TITLE: GCN CIRCULAR NUMBER: 21042 SUBJECT: GRB 170428A: Swift detection of a short burst DATE: 17/04/28 09:38:15 GMT FROM: David Palmer at LANL A. P. Beardmore (U Leicester), N. P. M. Kuin (UCL-MSSL), K. L. Page (U Leicester), D. M. Palmer (LANL), B. Sbarufatti (INAF-OAB/PSU) and R. L. C. Starling (U Leicester) report on behalf of the Swift Team: At 09:13:42 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 170428A (trigger=750298). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 330.063, +26.918 which is RA(J2000) = 22h 00m 15s Dec(J2000) = +26d 55' 03" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a single peak structure with a duration of about 0.25 sec. The peak count rate was ~22000 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 09:15:06.9 UT, 84.6 seconds after the BAT trigger. Swift then passed into the SAA, during which time no data are collected. No source was detected in 208 s of promptly downlinked data. We are waiting for the full dataset to detect and localise the XRT counterpart. UVOT took a finding chart exposure of 84 seconds with the U filter starting 866 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 19.2 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 18.0 mag. No correction has been made for the expected extinction corresponding to E(B-V) of 0.06. Burst Advocate for this burst is A. P. Beardmore (apb AT star.le.ac.uk). 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/too.html.)