, which had already identied the host galaxy as
an NLSy1 (Rakshit et al. 2017; Abolfathi et al. 2018).
The follow-up FLOYDS-N (Arcavi et al. 2019) and LDT
(PI: Gezari) spectra showed a steep blue continuum and a
strong H
II prole with Bowen uorescence features, indicating
it became a aring SMBH belonging to the observational class
established by Trakhtenbrot et al. (2019a).
AT2019pevspectroscopically identied as an NLSy1 on
2019 September 15 with LT SPRAT (PI: Perley) , based on the
width of the Balmer emission lines and the strength of the
[O
III] λ5007 emission line. Gezari et al. (2019) reported that
the LT spectrum showed evidence for blueshifted H
II λ4686
emission as well as N
III λ4640 emission, due to the Bowen
uorescence mechanism, placing it again in the observational
subclass of the Trakhtenbrot et al. (2019a) objects. Near peak it
was observed with the Keck 10 m Low Resolution Imaging
Spectrometer (LRIS; PI: Graham) as well as the LDT Deveny
Spectrograph (PI: Gezari) and Lick (PI: Foley), which
conrmed the strong blue continuum and clearly dened and
persistent Bowen uorescence features.
Figure 1. Comparison of the ZTF g- and r-band difference imaging light-curve shapes and absolute magnitudes of the sample. AT2019fdr decreases before reaching a
second plateau stage and undergoes signicant reddening after the rst plateau while the others never do. AT2019pev rises again symmetrically after decreasing to
preare levels, as does AT2019avd. 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 reects true colors such that the initial colors approach g r = 0 mag for all transients in the sample. Observations at other
wavelengths are shown in Figure 2. Spectroscopic epochs are labeled for each light curve with an S below AT2019fdr and AT2020hle and above the rest.
22
https://www.wiserep.org/
3
The Astrophysical Journal, 920:56 (23pp), 2021 October 10 Frederick et al.

AT2019fdrobserved 8 days after peak on 2019 July 03
with the P200 DBSP (PI: Yan). We measured a signicant
blue horn component of Hβ and marginally detected H
II.
The transient continuum of AT2019fdr faded to reveal an
underlying Fe
II complex in the NOT ALFOSC (PI: Sollerman)
spectrum taken nearly 368 days after peak on 2020 April 30,
with no evidence for H
II emission.
AT2019avdThe spectrum taken with NOT (PI: Sollerman)
on 2019 March 15 near the rst optical peak showed strong
Balmer line emission, no detection of a H
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 to the TNS by Trakhtenbrot et al. (2020) showed the
appearance of H
II and Bowen uorescence features and a blue
horn in Hβ. Again this event was classied as a member of the
Trakhtenbrot et al. (2019a) observational class of aring NLSy1s.
AT2020hleIn the LT (PI: Perley) spectrum of AT2020hle
taken on 2020 May 18, 8 days after peak, the narrow component
of the H
II prole is signicantly blueshifted. No Fe II line
complex was detected in the spectra of this transient.
2.3. UV Photometry
We triggered target-of-opportunity monitoring observations
with the Neil Gehrels Swift Telescope (Gehrels et al. 2004) for
all transients in the sample. Using the HEASOFT command
uvotsource, we extracted UVOT photometry within a 5
radius circular aperture and using an annular background
region centered on the coordinates of the optical transient.
Figure 2 shows the νL
ν
light curves of all ares in the sample.
We compare ZTF g- and r-band difference imaging, Wide-eld
Infrared Survey Explorer (WISE) difference imaging, Swift
X-ray Telescope (XRT) monitoring, and Swift UVOT detections
subtracted by the archival Galaxy Evolution Explorer (GALEX;
Martin et al. 2005) All-Sky Imaging Survey (AIS; Bianchi et al.
2017) near-UV (NUV; λ
eff
= 2310 Å) host measurements
(measured with a 6 radius aperture). The GALEX coadds
include observations of the host galaxies taken throughout the
survey duration between 2003 and 2013.
We found all transients in the sample to be UV detected,
but with varying UV colors. The UV color of AT2019avd
Table 1
Summary of Spectroscopic Follow-up Observations of the Sample
Name Obs UT Instrument Exposure (s) Reference
AT2019brs 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 work
2020 Mar 28 Palomar 60 SEDM 2250 This work
2020 Dec 06 LDT Deveny 900 This work
AT2019pev 2019 Sep 08 Palomar 60 SEDM 2250 This work
2019 Sep 15 LT SPRAT 500 This work
2019 Sep 22 Palomar 60 SEDM 2250 This work
2019 Sep 24 LDT Deveny 600 This work
2019 Sep 25 Keck LRIS 300 This work
2019 Sep 25 NICER 2000 Kara et al. (2019)
2019 Oct 01 Chandra LETG 45400 Miller et al. (2019)
2019 Oct 05 Lick 3 m KAST 1500 This work
2019 Oct 12 LT SPRAT 500 This work
2019 Oct 15 Chandra LETG 91000 Mathur et al. (2019)
2019 Oct 23 LDT Deveny 900 This work
2019 Nov 01 Palomar 60 SEDM 2250 This work
2019 Dec 03 LDT Deveny 2400 This work
2020 Jan 30 Swift XRT 94700 This work
2020 Feb 26 LDT Deveny 2600 This work
2020 Sep 13 LDT Deveny 3200 This work
AT2019fdr 2019 May 25 Palomar 60 SEDM 2250 This work
2019 Jun 17 LT SPRAT 900 This work
2019 Jun 22 LDT Deveny 900 This work
2019 Jul 03 Palomar 200 Hale 600 This work
2020 Apr 30 NOT ALFOSC 1750 This work
2020 Jun 04 Palomar 60 SEDM 2250 This work
2020 Jun 09 LDT Deveny 900 This work
2020 Sep 15 LDT Deveny 900 This work
AT2019avd 2020 Mar 15 NOT ALFOSC 1800 Malyali et al. (2021)
2020 Apr 28 SRG eROSITA 140 Malyali et al. (2021)
2020 May 10 FLOYDS-S 3600 Trakhtenbrot et al. (2020)
2019 Sep 18 Palomar 60 SEDM 2250 This work
2019 Sep 19 Palomar 60 SEDM 2250 This work
2019 Sep 22 Palomar 60 SEDM 2250 This work
AT2020hle 2020 May 16 Palomar 60 SEDM 2250 This work
2020 May 18 LT SPRAT 1000 This work
2020 Dec 06 LDT Deveny 2400 This work
4
The Astrophysical Journal, 920:56 (23pp), 2021 October 10 Frederick et al.

