J/ApJ/957/57 Finding tidal disruption event & flares in ZTF (Dgany+, 2023)
Needle in a haystack: finding supermassive black hole-related flares in the
Zwicky Transient Facility public survey.
Dgany Y., Arcavi I., Makrygianni L., Pellegrino C., Howell D.A.
<Astrophys. J., 957, 57 (2023)>
=2023ApJ...957...57D 2023ApJ...957...57D
ADC_Keywords: Transient; Supernovae; Active gal. nuclei; Redshifts; Surveys;
Black holes; Cross identifications
Keywords: Supernovae ; Tidal disruption ; Supermassive black holes ;
Active galactic nuclei ; Astronomical methods
Abstract:
Transient accretion events onto supermassive black holes (SMBHs), such
as tidal disruption events (TDEs), Bowen Fluorescence Flares (BFFs),
and active galactic nuclei (AGNs), which are accompanied by sudden
increases of activity, offer a new window onto the SMBH population,
accretion physics, and stellar dynamics in galaxy centers. However,
such transients are rare and finding them in wide-field transient
surveys is challenging. Here we present the results of a systematic
real-time search for SMBH-related transients in Zwicky Transient
Facility (ZTF) public alerts, using various search queries. We
examined 345 rising events coincident with a galaxy nucleus, with no
history of previous activity, of which 223 were spectroscopically
classified. Of those, five (2.2%) were TDEs, one (0.5%) was a BFF, and
two (0.9%) were AGN flares. Limiting the search to blue events, the
fraction of TDEs nearly doubles to 4.1%, and no TDEs are missed.
Limiting the search further to candidate post-starburst galaxies
increases the relative number of TDEs to 16.7%, but the absolute
numbers in such a search are small. The main contamination source is
supernovae (95.1% of classified events), of which the majority (82.2%
of supernovae) are of Type Ia. In a comparison set of 39 events with
limited photometric history, the AGN contamination increases to
∼30%. Host galaxy offset is not a significant discriminant of TDEs in
current ZTF data, but might be useful in higher-resolution data. Our
results can be used to quantify the efficiency of various SMBH-related
transient search strategies in optical surveys such as ZTF and the
Legacy Survey of Space and Time.
Description:
We searched the ZTF real-time alert stream for transients in galaxy
centers every day between 2020 November 3 and 2022 March 6 (UT dates),
with the exception of a ∼2 month break due to a ZTF technical outage
between 2021 December 5 and 2022 February 17. In total, our search
includes alerts from 414 days.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table8.dat 223 391 *Candidates of interest
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Note on table8.dat: Since some queries are subsets of other, here we list only
the most stringent query which produced each event.
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See also:
V/147 : The SDSS Photometric Catalogue, Release 12 (Alam+, 2015)
V/154 : Sloan Digital Sky Surveys (SDSS), Release 16 (DR16) (Ahumada+, 2020)
J/ApJ/673/999 : Supernova and host galaxies metallicities (Prieto+, 2008)
J/ApJ/753/106 : QSO variability with SDSS and POSS imaging (MacLeod+, 2012)
J/ApJ/793/38 : Palomar Transient Factory photometric obs. (Arcavi+, 2014)
J/MNRAS/438/L101 : First month on SN 2013ej (Valenti+, 2014)
J/MNRAS/463/296 : PS1 slow-blue nuclear hypervariables (Lawrence+, 2016)
J/ApJ/868/99 : Post-starburst galaxies identifications (French+, 2018)
J/ApJ/873/92 : Opt. & UV evolution of iPTF15af (Blagorodnova+, 2019)
J/ApJ/905/93 : Classif. of PS1-MDS SNe with Superphot (Hosseinzadeh+, 2020)
J/ApJ/905/94 : Classif. for PS1-MDS SNe with SuperRAENN (Villar+, 2020)
J/ApJ/924/121 : Transients in the centers of PS & QBS gal. (Arcavi+, 2022)
J/ApJ/949/120 : Extragalactic fast blue opt. transients from ZTF (Ho+, 2023)
J/ApJ/953/32 : AT 2021loi photometry & sp. follow-up (Makrygianni+, 2023)
http://www.wis-tns.org/ : Transient Name Server homepage
http://www.ztf.caltech.edu/ : Zwicky Transient Facility homepage
http://sites.astro.caltech.edu/ztf/bts/bts.php : ZTF Bright Transient Survey
Byte-by-byte Description of file: table8.dat
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Bytes Format Units Label Explanations
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1- 12 A12 --- ZTF ZTF identifier
14- 23 A10 --- TNS TNS identifier
25- 33 A9 --- TNS2 Other TNS identifier
35- 70 A36 --- Name Other common identifier(s)
72- 80 F9.5 deg RAdeg Right Ascension in decimal degrees (J2000)
82- 90 F9.5 deg DEdeg [-29.71/83.12] Declination (J2000)
92- 93 I2 --- Query [1/10] Query condition (1)
95-109 A15 --- Class SN classification (2)
111-121 F11.9 --- z [0.0014/0.23]? Spectroscopic redshift
123-123 A1 --- Note Additional note (3)
125-223 A99 --- Ref Bibcode(s) for reference(s)
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Note (1): We used the custom query builder on version 1.0 of the Lasair broker
(https://lasair.lsst.ac.uk) to filter the alerts. Lasair uses a
contextual classifier called Sherlock (https://lasair.readthedocs.io/en/
develop/core_functions/sherlock.html). Sherlock is a boosted decision
tree algorithm that provides an initial classification of every
nonmoving object by performing a spatial crossmatch against data from
historical and ongoing astronomical surveys.
