TITLE: GCN CIRCULAR NUMBER: 24913 SUBJECT: Swift Trigger 912102: possible GRB 190630B DATE: 19/06/30 06:24:47 GMT FROM: Hans Krimm at NSF/NASA-GSFC H. A. Krimm (NSF), N. J. Klingler (PSU), K. L. Page (U Leicester) and A. Tohuvavohu (PSU) report on behalf of the Neil Gehrels Swift Observatory Team: At 06:02:08 UT, the Swift Burst Alert Telescope (BAT) triggered and located possible GRB 190630B (trigger=912102). Swift slewed immediately to the location. The BAT on-board calculated location is RA, Dec 223.669, +41.521 which is RA(J2000) = 14h 54m 41s Dec(J2000) = +41d 31' 16" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a single-peaked structure with a duration of about 5 sec. The peak count rate was ~1000 counts/sec (15-350 keV), at ~0 sec after the trigger. This increase in flux occurred during a rise in background due to entry to the SAA, so it is not immediately clear from the BAT data alone that this is indeed a burst rather than a noise event. The XRT began observing the field at 06:03:10.7 UT, 62.1 seconds after the BAT trigger. No source was detected in the 2.5-s promptly available image. We are waiting for the full dataset to detect and localise the XRT counterpart. UVOT took a finding chart exposure of 66 seconds with the White filter starting 65 seconds after the BAT trigger. No credible afterglow candidate has been found in the initial data products. The 2.7'x2.7' sub-image covers 25% of the BAT error circle. The typical 3-sigma upper limit has been about 19.6 mag. The 8'x8' region for the list of sources generated on-board covers 100% of the BAT error circle. The list of sources is typically complete to about 18 mag. No correction has been made for the expected extinction corresponding to E(B-V) of 0.01. Due to the high background in the BAT and the lack of immediate counterpart detection in the XRT, the Swift team will not be able to confirm or deny whether this event is a GRB without further data. Burst Advocate for this burst is H. A. Krimm (hkrimm AT nsf.gov). Please contact the BA by email if you require additional information regarding Swift followup of this burst. In extremely urgent cases, after trying the Burst Advocate, you can contact the Swift PI by phone (see Swift TOO web site for information: http://www.swift.psu.edu/)