# Rapidly Evolving Transients in Archival ZTF Public Alerts

Wenxiong Li The School of Physics and Astronomy, Tel Aviv University, Tel Aviv
69978, Israel  Iair Arcavi The School of Physics and Astronomy, Tel Aviv
University, Tel Aviv 69978, Israel  CIFAR Azrieli Global Scholars program,
CIFAR, Toronto, Canada  Ehud Nakar The School of Physics and Astronomy, Tel
Aviv University, Tel Aviv 69978, Israel  Alexei V. Filippenko Department of
Astronomy, University of California, Berkeley, CA 94720-3411, USA  Thomas G.
Brink Department of Astronomy, University of California, Berkeley, CA
94720-3411, USA  WeiKang Zheng  Department of Astronomy, University of
California, Berkeley, CA 94720-3411, USA  Marco C. Lam The School of Physics
and Astronomy, Tel Aviv University, Tel Aviv 69978, Israel  Ido Keinan The
School of Physics and Astronomy, Tel Aviv University, Tel Aviv 69978, Israel
Seán J. Brennan Department of Astronomy, The Oskar Klein Center, Stockholm
University, AlbaNova, 106 91 Stockholm, Sweden  School of Physics, O’Brien
Centre for Science North, University College Dublin, Belfield, Dublin 4,
Ireland  Noi Shitrit The School of Physics and Astronomy, Tel Aviv University,
Tel Aviv 69978, Israel

###### Abstract

We search the archival Zwicky Transient Facility public survey for rapidly
evolving transient (RET) candidates based on well-defined criteria between
2018 May and 2021 December. The search yielded 19 bona-fide RET candidates,
corresponding to a discovery rate of $\sim 5.2$ events per year. Even with a
Galactic latitude cut of $20^{\circ}$, 8 of the 19 events ($\sim 42$%) are
Galactic, including one with a light-curve shape closely resembling that of
the GW170817 kilonova (KN). An additional event is a nova in M31. Four out of
the 19 events ($\sim 21$%) are confirmed extragalactic RETs (one confirmed
here for the first time) and the origin of 6 additional events cannot be
determined. We did not find any extragalactic events resembling the GW170817
KN, from which we obtain an upper limit on the volumetric rate of
GW170817-like KNe of $R\leq$ 2,400 Gpc-3 yr-1 (95% confidence). These results
can be used for quantifying contaminants to RET searches in transient alert
streams, specifically when searching for kilonovae independently of
gravitational-wave and gamma-ray-burst triggers.

High energy astrophysics; Supernovae; Transient sources; Time domain astronomy

††facilities: Keck I/LRIS (Oke et al., 1995; Rockosi et al., 2010)††software:
Astropy (Astropy Collaboration et al., 2013), Aspired (Lam et al., 2021; Lam,
2023), autophot Brennan & Fraser (2022), LPipe (Perley, 2019b), Matplotlib
(Caswell et al., 2021), Pandas (Reback et al., 2022), simsurvey (Feindt et
al., 2019).

##  1 Introduction

Most extragalactic transients discovered by wide-field optical surveys are
thermonuclear and core-collapse (CC) supernovae (SNe) with characteristic
timescales (i.e., rest-frame time spent above half of their peak brightness,
$t_{1/2}$) longer than 10 days (Perley et al., 2020). Over the last decade,
optical surveys have discovered a new class of transients with rapidly
evolving light curves (on timescales of just a few days). Initially, such
rapidly evolving transients (RETs) of a variety of spectral types were studied
individually, such as SN 2002bj (officially classified as an SN Ib but with a
very peculiar spectrum; Poznanski et al., 2010), SN 2010X (classified as a so-
called “point Ia” or SN .Ia; Bildsten et al., 2007; Kasliwal et al., 2010),
PTF09uj (SN IIn; Ofek et al., 2010), SN 2005ek (SN Ic; Drout et al., 2013),
iPTF15ul (SN Ibn; Hosseinzadeh et al., 2017), iPTF16asu (SN Ic-BL; Whitesides
et al., 2017), and SN 2018kzr (McBrien et al., 2019). The $Kepler$ space
telescope, during its K2 mission, also discovered a RET, KSN 2015K, which was
observed with a 30 min cadence (Rest et al., 2018).

With more discoveries, RETs started to be studied in samples. Drout et al.
(2014) presented 14 RETs from Pan-STARRS1 (PS1; Chambers et al., 2016). Arcavi
et al. (2016) reported four especially bright RETs with peak luminosities
between those of SNe and superluminous SNe (SLSNe; $M_{\rm peak}\approx-20$
mag) discovered by the Palomar Transient Factory (PTF; Law et al., 2009; Rau
et al., 2009) and the SN Legacy Survey (SNLS; Astier et al., 2006). Pursiainen
et al. (2018) discovered 72 RETs with the Dark Energy Survey (DES; Flaugher,
2005). Perley et al. (2020) classified 14 RETs with the Bright Transient
Survey of the Zwicky Transient Facility (ZTF; Bellm et al., 2019; Graham et
al., 2019; Fremling et al., 2020a). Recently, Ho et al. (2021) presented 42
RETs from Phase I of ZTF.

Many RETs were found in archival searches and thus have no spectroscopic data.
Even those found in real time are challenging to classify spectroscopically
given their rapid fading. Those that do have spectra show a variety of
characteristics, including blue continua and spectral features associated with
normal SNe of various types. This suggests that the RET population is likely
heterogeneous in nature.