(UVW1 g = 0.2 mag) was similar to that of AT2019pev
(UVW2 g = 0.2 mag) and AT2019brs (ranging from
UVW2 g = 0.1 mag to 0.7 mag in 80 days) but
AT2019fdr was the only transient in the sample with red
UV color ( UVW2 g = 0.8 mag). The UV light curves of
the sample tend to follow the shape of the optical.
Figure 2. We track the colors of the transients in the sample with a ν L
ν
light curve, comparing the ZTF and WISE data to concurrent high-cadence Swift UVOT and
XRT monitoring observations. The X-ray rise and fade of AT2019pev tracks the optical/UV with no signicant delay. Times are given in days since the rst ZTF
detection, and data have been host/reference-subtracted. The X-ray error bars are comparable to the size of the data points. See Figure 11 for pre-outburst forced
photometry.
5
The Astrophysical Journal, 920:56 (23pp), 2021 October 10 Frederick et al.

Citations
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Journal ArticleDOI
TL;DR: Recently, one high-energy neutrino was associated with a tidal disruption event (TDE) as mentioned in this paper , coincident with another high energy Neutrino source, and the probability of finding two such bright events by chance is just 0.034%.
Abstract: The origins of the high-energy cosmic neutrino flux remain largely unknown. Recently, one high-energy neutrino was associated with a tidal disruption event (TDE). Here we present AT2019fdr, an exceptionally luminous TDE candidate, coincident with another high-energy neutrino. Our observations, including a bright dust echo and soft late-time x-ray emission, further support a TDE origin of this flare. The probability of finding two such bright events by chance is just 0.034%. We evaluate several models for neutrino production and show that AT2019fdr is capable of producing the observed high-energy neutrino, reinforcing the case for TDEs as neutrino sources.Received 16 December 2021Accepted 9 March 2022DOI:https://doi.org/10.1103/PhysRevLett.128.221101© 2022 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasCosmic ray accelerationCosmic ray sourcesElectromagnetic radiation astronomyExtrasolar neutrino astronomyTransient & explosive astronomical phenomenaGravitation, Cosmology & Astrophysics

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Journal ArticleDOI
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Journal ArticleDOI
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9 citations

Journal ArticleDOI
TL;DR: In this article , it was shown that IC200530A may originate from the hydrogen-rich superluminous supernova AT2019fdr, which was initially suggested to be a tidal disruption event in a Narrow Line Seyfert 1 galaxy.
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Frequently Asked Questions (2)
Q1. What are the contributions mentioned in the paper "A family tree of optical transients from narrow-line seyfert 1 galaxies" ?

The authors report UV and X-ray follow-up of the flares and observe persistent UV emission, with two of the five transients detected with luminous X-ray emission, ruling out a supernova interpretation. The authors compare the properties of this sample to previously reported flaring NLSy1 galaxies and find that they fall into three spectroscopic categories: The latter are members of the growing class of AGN flares attributed to enhanced accretion reported by Trakhtenbrot et al. The authors consider physical interpretations in the context of related transients from the literature. 

Based on observed groupings of the sample, the authors propose the following naming scheme of spectroscopic classes of such transients for use in future optical surveys: “ NLSy1He II, ” “ NLSy1-He II+N III, ” and “ NLSy1-Fe II. ” they ruled out the possibility that these are Type IIn SNe occurring in NLSy1 systems. The authors suggest four different possible explanations for this enhancement: 1. A selection bias due to shorter timescales for lower-mass BH systems ( like NLSy1s ), which are therefore more likely to be captured using transient-detection methods that systematically ignore slower events within the baseline of wide-field optical surveys, 2. A systematic disregard of smooth flares in broad-line AGNs during transient searches, or 3. For two in the sample ( AT2019pev and AT2019avd ), the authors can rule out the simple TDE scenario from rebrightening in their light curves, and they determine that they, along with AT2019brs ( which had a preflare NLSy1 spectral classification and a BH mass estimate too large to host a canonical TDE ), are likely outbursts related to enhanced accretion in excess of typical AGN variability and with spectral features they classify as “ NLSy1-He II+N III, ” and members of the Trakhtenbrot et al. ( 2019a ) class of AGN flares. The authors hope this classification scheme will guide real-time predictions for the potential future behavior of large-amplitude flares in NLSy1s, which are clearly an interesting population for future study.