TITLE: GCN CIRCULAR NUMBER: 8742 SUBJECT: GRB 081228: Swift detection of a burst DATE: 08/12/28 01:33:10 GMT FROM: David Burrows at PSU/Swift K. L. Page (U Leicester), W. H. Baumgartner (GSFC/UMBC), P. J. Brown (PSU), D. N. Burrows (PSU), S. T. Holland (CRESST/USRA/GSFC), J. A. Kennea (PSU), H. A. Krimm (CRESST/GSFC/USRA), C. B. Markwardt (CRESST/GSFC/UMD) and T. N. Ukwatta (GSFC/GWU) report on behalf of the Swift Team: At 01:17:40 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 081228 (trigger=338338). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 39.467, +30.843 which is RA(J2000) = 02h 37m 52s Dec(J2000) = +30d 50' 33" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a single peak with a duration of about 2.5 sec. The peak count rate was ~1500 counts/sec (15-350 keV), at ~0.5 sec after the trigger. The XRT began observing the field at 01:18:58.7 UT, 78.0 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued X-ray source located at RA, Dec 39.46043, 30.85393 which is equivalent to: RA(J2000) = 02h 37m 50.50s Dec(J2000) = +30d 51' 14.1" with an uncertainty of 4.8 arcseconds (radius, 90% containment). This location is 44 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. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density consistent with the Galactic value of 8.8e+20 cm^-2 (Kalberla et al. 2005). UVOT took a finding chart exposure of 37 seconds with the White filter starting 82 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 0.16. Burst Advocate for this burst is K. L. Page (kpa 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.)