Our queries, which are based on the TDE queries by M. Nicholl on
version 1.0 of Lasair, filter ZTF alerts according to the following
criteria:
a = The candidate is within a certain threshold distance of the
nearest Sherlock catalog source. For 80% of our sample, we choose a
threshold of 0.5".
b = The nearest catalog source is likely a galaxy rather than a star:
objects.sgscore1<0.5.
c = The Sherlock classification of the candidate is either "SN" (Supernova)
or "NT" (Nuclear Transient)
d = The candidate does not have detections more than 100 days ago
(indicating that it might be a variable, rather than a transient source)
e = The candidate does not have a ZTF17 or ZTF19 name, meaning that it was
not created by ZTF in 2017 or 2019
f = The candidate is not a previously classified SN
g = The candidate has <three of its detections deemed unreliable
h = At least one of those detections was no more than 14 days ago
i = The candidate is more than 10° away from the Galactic plane
j = The latest g- or r-band magnitude of the candidate is brighter than
19 or 19.5.
h = The candidate has a g-r magnitude difference <0.05
In total, ten queries are created as follows:
1 = Conditions a-i, with a limiting magnitude <19, blue (Condition h), and
in a post-starburst (PS) galaxy.
2 = Conditions a-i, with a limiting magnitude <19 and blue
3 = Conditions a-i, with a limiting magnitude <19
4 = Conditions a-i, with a limiting magnitude <19.5, blue, and
in a post-starburst (PS) galaxy.
5 = Conditions a-i, with a limiting magnitude <19.5 and blue
6 = Conditions a-i, with a limiting magnitude <19.5
7 = Conditions a-i, with a limiting magnitude <19, ran on Lasair 3.0 (Iris)
between 2022 April 6 and 2022 August 2 (for a total of 118 days)
8 = Conditions a-i, with a limiting magnitude <19.5, ran on Lasair 3.0 (Iris)
between 2022 April 6 and 2022 August 2 (for a total of 118 days)
9 = Conditions a-i, with a limiting magnitude <19 and in a post-starburst
(PS) galaxy, ran on Lasair 3.0 (Iris) between 2022 April 6 and
2022 August 2 (for a total of 118 days)
10 = Conditions a-i, with a limiting magnitude <19.5 and in a post-starburst
(PS) galaxy, ran on Lasair 3.0 (Iris) between 2022 April 6 and
2022 August 2 (for a total of 118 days)
See Section 2 for further explanations.
Note (2): Classification as follows:
AGN = Flaring active galactic nuclei (8 occurrences)
BFF = Bowen Fluorescence Flare (1 occurrence: AT 2021seu)
TDE = tidal disruption event (5 occurrences: AT 2020vwl,
AT 2021ehb, AT 2022bdw, AT 2022csn and AT 2022dbl)
CV = Cataclysmic Variable (1 occurrence)
Varstar = Variable Star (1 occurrence)
Galaxy = "galaxy" (which means that it was either an artifact or
it faded before the spectrum was obtained; 1 occurrence)
Other = 1 occurrence: AT 2022amc, which displays a featureless blue
continuum. This could have been a young core-collapse SN
or some other hot flare, including an SMBH-related one,
such as a TDE or BFF. Unfortunately, no follow-up spectra
were posted to TNS or, to our knowledge, published
elsewhere, so its nature remains undetermined.
SLSN-I = (1 occurrence)
SLSN-II = (1 occurrence)
SN = (1 occurrence)
SN I = (1 occurrence)
SN II = (24 occurrences)
SN IIP = (1 occurrence)
SN IIb = (1 occurrence)
SN IIn = (6 occurrences)
SN Ia = (171 occurrences)
SN Ia-91T-like = (15 occurrences)
SN Ia-91bg-like = (1 occurrence)
SN Ia-CSM = (1 occurrence)
SN Ia-pec = (1 occurrence)
SN Ib = (2 occurrences)
SN Ic = (3 occurrences)
SN Ic-BL = (4 occurrences)
See Section 3.
Note (3): Note as follows:
a = Not clear if indeed rising at discovery.
b = Lower than expected signal to noise in spectrum attempt.
c = Faded before spectrum was attempted (could have been rapidly evolving
or an artifact.
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History:
From electronic version of the journal
(End) Prepared by [AAS], Emmanuelle Perret [CDS] 15-Jan-2026