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GCN Circular 28293

Subject
GRB200826A: Zwicky Transient Facility Follow-Up of a Fermi Short GRB (Trigger 620108997)
Date
2020-08-26T22:32:02Z (4 years ago)
From
Tomas Ahumada at U. of Maryland <tahumada@astro.umd.edu>
Ana Sagues Carracedo (OKC), Harsh Kumar (IITB), Tomas Ahumada (UMD), Igor
Andreoni (Caltech), Shreya Anand (Caltech),  Robert Stein (DESY), Michael
Coughlin (UMN), Leo Singer (NASA/GSFC) on behalf of the Zwicky Transient
Facility (ZTF) and Global Relay of Observatories Watching Transients Happen
(GROWTH) collaborations


We observed the localization region of the short GRB200826A (trigger
620108997) 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) camera.
We obtained a series of g- and r-band images covering 185.5 square degrees
beginning at 09:22:55 UT on 2020 Aug 26 (about 5 hours after the burst
trigger time). This corresponds to ~77.5% of the probability enclosed in
the GRB localization map (GCN 28284). Each exposure was 300s, reaching a
g-band median depth of 22.3 mag and r-band median depth of 22.1 mag. The
median limiting magnitude of the last 5 survey-mode nights (30 sec
exposures) was g ~ 20.6 mag and r ~ 20.0 mag, limiting our capability to
temporally constrain some of our faintest transients. 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) and AMPEL
(Nordin 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. The candidates within the 95% probability contour
of the GRB localization map that passed the automatic selection criteria
and human vetting are presented in the table below.


+----------------------------------------------------------------------------------------------------+
| ZTF Name      | IAU Name  | RA (deg)  | DEC (deg)  | Filter | Mag   |
MagErr | g-r |MJD      | Notes |
+----------------------------------------------------------------------------------------------------+
| ZTF20abwyple  | AT2020sbi | 1.3824038 | 37.3638915 | r | 21.2 | 0.11 |
0.13 | 2459087.9109491 | (a) |
| ZTF20abwyueq  | AT2020sbn | 9.2615748 | 39.6340049 | r | 21.8 | 0.13 |
0.08 | 2459087.9219444 | (a) |
| ZTF20abwyzdb  | AT2020sbp | 9.2046817 | 43.6712369 | r | 21.7 | 0.17 |
0.4  | 2459087.9219444 | (b) |
| ZTF20abwytgj  | AT2020sbc | 5.7051754 | 29.8539268 | r | 22.0 | 0.19 |
0.09 | 2459087.9181366 | (a) |
| ZTF20abwysgz  | AT2020sbm | 8.2746333 | 30.7142518 | r | 20.2 | 0.05 |
0.1  | 2459087.9181366 | (a) |
| ZTF20abwywiz  | AT2020sbv | 1.4094587 | 36.2573281 | r | 21.9 | 0.21 |
0.45 | 2459087.9145486 | (b) |
| ZTF20abwyrop  | AT2020sbd | 359.33861 | 31.1190224 | r | 21.5 | 0.12 |
0.22 | 2459087.9145486 | (c) |
| ZTF20abwyxpj  | AT2020sbs | 12.411974 | 31.7868639 | r | 21.8 | 0.16 |
0.35 | 2459087.9181366 | (c)(d) |
| ZTF20abwypor  | AT2020sbk | 6.8956443 | 37.2409844 | r | 21.5 | 0.13 |
0.15 | 2459087.9109491 | (c) |
| ZTF20abwyvrl  | AT2020sbj | 2.7749043 | 37.6768649 | r | 21.9 | 0.21 |
0.34 | 2459087.9109491 | (a) |
| ZTF20abwypqf  | AT2020sbl | 5.1321015 | 39.4099175 | r | 21.8 | 0.21 |
0.0  | 2459087.9109491 | (a) |
| ZTF20abwyrzs  | AT2020sbo | 359.79701 | 32.1773094 | r | 21.6 | 0.17 |
0.09 | 2459087.9145486 | (a)(d)(e) |
| ZTF20abwzabz  | AT2020sbr | 12.580528 | 42.6494866 | r | 21.9 | 0.16 |
0.02 | 2459087.9219444 | (b)(e) |
| ZTF20abwysmm  | AT2020sbt | 11.333202 | 33.4189371 | r | 21.5 | 0.14 |
0.0  | 2459087.9181366 | (d)(e) |
+----------------------------------------------------------------------------------------------------+
Notes:
(a) offset from a galaxy
(b) nuclear
(c) hostless
(d) PS1 detection at source position
(e) possible point-like source underneath


We checked for galaxies in the GLADE catalog (Dalya et al. 2018) around 100
arcsec of each candidate in the table above. We found crossmatches for
ZTF20abwysgz, which is 25.8 arcsec or 81.7 kpc away from a GLADE galaxy,
and ZTF20abwyrzs, 35.9 arcsec or 163.1 kpc offset. However, there are
several ZTF sources closer in the image to both candidates, which makes an
association between the ZTF candidates and GLADE galaxies improbable. In
fact, many candidates (including ZTF20abwysgz and ZTF20abwyrzs) appear to
be a few arcseconds offset from their possible host galaxies:

name         | separation  |
-------------+--------------
ZTF20abwyple | ~2.7 arcsec |
ZTF20abwypqf | ~1.1 arcsec |
ZTF20abwyple | ~2.6 arcsec |
ZTF20abwyueq | ~1.9 arcsec |
ZTF20abwytgj | ~2.0 arcsec |
ZTF20abwysgz | ~1.6 arcsec |
ZTF20abwyvrl | ~2.3 arcsec |
ZTF20abwypqf | ~5.3 arcsec |
ZTF20abwyrzs | ~5.0 arcsec |

Follow-up observations of these candidates is encouraged.

ZTF and GROWTH are worldwide collaborations comprising Caltech, USA; IPAC,
USA, WIS, Israel; OKC, Sweden; JSI/UMd, USA; U Washington, USA; DESY,
Germany; MOST, Taiwan; UW Milwaukee, USA; LANL USA; Tokyo Tech, Japan;
IITB, India; IIA, India; LJMU, UK; TTU, USA; SDSU, USA and USyd, Australia.
ZTF acknowledges the generous support of the NSF under AST MSIP Grant No
1440341.
GROWTH acknowledges generous support of the NSF under PIRE Grant No 1545949.
Alert distribution service provided by DIRAC@UW (Patterson et al. 2019).
Alert database searches are done by AMPEL (Nordin et al. 2019) and Kowalski
(Duev et al. 2019).
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