[vsnet-grb-info 20773] GRB180626A: Zwicky Transient Facility Follow-Up of a Fermi Short GRB (Trigger 548793993)

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  22871
SUBJECT: GRB180626A: Zwicky Transient Facility Follow-Up of a Fermi Short GRB (Trigger 548793993)
DATE:    18/06/30 03:46:01 GMT
FROM:    Michael Coughlin at Caltech/LIGO  <mcoughli at caltech.edu>

Michael W. Coughlin (Caltech), Leo P. Singer (NASA GSFC), Tomás Ahumada
(UMD), S. Bradley Cenko (NASA GSFC), Shaon Ghosh (UWM), Mansi M. Kasliwal
(Caltech), Eric C. Bellm (UW), V. Zach Golkhou (UW), Ludwig Rauch (DESY),
Robert Stein (DESY), on behalf of the ZTF and GROWTH collaborations and the
KPED team

We observed the localization region of the short GRB 180626 (trigger
551697835) 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 r-
and g-band images covering 275 square degrees beginning at 10:52 UT on 2018
June 26 (1:29 hours after the burst trigger time), corresponding to ~ 36%
of the probability enclosed in the localization region.  Using the IPN
updated localization of GRB 180626 available the next day, we observed the
new region with ZTF beginning at 05:01 UT on 2018 June 27 (19:43 hours
after the trigger time). The observations covered 230 square degrees,
corresponding to ~ 87% 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. 45
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). Out of the 45 transients, only 1 transient was
within the IPN localization region, but the object has previous detections
at similar magnitudes. No viable optical counterparts were thus identified.

We would like to highlight one interesting object discovered in the ZTF
fields on the first night (it does not fall into the IPN localization
region) and later followed up with the Kitt Peak EMCCD Demonstrator (KPED)
on the Kitt Peak 84 inch telescope on June 27. Located at RA: 19:48:49.1 ,
DEC: +46:30:36.1, ZTF18aauebur was first detected by ZTF June 25.30 (1 day
before the trigger); it is a rapidly evolving transient that has gone from
g = 18.4 to g = 20.5 in 1.92 days.  An underlying source is present at this
location in Pan-STARRS DR1 and GALEX. Given the low Galactic latitude, it
may be a stellar flare.

The median 5 sigma upper limit for an isolated point source in our images
was r > 20.9 and g > 20.9 mag for the observations made on June 26 and r >
21.2 and g > 21.0 mag for the observations made on June 27.

ZTF is a project led by PI S. R. Kulkarni at Caltech (see ATEL #11266
<http://www.astronomerstelegram.org/?read=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 at UW. Alert filtering is being undertaken by the GROWTH marshal system,
supported by NSF PIRE grant 1545949.