Several scenarios have been proposed to explain the photometric and
spectroscopic properties of RETs, including shock breakout or post-shock
cooling emission from a dense wind circumstellar medium (CSM; Ofek et al.,
2010) or an extended stellar envelope (Drout et al., 2013; Whitesides et al.,
2017), the explosion of a stripped massive star (Tauris et al., 2013; Kleiser
& Kasen, 2014; Kleiser et al., 2018), white dwarf detonation (Arcavi et al.,
2016; McBrien et al., 2019), magnetar spin-down (Arcavi et al., 2016;
Whitesides et al., 2017), and CSM interaction (Arcavi et al., 2016). However,
limited by the low number of events and their heterogeneity, it is challenging
to constrain their distribution among different possible physical origins.

The identification of an electromagnetic counterpart to the gravitational wave
(GW) event GW170817 (see LIGO Scientific Collaboration, 2017, and references
therein) confirmed binary neutron star (BNS) mergers as yet another RET
channel, termed a “macronova” (Kulkarni, 2005) or “kilonova” (KN; e.g., Li &
Paczyński, 1998; Rosswog, 2005; Metzger et al., 2010; Metzger & Berger, 2012;
Barnes & Kasen, 2013; Tanaka & Hotokezaka, 2013; Metzger, 2019). The
optical/infrared emission of the electromagnetic transient, known as AT
2017gfo, provided evidence for $r$-process nucleosynthesis in the BNS merger
ejecta (e.g., LIGO Scientific Collaboration 2017; Arcavi et al. 2017; Chornock
et al. 2017; Coulter et al. 2017; Cowperthwaite et al. 2017; Drout et al.
2017; Díaz et al. 2017; Evans et al. 2017; Hu et al. 2017; Kasliwal et al.
2017; Lipunov et al. 2017; Pian et al. 2017; Shappee et al. 2017; Kilpatrick
et al. 2017; Smartt et al. 2017; Tanvir et al. 2017; Utsumi et al. 2017;
Valenti et al. 2017; see Nakar 2020 for a recent review). However, AT 2017gfo
also left many open questions unanswered, such as the nature of its early blue
emission (shock cooling, boosted relativistic ejecta, or low-opacity material;
see, e.g., Arcavi, 2018, and references therein), and how common the emission
properties of this single event are. Recently, Rastinejad et al. (2022) and
Troja et al. (2022) discovered a KN candidate in the afterglow of an extended-
emission gamma-ray burst (GRB; but see Waxman et al. 2022 for an alternative
explanation). In this case, disentangling the GRB afterglow from the KN
emission is not trivial, especially at early times. Therefore, stringent
constraints on the early blue emission of this event cannot be readily
obtained without making assumptions about the afterglow component.

Transient surveys have been optimizing their search strategies to maximize the
probability of a KN discovery without GW or GRB triggers. McBrien et al.
(2021) presented a marginal KN candidate, AT 2017des, discovered by PS1,
though its identification as a KN remains uncertain. Andreoni et al. (2020)
searched the first 23 months of ZTF data for candidate KNe, none of which were
deemed possible KNe after thorough vetting. Andreoni et al. (2021) introduced
a software infrastructure to identify KNe and RETs in real-time ZTF data. They
independently found several extragalactic RETs but no KN candidates in 13
months of observations.

Here we perform an independent archival search for unreported KNe and other
RETs in Phases I and II of the ZTF public “Northern Sky Survey” which covers
declinations $\delta\geq-31^{\circ}$ and Galactic latitudes $\mid b_{\rm
Gal}\mid>7^{\circ}$ (Bellm et al., 2019) from May 2018 to December 2021 (Phase
II started in October 2020). Each visible field was scheduled to be observed
once in the $g$ band and once in the $r$ band every third night during Phase I
and every other night during Phase II. To reduce the number of Galactic
variable sources as contaminants, we further restrict the selection here to
$\mid b_{\rm Gal}\mid>20^{\circ}$.

We use the Planck Collaboration et al.2020A&A…641A…6P (2020) cosmological
model throughout. Times are presented in UTC and magnitudes are given in the
AB system (Oke & Gunn, 1983).

##  2 Methods

We query the ZTF public alert stream between 2018 May 04 and 2021 December 18
using the criteria detailed below. Several public astronomical alert brokers
filter, store, and deliver ZTF alerts together with contextual information for
each alert. Among them are the Automatic Learning for the Rapid Classification
of Events (ALeRCE; Förster et al., 2021) broker111https://alerce.online/, the
Arizona-NOAO Temporal Analysis and Response to Events System (ANTARES; Narayan
et al., 2018)222https://antares.noirlab.edu/, Lasair (Smith et al.,
2019)333https://lasair-ztf.lsst.ac.uk/, FINK (Möller et al.,
2021)444https://fink-broker.org/, and Make Alerts Really Simple (MARS; which
has been discontinued)555https://mars.lco.global/. After experimenting with
several brokers, we chose ALeRCE for this work (though it could be undertaken
with most other brokers as well) owing to its rapid and complete databases,
informative website, connections to external archives, and user-friendly
python API access.

###  2.1 Selection Criteria

Since there is no exact definition of RETs, we need to determine a set of
selection criteria to efficiently separate them from the stream of roughly
100,000 alerts per day produced by ZTF. These criteria are based on the
information provided in the alerts. First, the time span ($t$) during which
RETs are detectable is relatively short. We choose the maximum $t$ to be 30
days (shorter than typical SNe, but longer than the KN AT 2017gfo, in the
optical bands). Second, we require that the number of positive subtraction
detections (ndet), with a real-bogus score (rb; Mahabal et al. 2019) $>0.5$,
be $>3$ to exclude likely artifacts with few sporadic detections.

