TITLE:   GCN CIRCULAR
NUMBER:  23379
SUBJECT: GRB 180728B: Zwicky Transient Facility Follow-Up of a Fermi Short GRB (Trigger 554505003)
DATE:    18/10/24 19:40:05 GMT
FROM:    Michael Coughlin at Caltech/LIGO  <mcoughli@caltech.edu>

Authors: Michael W. Coughlin (Caltech), Tomás Ahumada (UMD), S.
Bradley Cenko (NASA GSFC), Shaon Ghosh (UWM), Virginia Cunningham
(UMD), Eric C. Bellm (UW), Mansi M. Kasliwal (Caltech) and Leo P.
Singer (NASA GSFC) on behalf of the ZTF and GROWTH collaborations

We observed the localization region of the short GRB 180728B (trigger
554505003) 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. The observations
taken during the night of July 29 did not cover the IPN updated
localization of GRB 180628B available the next day.  The IPN region
was observed with ZTF beginning at 04:06 UT on 2018 July 30 (30:58
hours after the trigger time). The observations covered 334 square
degrees, corresponding to ~ 76% of the probability enclosed in the
localization region.

The images were processed through the ZTF reduction and image
subtraction pipelines at IPAC to search for potential counterparts. 7
high-significance transient and variable candidates were identified by
our pipeline in the area observed, all of which had previous
detections with ZTF in the days and weeks prior to the GRB trigger
time (e.g., supernovae, active galactic nuclei). No viable optical
counterparts were thus identified.

The median 5-sigma upper limit for an isolated point source in our
images was r > 18.7 and g > 20. mag for the observations made on July
30.

ZTF is a project led by PI S. R. Kulkarni at Caltech (see ATEL
#11266), and includes IPAC; WIS, Israel; OKC, Sweden; JSI/UMd, USA;
UW, USA; DESY, Germany; NRC, Taiwan; UW Milwaukee, USA and LANL USA.
ZTF acknowledges the generous support of the NSF under AST MSIP Grant
No 1440341. Alert distribution service provided by DIRAC@UW. Alert
filtering is being undertaken by the GROWTH marshal system, supported
by NSF PIRE grant 1545949.