TITLE:   GCN CIRCULAR
NUMBER:  30109
SUBJECT: GRB210529B: Zwicky Transient Facility Follow-Up of a Fermi Short GRB (Trigger 644025222)
DATE:    21/06/01 10:04:08 GMT
FROM:    Tomas Ahumada at U. of Maryland  <tahumada@astro.umd.edu>

Tomas Ahumada (UMD), Shreya Anand (Caltech), Harsh Kumar (IIT-B), Igor
Andreoni (Caltech), Albert Kong (NTHU), Erik Kool (SU), Ana
Sagues-Carracedo (SU), Simeon Reusch (DESY), , Michael Coughlin (UMN),
Mansi Kasliwal (Caltech), Robert Stein (DESY), Leo Singer (NASA/GSFC), and
Matthew Graham (Caltech) report on behalf of the ZTF collaboration:

We observed the localization region of the short GRB210529B (trigger
644025222.097546) detected by the Gamma-Ray Burst Monitor (GBM) on the
Fermi satellite with the Palomar 48 inch telescope equipped with the 47
square degree Zwicky Transient Facility (ZTF; Bellm et al., 2019; Graham et
al., 2019) camera. We obtained a series of g- and r-band images covering
about 686 square degrees beginning at 05:06:36.18 UT on 2021-05-30 (~ 5.1
hours after the burst trigger time). This corresponds to ~85% of the
probability enclosed in the region of the GRB localization that was
unocculted by the Earth for Fermi GBM (GCN 30101). Exposures reached a
median depth of 22.2 mag in the g-band and 22.3 mag in the r-band.
On 2021-05-31, we obtained another epoch of g- and r-band follow-up
observations, beginning at 05:07:50.96 UT with exposures reaching a median
depth of 22.2 mag and 22.2 mag respectively, and covering ~85% of the
enclosed probability.

The images were processed in real-time through the ZTF reduction and image
subtraction pipelines at IPAC (Masci et al. 2019). We queried the ZTF alert
stream using Kowalski (Duev et al. 2019). We required at least 2 detections
separated by at least 15 minutes to select against moving objects.
Furthermore, we cross-matched our candidates with the Minor Planet Center
to flag known asteroids, reject stellar sources (Tachibana and Miller
2018), and apply machine learning algorithms (Duev et al. 2019, Mahabal et
al. 2019). We require no spatially coincident ZTF alert to be issued before
the detection time of the GBM trigger.

We identified 146 transient candidates located within the 95% probability
contour of the skymap that passed automatic selection criteria. The
majority of the transients showed no evolution between the two nights. We
performed forced photometry (Masci et al. 2019) on the candidates reported
below and found no previous history of variability.

+--------------+-----------+-------------+--------------+--------+-------+-----------+
|     Name     |  IAU Name |      RA     |     Dec      | disc. mag |
 notes     |
+--------------+-----------+-------------+--------------+--------+-------+-----------+
| ZTF21abcwmzx | AT2021nzo | 14:42:28.10 | +31:56:34.19 | g = 21.51 |
(1)(e)  |
| ZTF21abcwnbm | AT2021nzp | 14:43:43.24 | +33:44:52.20 | g = 21.54 |
(1)(d)  |
| ZTF21abcwuxv | AT2021nzq | 15:30:34.50 | +43:05:52.27 | r = 21.82 |
(a)(b) |
| ZTF21abcwvzr | AT2021nzr | 14:49:49.23 | +35:05:43.32 | r = 21.9  |
(a)(e) |
| ZTF21abcwwaj | AT2021nzs | 14:40:59.49 | +32:07:24.64 | r = 21.24 |
(a)(c) |
| ZTF21abcwyoe | AT2021nzt | 15:58:56.54 | +33:08:01.79 | r = 21.83 |
(a)(d) |
| ZTF21abcwyvi | AT2021nzu | 15:58:53.27 | +33:39:12.74 | r = 21.98 |
(a)(b) |

Notes:
(1) shows rise and rapid decay
(a) Non-detection during the second night (2021-05-31). These have two or
more detections separated by >50 min during the first night.
(b) hostless
(c) host at z=0.1
(d) nuclear
(e) likely offset from host

Based on observations obtained with the Samuel Oschin Telescope 48-inch and
the 60-inch Telescope at the Palomar Observatory as part of the Zwicky
Transient Facility project. ZTF is supported by the National Science
Foundation under Grant No. AST-2034437 and a collaboration including
Caltech, IPAC, the Weizmann Institute for Science, the Oskar Klein Center
at Stockholm University, the University of Maryland, Deutsches
Elektronen-Synchrotron and Humboldt University, the TANGO Consortium of
Taiwan, the University of Wisconsin at Milwaukee, Trinity College Dublin,
Lawrence Livermore National Laboratories, and IN2P3, France. Operations are
conducted by COO, IPAC, and UW.


-- 
Tomas Ahumada (he/him)
Ph.D. Candidate
Department of Astronomy
University of Maryland, College Park
NASA Goddard Space Flight Center, Code 661
B.Sc. Astronomy, Pontificia Universidad Católica de Chile