TITLE: GCN CIRCULAR NUMBER: 23324 SUBJECT: GRB180913A: Zwicky Transient Facility Follow-Up of a Fermi Short GRB (Trigger 558557292) DATE: 18/10/11 15:49:25 GMT FROM: Michael Coughlin at Caltech/LIGO Michael W. Coughlin (Caltech), S. Bradley Cenko (NASA GSFC), Tomás 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. . ---------------------------------------------------------------------------------------------------------------------------- ZTF ID RA DEC last non-detection discovery mag KPED mag ---------------------------------------------------------------------------------------------------------------------------- 2018/09/14 2018/09/17 ---------------------------------------------------------------------------------------------------------------------------- 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 ------------------------------------------------------------------------------------------------------------------------- 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.