The most important parameter of a RET is its magnitude rate of change,
$\mid\Delta m/\Delta t\mid$, where $\Delta m=m_{1}-m_{2}$ and $\Delta
t=t_{1}-t_{2}$ are the magnitude and time differences between two detections
or the first detection and the last nondetection of a candidate. Dietrich et
al. (2020) and Anand et al. (2021) modeled KN light curves from BNS and
neutron-star–black-hole (NSBH) mergers, respectively. Their derived 95%
thresholds for the expected decline rates between peak and six days post-peak
of NSBH (BNS) KNe are 0.57 (0.60) and 0.39 (0.49) mag day-1 in the $g$ and $r$
bands, respectively. To avoid missing KN-like events, we set the lower
threshold of the maximum $\mid\Delta m/\Delta t\mid$ of each candidate to 0.4
mag day-1. We further require $\Delta t>1$ hr and $\Delta m>\left(\delta
m_{1}+\delta m_{2}\right)$, where $\delta m_{i}$ is the uncertainty in each
magnitude measurement. Our approach identifies both rapidly rising and rapidly
declining transients, which sets it apart from some previous searches (which
focused only on rapid decline rates).

Finally, since we aim to discover extragalactic transients, we require that
there is a galaxy-like object in the vicinity of each candidate. We achieve
this via two approaches: (a) cross-matching each candidate position with the
Galaxy List for the Advanced Detector Era (GLADE; Dálya et al., 2018) version
2.3 catalog; and (b) cross-matching with galaxy-like sources (sgscore $<0.5$,
i.e., more likely a galaxy than a star, based on the star/galaxy separation
algorithm of Tachibana & Miller 2018) in PS1 images, which cover the entire
footprint of the ZTF survey. In both cases, we use a 100″ radius for the
cross-match.

To summarize, we select events that

  1. 1.

are detected for $<30$ days;

  2. 2.

have ndet $>3$ detections with a real-bogus score rb $>0.5$;

  3. 3.

have $\mid\Delta m/\Delta t\mid_{\rm max}\geq 0.4$ mag day-1;

  4. 4.

    1. (a)

are coincident within 100″ with a GLADE galaxy, or

    2. (b)

are coincident within 100″ with a PS1 object having an sgscore $<0.5$.

##  3 Results

A total of 730 candidates passed the selection criteria detailed in Section
2.1. We examined all of them by eye and found that most have erratic light
curves, indicating they are likely subtraction artifacts. We remove these
events and are left with 60 candidates with coherent light curves. For these
candidates, we request forced photometry through the public ZTF forced-
photometry service666https://ztfweb.ipac.caltech.edu/cgi-
bin/requestForcedPhotometry.cgi ($g$ and $r$ bands; Masci et al., 2019), the
Asteroid Terrestrial-impact Last Alert System (ATLAS) forced photometry
service777https://fallingstar-data.com/forcedphot/ ($o$ and $c$ bands; Tonry
et al., 2018; Smith et al., 2020), and the All-Sky Automated Survey for
SuperNovae (ASAS-SN) forced photometry service888https://fallingstar-
data.com/forcedphot/ ($g$ band; Shappee et al., 2014; Kochanek et al., 2017)
at the median coordinates of each candidate. We request forced photometry from
the start of available data for each survey (2018 March for ZTF, 2015 July for
ATLAS, and 2013 April for ASAS-SN) to 2021 December. The forced light curves
revealed that some candidates are in fact recurrent events (i.e., events with
historical outbursts), thus ruling them out as valid extragalactic RETs.
Furthermore, some candidates rapidly evolve only in certain phases (e.g.,
early days of an SN), but later evolve more slowly. Thus, we further remove
such events by requiring $t_{1/2}\leq 15$ days.

After applying all these filters, a total of 19 candidates remain; their
information is listed in Table 3.2. Except for ZTF19accjfgv (SN 2019rta),
which appears in the sample of Ho et al. (2021), none of our other candidates
are in the samples of Andreoni et al. (2020), Andreoni et al. (2021), or Ho et
al. (2021) (see Section 4 for discussion). We further exclude 10 events with
Transient Name Server999https://www.wis-tns.org/ (TNS) classifications. These
include one SN Ibn (SN 2018fmt), one SN Icn (SN 2021ckj), one SN IIb (SN
2019rta, probably detected during its rapidly evolving shock-cooling phase),
and seven Galactic objects. The light curves of the remaining nine
unclassified candidates are presented in Figure 1 and are discussed
individually below. We calculate peak magnitudes for each candidate using a
polynomial fit to their light curves around peak (taking the value of the
brighter band if there are observations in more than one band).

Figure 1: Comparison of the light curves of our RET candidates to the SN Ibc
$r$-band template (gray solid line) from Drout et al. (2011), the SN Ibn
$r$-band template (blue solid line) from Hosseinzadeh et al. (2017), and
$r$-band light curves of a few representative RETs: AT 2018cow (red solid
line; Perley et al., 2019a), and AT 2017gfo (the KN; black solid line;
Andreoni et al., 2017; Arcavi et al., 2017; Coulter et al., 2017;
Cowperthwaite et al., 2017; Díaz et al., 2017; Drout et al., 2017; Valenti et
al., 2017; Kasliwal et al., 2017; Pian et al., 2017; Shappee et al., 2017;
Smartt et al., 2017; Tanvir et al., 2017; Utsumi et al., 2017). In all cases
we shift the comparison light curves to match the peak apparent magnitude of
the RET candidates, except AT 2020ohw, for which we have a spectroscopic
redshift and can thus display the light curves in absolute magnitude. For that
event we also plot the $r$-band light curve of Type Ib SN iPTF13bvn (red
dashed line; Fremling et al., 2016) and the $R$-band light curve of Type Ic SN
1994I (red dash-dotted line; Richmond et al., 1996). In all light curves,
triangles denote 3$\sigma$ nondetection limits. A high-cadence oscillating
portion of the light curve is seen in the inset of AT 2019wjk.

