TITLE: GCN CIRCULAR NUMBER: 11782 SUBJECT: GRB 110312A: Swift detection of a burst DATE: 11/03/12 18:14:48 GMT FROM: Scott Barthelmy at NASA/GSFC S. R. Oates (UCL-MSSL), S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC), A. P. Beardmore (U Leicester), M. M. Chester (PSU), N. Gehrels (NASA/GSFC), S. T. Holland (CRESST/USRA/GSFC), C. B. Markwardt (NASA/GSFC), F. E. Marshall (NASA/GSFC), D. M. Palmer (LANL), M. H. Siegel (PSU), M. Stamatikos (OSU/NASA/GSFC), R. L. C. Starling (U Leicester), E. Troja (NASA/GSFC/ORAU) and T. N. Ukwatta (MSU) report on behalf of the Swift Team: At 17:55:37 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 110312A (trigger=449074). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 157.479, -5.255 which is RA(J2000) = 10h 29m 55s Dec(J2000) = -05d 15' 18" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). As is typical for image triggers, the BAT lightcurve does not show anything significant. The XRT began observing the field at 17:58:16.3 UT, 159.1 seconds after the BAT trigger. Using promptly downlinked data we find a bright, fading, uncatalogued X-ray source with an enhanced position: RA, Dec 157.4813, -5.2628 which is equivalent to: RA(J2000) = 10h 29m 55.50s Dec(J2000) = -05d 15' 46.0" with an uncertainty of 2.3 arcseconds (radius, 90% containment). This location is 28 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 3.72 x 10^20 cm^-2 (Kalberla et al. 2005). The initial flux in the 2.5 s image was 4.19e-10 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 167 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 100% of the XRT 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 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 0.04. Burst Advocate for this burst is S. R. Oates (sro AT mssl.ucl.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.)