TITLE: GCN CIRCULAR NUMBER: 12737 SUBJECT: GRB 111228A: Swift detection of a burst with an optical counterpart DATE: 11/12/28 16:00:13 GMT FROM: David Burrows at PSU/Swift T. N. Ukwatta (MSU), W. H. Baumgartner (GSFC/UMBC), D. N. Burrows (PSU), S. Campana (INAF-OAB), P. D'Avanzo (INAF-OAB), B. Gendre (ASDC), C. Guidorzi (U Ferrara), S. T. Holland (CRESST/USRA/GSFC), E. A. Hoversten (PSU), H. A. Krimm (CRESST/GSFC/USRA), A. Y. Lien (NASA/GSFC/ORAU), C. B. Markwardt (NASA/GSFC), A. Melandri (INAF-OAB), J. P. Osborne (U Leicester), B. Sbarufatti (INAF-OAB/PSU), M. H. Siegel (PSU), E. Sonbas (NASA/GSFC/Adiyaman Univ.) and E. Troja (NASA/GSFC/ORAU) report on behalf of the Swift Team: At 15:44:43 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 111228A (trigger=510649). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 150.079, +18.293 which is RA(J2000) = 10h 00m 19s Dec(J2000) = +18d 17' 35" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a strong triple-peaked structure with a duration of about 100 sec. The peak count rate was ~5000 counts/sec (15-350 keV), at ~93 sec after the trigger. The XRT began observing the field at 15:47:08.6 UT, 145.1 seconds after the BAT trigger. Using promptly downlinked data we find a bright, uncatalogued X-ray source located at RA, Dec 150.06478, 18.29912 which is equivalent to: RA(J2000) = 10h 00m 15.55s Dec(J2000) = +18d 17' 56.8" with an uncertainty of 4.2 arcseconds (radius, 90% containment). This location is 53 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. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density in excess of the Galactic value (2.95 x 10^20 cm^-2, Kalberla et al. 2005), with an excess column of 2.4 (+2.09/-1.76) x 10^21 cm^-2 (90% confidence). The initial flux in the 0.1 s image was 8.82e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 155 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 10:00:16.01 = 150.06669 DEC(J2000) = +18:17:51.8 = 18.29773 with a 90%-confidence error radius of about 0.75 arc sec. This position is 8.3 arc sec. from the center of the XRT error circle. The estimated magnitude is 17.19 with a 1-sigma error of about 0.14. No correction has been made for the expected extinction corresponding to E(B-V) of 0.03. Burst Advocate for this burst is T. N. Ukwatta (tilan.ukwatta 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.)