###  3.1 ZTF18achdhvu

The rising rate of the light curve of ZTF18achdhvu is similar to the SN Ibn
template from Hosseinzadeh et al. (2017), while the decline rate resembles
that of the SN Ibc template from Drout et al. (2011). The $g$-band light curve
has a break at $\sim 20$ days after maximum brightness. There is an underlying
source, SDSS J122558.34+570930.3, in Sloan Digital Sky Survey (SDSS; York et
al., 2000) and PS1 images, 0.4″ from the position of ZTF18achdhvu with
$g=21.89\pm 0.07$ mag, sgscore = 0.04 (i.e., a galaxy-like source), and
$E(B-V)_{\rm MW}=0.011$ mag (Schlafly & Finkbeiner, 2011). It is classified as
a galaxy with a photometric redshift of $z_{\rm ph}=0.215\pm 0.072$ (Alam et
al., 2015) by SDSS.

Considering the high Galactic latitude ($b=59.6^{\circ}$) and the galaxy-like
morphology of the underlying source, it is reasonable to assume ZTF18achdhvu
is an extragalactic transient. Taking the photometric redshift, we derive a
distance modulus of $40.2^{+0.7}_{-1.0}$ mag, corresponding to Galactic-
extinction-corrected absolute peak magnitudes of
$M_{g}\approx-21.0^{+1.0}_{-0.7}$ mag and $M_{r}\approx-20.8^{+1.0}_{-0.7}$
mag.

###  3.2 AT 2019kzf (ZTF19abdvsjg)

AT 2019kzf was reported to the TNS through Gaia alerts (Hodgkin et al., 2019)
as Gaia19cxh, and was independently detected by ASAS-SN as ASASSN-19px. ATLAS
detected the event 15 days before the ZTF discovery. Assuming the first ATLAS
detection was at peak brightness, the evolution of the light curve of AT
2019kzf is slightly slower than AT 2018cow while faster than the SN Ibn
template. Its rise is consistent with that of AT 2018cow. A persistent source
is identified in SDSS and PS1 images at the position of AT 2019kzf with
$g\approx 21.8$ mag and sgscore = 0.47. We cannot determine the nature of this
source based on current data. If it is not a galaxy but rather a stellar
source, AT 2019kzf could be a cataclysmic variable (CV) as proposed by Necker
et al. (2022).

Table 1: RET Candidates that Passed Our Selection Criteria.

ZTF Name | Other Name(s) |  $\alpha$ (J2000) |  $\delta$ (J2000) | $b_{\rm Gal}$ | Classification | Reference
---|---|---|---|---|---|---
|  | (degrees) | (degrees) | (degrees) |  |
ZTF18achdhvu | — | 186.49321 | 57.1585 | 59.6 | Undetermined | (1)  |  |
ZTF19abdvsjg | AT 2019kzf/ | 255.83079 | 12.76519 | 29.7 | Undetermined | (2)  |  |
| ASASSN-19px |  . | . |  . |  |  |  |
ZTF19abfpwia | AT 2019lbw | 318.43609 |  $-$6.55881 |  $-$34.5 | Undetermined | (3)  |  |
ZTF19acxmocr | AT 2019wjk | 109.08012 | 37.45094 | 20.7 | Likely nova | (4)  |  |
ZTF20aapcjjo | AT 2020ddu | 290.48945 | 65.91445 | 21.6 | Undetermined | (5)  |  |
ZTF20abezywi | AT 2020mnt | 245.72241 |  $-$18.37980 | 21.5 | Undetermined | (6)  |  |
ZTF20abkdtvy | AT 2020ohw | 219.85853 | 13.0189 | 61.1 | Extragalactic | (7)  |  |
ZTF20ablatmf | AT 2020oor | 331.18954 | 12.48679 |  $-$33.4 | Undetermined | (8)  |  |
ZTF21abcyioo | — | 278.26289 | 59.02385 | 25.3 | Galactic | (9)  |  |
Classified candidates
ZTF18abqbuaj | SN 2018fmt | 20.25363 |  $-$13.86293 |  $-$75.0 | SN Ibn | (10)  |  |
ZTF18absrffm | AT 2018ftw | 260.23592 | 7.80938 | 23.6 | CV | (11)  |  |
ZTF18abulqzb | AT 2018fzl | 342.04971 | 16.23608 |  $-$37.3 | CV | (12)  |  |
ZTF19accjfgv | SN 2019rta | 127.20543 | 75.32802 | 32.3 | SN IIb | (13)  |  |
ZTF19acgfhfd | AT 2019sxc | 10.75647 | 41.29992 |  $-$21.5 | Nova in M31 | (14)  |  |
ZTF20acszhad | AT 2020aavs | 76.31652 |  $-$7.28544 |  $-$26.9 | Variable Star | (15)  |  |
ZTF21aajbgol | SN 2021ckj | 136.46285 |  $-$8.58543 | 24.8 | SN Icn | (16)  |  |
ZTF21abexegc | AT 2021plg | 219.82142 |  $-$18.07749 | 37.8 | CV | (17)  |  |
ZTF21abzydnp | — | 252.51872 | 24.563 | 36.8 | CV | (18)  |  |
ZTF21accbxzk | AT 2021zdk | 331.92443 | 17.87362 |  $-$30.0 | CV | (19)  |  |

References. — (1) This work; (2) this work; Hodgkin et al. (2019); (3) this
work; Nordin et al. (2019a); (4) this work; Nordin et al. (2019b); (5) this
work; Fremling (2020b); (6) this work; Nordin et al. (2020a); (7) this work;
Forster et al. (2020); (8) this work; Nordin et al. (2020b); (9) this work;
(10) Gromadzki et al. (2018); Fremling et al. (2018a); (11) Fremling et al.
(2018b); (12) Wevers et al. (2018); (13) Dahiwale & Fremling (2019); Ho et al.
(2021); (14) Dahiwale & Fremling (2020); (15) Díaz et al. (2020); (16)
Pastorello et al. (2021a, b); Gal-Yam et al. (2021); (17) Do (2021); (18)
Coppejans et al. (2016); (19) Payne (2021).

