GCN Circular 23324

Michael W. Coughlin (Caltech), S. Bradley Cenko (NASA GSFC), Tom��s
Ahumada (UMD), Leo P. Singer (NASA GSFC), Shaon Ghosh (UWM), Igor
Andreoni (Caltech), Mansi M. Kasliwal (Caltech), Eric C. Bellm (UW),
V. Zach Golkhou (UW), on behalf of the ZTF and GROWTH collaborations
and the KPED team

We observed the localization region of the short GRB 180913A (trigger
558557292) 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 180 second g-, r- and g-band images covering 546 square
degrees beginning at 3:06 UT on 2018 September 14 (8:18 hours after
the burst trigger time). Accounting for area lost to processing
failures due to the overlap of the region with the Galactic plane,
this corresponds to analyzing 72% of the probability enclosed.  Using
the IPN updated localization of GRB 180913B available the next day, we
observed the IPN region with ZTF beginning at 3:08 UT on 2018
September 15, using 300 second g-, r- and g-band exposures. The
observations covered 403 square degrees. Once again accounting for
processing failures, 53% of the enclosed probability was analyzed. A
third set of the same epochs was taken beginning at 3:06 UT on 2018
September 19, using 300 second g-band and 180 second r-band exposures.

The images were processed through the ZTF reduction and image
subtraction pipelines at IPAC to search for potential counterparts.
296 high-significance transient and variable candidates were
identified by our pipeline in the area observed,  most (284) of which
had previous detections with ZTF in the days and weeks prior to the
GRB trigger time (e.g., supernovae, active galactic nuclei). The 12
transients discovered the night of September 14 are listed in the
table below. These candidates were followed up with the Kitt Peak
EMCCD Demonstrator (KPED) on the Kitt Peak 84 inch telescope on
September 16 and with the fl05 Sinistro Camera at Las Cumbres
Observatory (LCO) node at McDonald Observatory on September 23. .

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       ZTF ID             RA DEC      last non-detection discovery mag
     KPED mag

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                       2018/09/14 2018/09/17

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ZTF18abvzgms     354.4607 47.8892      2018/09/12 g>20.96 g = 21.96
       g = 19.65
ZTF18abwiios        348.0586 39.4641     2018/09/12 g>20.92 g = 22.04
         g = 20.24

ZTF18abvzfgy        349.0633 43.5331    2018/09/12 g>20.98  g = 20.98 g = 20.36

ZTF18abvzjwk       337.6354 39.8374     2018/09/12 g>20.72   g = 21.70 g = 21.80

ZTF18abvwkbw     344.1347 37.0509     2018/09/13 r>20.64    r = 21.19
  r > 23.12

ZTF18abvwhkl      346.4340 45.543      2018/09/12 r>20.60   r = 21.44 r = 21.15

ZTF18abvucnv       337.8832 39.501      2018/09/12 g>20.85 g = 21.15
         g = 20.88

ZTF18abwiitm        348.8650 39.9529    2018/09/12 g>20.91   g = 21.71 g = 20.9

ZTF18abvubdm     344.6185 47.1011     2018/09/12 g>20.86 g = 21.01
      g = 20.19

ZTF18abvzsld            3.988 49.4808 2018/09/13  g>20.99 g = 21.50 g = 21.14

ZTF18abwiivr         343.0658 37.3748      2018/09/14 g=21.73 2018/09/12 g>20.89

ZTF18abvzmtm     358.8045 48.3605        2018/09/14 g=21.65 2018/09/13 g>21.01

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The median 5 sigma upper limit for an isolated point source in our
images was g > 21.8 and r > 22.2 mag for the observations made on
September 14 and g > 21.9 and g > 22.1 mag for the observations made
on September 15. For the LCO observations, the median 5 sigma upper
limit for an isolated point source in our images was g> 20.2 and
r>19.9 , no transient had risen to these magnitudes by September 23.

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.