TITLE: GCN CIRCULAR NUMBER: 23570 SUBJECT: GRB 181228A: Swift detection of a burst DATE: 18/12/28 03:07:38 GMT FROM: Scott Barthelmy at NASA/GSFC M. J. Moss (George Washington University), S. D. Barthelmy (GSFC), A. D'Ai (INAF-IASFPA), V. D'Elia (SSDC), J. A. Kennea (PSU), A. Y. Lien (GSFC/UMBC) and A. Tohuvavohu (PSU) report on behalf of the Neil Gehrels Swift Observatory Team: At 02:53:17 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 181228A (trigger=879909). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 49.831, +13.195, which is RA(J2000) = 03h 19m 19s Dec(J2000) = +13d 11' 42" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows multiple overlapping peaks with a duration of about 25 sec. The peak count rate was ~7000 counts/sec (15-350 keV), at ~13 sec after the trigger. The XRT began observing the field at 02:54:40.1 UT, 82.5 seconds after the BAT trigger. Using promptly downlinked data we find a bright, uncatalogued X-ray source with an enhanced position: RA, Dec 49.8114, 13.2000 which is equivalent to: RA(J2000) = 03h 19m 14.75s Dec(J2000) = +13d 12' 00.1" with an uncertainty of 2.6 arcseconds (radius, 90% containment). This location is 71 arcseconds from the BAT onboard position, within the BAT error circle. This position may be improved as more data are received; the latest position is available at http://www.swift.ac.uk/sper. We cannot determine whether the source is fading at the present time. No spectrum from the promptly downlinked event data is yet available to determine the column density. The initial flux in the 2.5 s image was 5.46e-10 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of nominal 150.000 seconds with the White filter starting 93 seconds after the BAT trigger. No credible afterglow candidate has been found in the initial data products. Data from the 2.7'x2.7' sub-image are not available at this time. The 8'x8' region for the list of sources generated on-board covers 100% of the XRT 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 0.51. Burst Advocate for this burst is M. J. Moss (mikejmoss3 AT gmail.com). 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.)