###  3.3 AT 2019lbw (ZTF19abfpwia)

AT 2019lbw was reported to the TNS by Nordin et al. (2019a). It has a very
rapid rise with $\mid\Delta m/\Delta t\mid=0.9$ mag day-1 and a post-peak
decline rate between that of AT 2018cow and the SN Ibn template. Approximately
20 days post-peak, the $r$-band light curve shows a steep drop. There is no
obvious host-galaxy candidate or another underlying optical source in the
vicinity of AT 2019lbw. We obtained deep imaging at the position of AT 2019lbw
using Keck I/LRIS (Oke et al., 1995; Rockosi et al., 2010) on 2022 September
21.

Forced point-spread-function (PSF) photometry at the position of AT 2019lbw
was performed using the autophot pipeline Brennan & Fraser
(2022)101010https://github.com/Astro-Sean/autophot, with a PSF model built
from bright, isolated sources in the image. Optical magnitudes were calibrated
against SDSS field stars. We detect a source with $g=24.33\pm 0.19$ mag and
$r=24.25\pm 0.34$ mag (Figure 5) at the position of AT 2019lbw, but cannot
determine if it is a Galactic or extragalactic object.

The Catalina Real-time Transient Survey (CRTS; Drake et al., 2009) detected an
outburst which was 1″ away from AT 2019lbw on 2009 May 27 with an unfiltered
magnitude of $\sim 17.5$, $\sim 1$ mag fainter than the peak of AT 2019lbw.
This event might be related to AT 2019lbw, with both events being flares of
the same Galactic source whose proper motion between 2009 to 2019 is $\sim
1$″. Alternatively, both events could be extragalactic transients from an
undetected host galaxy, or they could be two spatially coincident events
without any physical relation. Since we do not have proper-motion measurements
or spectroscopic observations of the source, we cannot distinguish between
these scenarios.

###  3.4 AT 2019wjk (ZTF19acxmocr)

AT 2019wjk was reported to the TNS by Nordin et al. (2019b). It met our RET
criteria owing to its $g$-band drop of 1.51 mag day-1, at $\sim 25$ days after
peak light. ATLAS $o$-band and ASAS-SN $g$-band data reveal a very rapid rise
of $\sim 1.5$ mag day-1, faster than that of AT 2018cow. With a low Galactic
latitude of $b_{\rm Gal}=20.7^{\circ}$ (very close to our cutoff), AT 2019wjk
is located within the ZTF high-cadence Galactic-plane survey fields (Kupfer et
al., 2021), with the data during the outburst published in ZTF Public Data
Release 7111111https://www.ztf.caltech.edu/ztf-public-releases.html (Masci et
al., 2019). The $\sim 5$ min cadence $r$-band light curve from this survey,
taken between 10 and 11 days after peak, clearly shows an oscillation pattern
between 16.6 and 17.0 mag (see inset in Figure 1), typical of post-peak light
curves of novae (Strope et al., 2010; Munari, 2012). A persistent source is
identified in PS1 images at the position of AT 2019wjk with $g=21.74\pm 0.04$
mag and sgscore = 0.30. To determine the nature of this source, we obtained a
Keck I/LRIS spectrum of the position of AT 2019wjk on 2022 March 03. The
spectrum was processed with the LPipe data-reduction pipeline (Perley, 2019b).
Figure 2 displays the reduced spectrum and the binned spectrum. It shows
double-peaked Balmer lines around a redshift of $z=0$. The low Galactic
latitude, rapid oscillations, and redshift zero double spectral lines are all
consistent with AT 2019wjk being a nova outburst of a CV.

Figure 2: The observed (light gray) and binned (black) spectrum taken at the
position of AT 2019wjk in 2022. The double-peaked $z\approx 0$ H$\alpha$ and
H$\beta$ emission lines (black dashed lines) are consistent with an accreting
Galactic source, making the 2019 outburst a possible nova. The continuum-
normalized spectra of H$\alpha$ and H$\beta$ in velocity space are displayed
in the inset.

###  3.5 AT 2020ddu (ZTF20aapcjjo)

AT 2020ddu was reported to the TNS by Fremling (2020b). It displays a rapid
rise, similar to that of AT 2018cow. After peak light, however, it evolves
slower, following the SN Ibn template until $\sim 15$ days after peak when the
light curve shows a drop in both the $g$ and $r$ bands. The decline rate
during this drop is 0.59 mag day-1 in $r$. No source is detected in PS1 at the
position of AT 2020ddu. We obtained deep imaging at this position using Keck
I/LRIS on 2022 July 01. The data-reduction process is the same as that
described in Section 3.3.

We marginally detect a source with $g=24.1\pm 0.3$ mag and $r=24.3\pm 0.5$ mag
(Figure 6) but cannot determine whether it is a Galactic or extragalactic
object.

###  3.6 AT 2020mnt (ZTF20abezywi)

AT 2020mnt was reported to the TNS by Nordin et al. (2020a). Assuming AT
2020mnt was discovered at peak brightness, the decline rate during the first
three days from discovery is comparable to that of AT 2018cow. After five days
the decline becomes slower and more similar to that of the SN Ibn template.
There is an underlying source visible in archival PS1 images 0.39″ from AT
2020mnt that has $g\approx 24$ mag, $r\approx 23$ mag (Bouy et al., 2013) and
sgscore = 0.23. We cannot determine the nature of this source based on current
data.

