TITLE: GCN CIRCULAR NUMBER: 13602 SUBJECT: GRB 120807A: Swift detection of a burst DATE: 12/08/07 07:23:44 GMT FROM: David Palmer at LANL F. E. Marshall (NASA/GSFC), P. A. Evans (U Leicester), N. Gehrels (NASA/GSFC), B.P. Gompertz (U Leicester), C. Guidorzi (U Ferrara), J. A. Kennea (PSU), O. M. Littlejohns (U Leicester), C. B. Markwardt (NASA/GSFC), K. L. Page (U Leicester), D. M. Palmer (LANL) and E. Troja (NASA/GSFC/ORAU) report on behalf of the Swift Team: At 07:09:37 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 120807A (trigger=530267). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 241.211, -47.490 which is RA(J2000) = 16h 04m 51s Dec(J2000) = -47d 29' 23" 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 15 sec. The peak count rate was ~2000 counts/sec (15-350 keV), at ~2 sec after the trigger. The XRT began observing the field at 07:10:49.4 UT, 71.6 seconds after the BAT trigger. Using promptly downlinked data we find a fading, uncatalogued X-ray source with an enhanced position: RA, Dec 241.2594, -47.4797 which is equivalent to: RA(J2000) = 16h 05m 02.26s Dec(J2000) = -47d 28' 47.0" with an uncertainty of 2.3 arcseconds (radius, 90% containment). This location is 123 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 (4.94 x 10^21 cm^-2, Kalberla et al. 2005), with an excess column of 3.5 (+4.80/-2.19) x 10^21 cm^-2 (90% confidence). UVOT took a finding chart exposure of 150 seconds with the White filter starting 75 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 none of the XRT error circle. 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 large, but uncertain extinction expected. Burst Advocate for this burst is F. E. Marshall (marshall AT milkyway.gsfc.nasa.gov). 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.)