TITLE: GCN CIRCULAR NUMBER: 22871 SUBJECT: GRB 180626C: Zwicky Transient Facility Follow-Up of a Fermi Short GRB (Trigger 551697835) DATE: 18/06/30 03:46:01 GMT FROM: Michael Coughlin at Caltech/LIGO 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 180626C (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 180626C 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 ), 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. [GCN OPS NOTE(01jul18): Per author's request, the "A"s in the Subject-line and in the two places in the first paragraph were changed to "C"s. And the Trigger number in the Subject-line was changed to 551697835.]