###  3.7 AT 2020ohw (ZTF20abkdtvy)

AT 2020ohw was reported to the TNS by Forster et al. (2020) and is the only
event in our sample with an archival spectrum of its host galaxy. The host,
SDSS J143926.04+130108.2, is at $z=0.02886\pm 0.00001$ (Tago et al., 2010),
with AT 2020ohw located at its center. The corresponding luminosity distance
is $134\pm 9$ Mpc and the distance modulus is $35.6\pm 0.2$ mag. The peak
absolute $r$-band magnitude of AT 2020ohw after Galactic extinction correction
is $M_{r}=-16.9\pm 0.3$ mag. The light-curve decline rate of AT 2020ohw is
between those of AT 2018cow and the SN Ibn template, with a maximum $g$-band
decline rate of $\mid\Delta m/\Delta t\mid_{\rm max}=0.50$ mag day-1 at $\sim
5$ days after peak. Its luminosity, however, is much lower than those of AT
2018cow and the SN Ibn template. In Figure 1, we further compare the $r$-band
light curve of AT 2020ohw with that of iPTF13bvn (Fremling et al., 2016), a
low-luminosity SN Ib, and with the $R$-band light curve of SN 1994I (Richmond
et al., 1996), a rapidly evolving SN Ic. AT 2020ohw is fainter than SN 1994I,
although both events have similar rising rates. The peak of the light curve of
AT 2020ohw is similar to that of iPTF13bvn but AT 2020ohw rises faster to
peak. We propose that AT 2020ohw is a RET, possibly a relatively low-
luminosity and rapidly evolving SN Ib/c.

###  3.8 AT 2020oor (ZTF20ablatmf)

AT 2020oor was reported to the TNS by Nordin et al. (2020b). The rise has a
similar rate to that of AT 2018cow, and the initial decline is even faster. In
the first four days after peak, the decline rate is similar to that of AT
2017gfo, but later slows down and even flattens. No source is detected in the
vicinity of AT 2020oor in archival images. We obtained $g$ and $r$ imaging at
the position of AT 2020oor with Keck I/LRIS on 2022 June 28. The data-
reduction process is the same as that described in Section 3.3. A possible
source is detected by autophot with a signal-to-noise ratio of $\sim 2$ in the
$g$ and $r$ images (Figure 7). We perform artificial source injection using
the PSF model of each image to determine the limiting magnitudes of the image
(see Brennan & Fraser (2022) for further details), obtaining a 3$\sigma$ upper
limit of 25.00 mag in $g$ and 24.95 mag in $r$. Unfortunately, we cannot
determine the nature of this source based on the current data.

###  3.9 ZTF21abcyioo

ZTF21abcyioo is the most promising KN candidate among our sample, having a
light curve similar in shape to that of AT 2017gfo (Figure 1). The last ZTF
$g$-band detection and adjacent upper limit, at six days after discovery, show
an even faster late-time decline than that of AT 2017gfo. The Galactic-
extinction-corrected $g-r$ color of ZTF21abcyioo is between $\sim-0.2$ and
$-0.3$ mag, not as red as that of AT 2017gfo which starts from $\sim 0$ mag
and evolves redward to $\sim 1.5$ mag at five days after peak. There is an
underlying source in archival PS1 images at the position of ZTF21abcyioo that
has $g=21.71\pm 0.04$ mag, $r=20.87\pm 0.03$ mag (Chambers et al., 2016), and
sgscore = 0.38. The Galactic latitude of ZTF21abcyioo is $b_{\rm
Gal}=25.3^{\circ}$, close to the limits of our search. The Gaia Data Release 3
(DR3) does not provide a measurable proper motion of this source (Gaia
Collaboration et al., 2022). The DESI Legacy Imaging Surveys catalog
classifies the archival source as a stellar object but does not provide an
ellipticity measurement (Zou et al., 2019; Dey et al., 2019).

To reveal the nature of ZTF21abcyioo, we obtained spectra at the position of
ZTF21abcyioo with Keck I/LRIS on 2022 September 23. The data were optimally
extracted with aspired121212https://github.com/cylammarco/ASPIRED (Lam et al.,
2021; Lam, 2023). We identify the prominent absorption doublet of sodium Na i
D $\lambda\lambda$5890 (D2), 5896 (D1) (Figure 3) at $z=0$. According to the
relation from Poznanski et al. (2012), the Galactic extinction of
$E(B-V)=0.04$ mag at this position (Schlafly & Finkbeiner, 2011) corresponds
to an equivalent width of EW(D1 \+ D2) $=0.39~{}\pm~{}0.07$ Å for the Na i D
doublet. However, we measure the observed EW(D1 \+ D2) from the spectrum to be
$\sim 17.6$ Å, much larger than the Galactic contribution. Therefore, we
suggest that the major contribution to these features is from the underlying
source, making it a $z=0$ Galactic object. Unfortunately, we cannot determine
the nature of the source or its outburst. To calculate the probability that
the spatial coincidence between ZTF21abcyioo and a foreground Galactic source
could be due to chance, we count the number of stellar sources in the PS1
catalog down to its limiting magnitude of $g=23.3$ mag (Chambers et al., 2016)
in a $0.1^{\circ}$ radius region around ZTF21abcyioo, finding 51 sources.
Conservatively, assuming we would associate ZTF21abcyioo with a stellar source
if it were within a radius of 2″ from it, we find that the probability of a
chance coincidence in this field is only 2%. This reaffirms our conclusion
that ZTF21abcyioo is probably of Galactic origin.

Figure 3: The observed (light gray) and binned (black) spectrum taken at the
position of ZTF21abcyioo in 2022. The $z=0$ Na i D absorption doublet is
labeled (dashed line), and the continuum-normalized spectrum of it is
displayed in the inset.

