TITLE: GCN CIRCULAR NUMBER: 13408 SUBJECT: GRB 120703A: Swift detection of a bright burst with an optical afterglow DATE: 12/07/03 17:34:05 GMT FROM: David Palmer at LANL B.-B. Zhang (PSU), S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC), D. N. Burrows (PSU), M. M. Chester (PSU), N. Gehrels (NASA/GSFC), D. Grupe (PSU), F. E. Marshall (NASA/GSFC), D. M. Palmer (LANL), J. L. Racusin (NASA/GSFC) and T. N. Ukwatta (MSU) report on behalf of the Swift Team: At 17:25:22 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 120703A (trigger=525671). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 339.357, -29.736 which is RA(J2000) = 22h 37m 26s Dec(J2000) = -29d 44' 08" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a multiply-peaked structure with a duration of about 50 sec. The peak count rate was ~14000 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 17:26:48.9 UT, 86.7 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued X-ray source located at RA, Dec 339.35607, -29.72444 which is equivalent to: RA(J2000) = 22h 37m 25.46s Dec(J2000) = -29d 43' 28.0" with an uncertainty of 3.8 arcseconds (radius, 90% containment). This location is 41 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 in excess of the Galactic value (1.21 x 10^20 cm^-2, Kalberla et al. 2005), with an excess column of 3.5 (+1.97/-1.72) x 10^21 cm^-2 (90% confidence). UVOT took a finding chart exposure of 150 seconds with the White filter starting 91 seconds after the BAT trigger. There is a candidate afterglow in the list of sources generated on-board at RA(J2000) = 22:37:25.71 = 339.35711 DEC(J2000) = -29:43:23.5 = -29.72320 with a 90%-confidence error radius of about 1.10 arc sec. This position is 5.5 arc sec. from the center of the XRT error circle. The estimated magnitude is 18.51. No correction has been made for the expected extinction corresponding to E(B-V) of 0.02. Burst Advocate for this burst is B.-B. Zhang (bbzhang AT psu.edu). 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.)