##  4 Discussion

Here we discuss the implications of our search results for future surveys.
First, we wish to quantify potential contaminants to RET searches in general,
and to KN searches in particular (both following, and independently of, GW
alerts). During the 3.75 yr period of our search, we found 60 RET candidates
with coherent light curves (see Section 3), or 16 events per year. Since we
required a limit on the duration of RET candidates, transients that start
rising quickly but are detected for a total duration of more than 30 days in
the ZTF survey are not included in our sample. Those transients could be
contaminants to real-time searches. Therefore, our estimate of 16 events per
year can only be considered as a lower limit for what can be found in real-
time searches. Of those, we identified 19 valid RET candidates in 3.6 yr,
corresponding to 5.2 events per year.

The distributions of Galactic latitude, $|b_{\rm Gal}|$, for our events are
displayed in Figure 4. As expected, Galactic objects are concentrated in lower
$b_{\rm Gal}$; however, there is still a significant number of these events
out to $|b_{\rm Gal}|=40^{\circ}$. Most undetermined events are also at
relatively low $|b_{\rm Gal}|$, indicating that they are likely Galactic as
well, with only one above $40^{\circ}$. RETs are more evenly distributed
compared to the other two groups. These results can help fine tune future
search criteria and contamination expectations.

Figure 4: The distribution of $|b_{\rm gal}|$ for undetermined events (blue),
Galactic objects (red), RETs (green), and an extragalactic nova (yellow).
Significant Galactic contamination extends out to $|b_{\rm gal}|$ = 40∘.

Next, we compare our search results with previous studies. Only one of our 19
RET candidates, SN 2019rta, overlaps with samples found by previous searches
(Ho et al., 2021). One main reason that we do not recover some of the existing
candidates is that we use data only from the public ZTF survey, which is a
subset of the full survey data available to Andreoni et al. (2020), Andreoni
et al. (2021), and Ho et al. (2021). Additionally, there are several
differences between our search criteria and those in the literature. Andreoni
et al. (2020) and Andreoni et al. (2021) selected only rapidly declining
transients while our search selected both rapidly rising and rapidly declining
events. Also, Andreoni et al. (2020) and Andreoni et al. (2021) rejected
candidates with a total time span of $>14$ days, which is much shorter than
our threshold of 30 days. Ho et al. (2021) required that the light curves of
the candidates be well sampled; there had to be an observation within 5.5 days
both before and after peak brightness in both the $g$ and $r$ bands, a
criterion that some of our candidates do not fulfill. Finally, all searches in
the literature ended before 2020 November; thus, all of our candidates from
after that epoch could not have been identified previously.

Our search yielded six undetermined events, four RETs (of which three were
previously known, and AT 2020ohw is newly identified here), but no valid KN
candidates. We can utilize these results to constrain the volumetric rate of
RETs and KNe. We use simsurvey (Feindt et al., 2019), a software package for
simulating transient light curves detected based on a given survey strategy,
to calculate the occurrence rates. The details of the ZTF CCDs and filters are
embedded in simsurvey. We use the actual observation schedule of the ZTF
public survey from our search period using the ZTF Public Data Release 14
(Masci et al., 2019) which includes coordinates, time, and filter information
for each exposure. Milky Way extinction was applied to each survey pointing
based on the measurements from Schlegel et al. (1998).

For KNe, we use the best-fit radiative transport parameters derived by possis
(Bulla, 2019) for AT 2017gfo in Dietrich et al. (2020) and assume the
distribution of viewing angle $i$ is uniform in cos($i$), following Andreoni
et al. (2020). We injected the simulated spectra into simsurvey in a redshift
range $z~{}\in$ (0, 0.03) with a given rate. We filter the simulated light
curves with the criteria described in Section 2.1. According to Poisson small-
number statistics, using the method of Gehrels (1986), and given that we
detected no KNe in our search, we find that the 95% confidence upper limit to
the number of actual KNe during our search period is three. Therefore, our
upper limit on the KN rate is the rate at which three KNe are recovered from
the simulation. Combining the survey schedule with the KN model, we derive a
rate of $R~{}\leq$ 2,400 Gpc-3 yr-1. This is consistent with but less
constraining than the rate limit of Andreoni et al. (2021) ($R~{}\leq$ 900
Gpc-3 yr-1 at 95% confidence) for GW170817-like KNe.

The rate calculation for RETs is slightly more complicated. Our search yielded
four RETs of varying peak $r$-band magnitudes ($-18.8$ for the Type Ibn SN
2018fmt, $-17.4$ for the Type IIb SN 2019rta, $-$19.8 for the Type Icn SN
2021ckj, and $-$16.6 for AT 2020ohw, the new RET identified here). Since the
assumed light-curve luminosity has a strong effect on the rate estimates, and
since we do not know the true luminosity function of RETs, here we estimate
the rate only of AT 2020ohw-like RETs (i.e., with a similar peak luminosity).
simsurvey requires a time-dependent spectral energy distribution (SED)
template, which we do not have for AT 2020ohw; we therefore use AT 2018cow as
a model, building the SED template from the AT 2018cow spectra of Perley et
al. (2019a) and Xiang et al. (2021) assuming the emission is isotropic. We
then scale the templates to match the light curve of AT 2020ohw around peak
and use that as input for simsurvey. The process described above is followed
to calculate the volumetric rate of RETs based on a single event discovered in
our search. With a 95% confidence interval, the number of actual AT 2020ohw-
like RETs during our search period is $1^{+3.74}_{-0.95}$, and the resulting
volumetric rate estimate is 1050${}^{+3750}_{-950}$ Gpc-3 yr-1. This is a
lower limit, even just on the rate of AT 2020ohw-like RETs, since we do not
know if any of the six undetermined events in our search are also of this
type. This result is therefore broadly consistent with the $\geq 1000$ Gpc-3
yr-1 rate calculated by Pursiainen et al. (2018) for FBOTs peaking between
$M_{g}=-15.8$ and $M_{g}=-22.2$ mag, and the 4800–8000 Gpc-3 yr-1 rate
calculated by Drout et al. (2011) for FBOTs peaking between $M_{g}=-16.5$ and
$M_{g}=-20$ mag.

##  5 Conclusions

Searching the archival ZTF public survey for RET candidates based on well-
defined criteria between 2018 May and 2021 December yielded 19 bona-fide RET
candidates, corresponding to a discovery rate of five events per year. To try
to determine their physical origin, we obtained observations of some
candidates using Keck I/LRIS. Of nine previously unclassified RET candidates,
we find that one is extragalactic, two are Galactic, and the nature of the
remaining six events remains undetermined. We derive a volumetric rate of RETs
similar to our one extragalactic confirmed candidate (which peaks at
$M_{r}=-16.6$ mag) of $R$ = 1050${}^{+3750}_{-950}$ Gpc-3 yr-1. No valid KN
candidates were detected, from which we are able to obtain an upper limit on
the volumetric rate of GW170817-like KNe of $R~{}\leq$ 2,400 Gpc-3 yr-1.

This work quantifies the contamination to KN and RET searches. Even with a
Galactic latitude cut of $20^{\circ}$, at least 8 and possibly up to 14 of the
19 events ($\sim 42$–74%) are Galactic, with a substantial number of them
extending out to a Galactic latitude of $40^{\circ}$. This includes one
Galactic event with a light-curve shape closely mimicking that of the GW170817
KN. Between 4 and 10 of the 19 events ($\sim 21$–53%) are RETs. These results
can be used for estimating contamination rates in searches for RETs in
transient alert streams and specifically for searching for KNe independently
of GW and GRB triggers.

## Acknowledgments

W.L. is supported by an Israel Science Foundation (ISF) grant (number 2752/19)
and a European Research Council (ERC) grant (JetNS) under the European Union’s
Horizon 2020 research and innovation program. I.A. is a CIFAR Azrieli Global
Scholar in the Gravity and the Extreme Universe Program and acknowledges
support from that program, from the ERC (grant agreement number 852097), from
the ISF (grant number 2752/19), from the United States – Israel Binational
Science Foundation (BSF), and from the Israeli Council for Higher Education
Alon Fellowship. M.C.L. is supported by the ERC (grant agreement number
852097). S.J.B. acknowledges support from Science Foundation Ireland and the
Royal Society (RS-EA/3471). A.V.F.’s group at UC Berkeley has been supported
by the Christopher R. Redlich Fund, Alan Eustace (W.Z. is a Eustace Specialist
in Astronomy), Frank and Kathleen Wood (T.G.B. is a Wood Specialist in
Astronomy), and numerous individual donors.

This work is based on observations obtained with the Samuel Oschin 48-inch
Telescope at Palomar Observatory as part of the Zwicky Transient Facility
project. ZTF is supported by the U.S. National Science Foundation (NSF) under
grant AST-1440341 and a collaboration including Caltech, IPAC, the Weizmann
Institute for Science, the Oskar Klein Center at Stockholm University, the
University of Maryland, the University of Washington, Deutsches Elektronen-
Synchrotron and Humboldt University, Los Alamos National Laboratories, the
TANGO Consortium of Taiwan, the University of Wisconsin at Milwaukee, and
Lawrence Berkeley National Laboratories. Operations are conducted by COO,
IPAC, and UW.

We thank Eric C. Bellm for sharing the ZTF observed pointing history used in
this paper.

Some of the data presented herein were obtained at the W. M. Keck Observatory,
which is operated as a scientific partnership among the California Institute
of Technology, the University of California, and the National Aeronautics and
Space Administration (NASA). The Observatory was made possible by the generous
financial support of the W. M. Keck Foundation. The authors wish to recognize
and acknowledge the very significant cultural role and reverence that the
summit of Maunakea has always had within the indigenous Hawaiian community. We
are most fortunate to have the opportunity to conduct observations from this
mountain.

The Pan-STARRS1 Surveys (PS1) and the PS1 public science archive have been
made possible through contributions by the Institute for Astronomy, the
University of Hawaii, the Pan-STARRS Project Office, the Max-Planck Society
and its participating institutes, the Max Planck Institute for Astronomy,
Heidelberg and the Max Planck Institute for Extraterrestrial Physics,
Garching, The Johns Hopkins University, Durham University, the University of
Edinburgh, the Queen’s University Belfast, the Harvard-Smithsonian Center for
Astrophysics, the Las Cumbres Observatory Global Telescope Network
Incorporated, the National Central University of Taiwan, the Space Telescope
Science Institute, NASA under grant NNX08AR22G issued through the Planetary
Science Division of the NASA Science Mission Directorate, NSF grant
AST-1238877, the University of Maryland, Eotvos Lorand University (ELTE), the
Los Alamos National Laboratory, and the Gordon and Betty Moore Foundation.

This research has made use of the VizieR catalogue access tool, CDS,
Strasbourg, France (DOI: 10.26093/cds/vizier). The original description of the
VizieR service was published in 2000, A&AS 143, 23.

##  Appendix A Deep Photometry

We obtained imaging at the positions of AT 2019lbw, AT 2020ddu, and AT 2020oor
with Keck I/LRIS. The PSF subtractions at these positions generated by
autophot (see Section 3.3, 3.5, and 3.8) are presented in Figures 5–7.

Figure 5: PSF subtraction of the source at the position of AT 2019lbw in $g$
(upper panels) and $r$ (lower panels) imaging, produced with autophot. The
main panels show a cutout of the target position before (left) and after
(right) PSF subtraction, while projections along the $X$ and $Y$ axes are also
shown for each panel.

Figure 6: Same as Figure 5 but for AT 2020ddu.

Figure 7: Same as Figure 6 but for AT 2020oor.

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