TITLE: GCN CIRCULAR NUMBER: 13123 SUBJECT: GRB 120327A: Swift detection of a burst with an optical counterpart DATE: 12/03/27 03:17:21 GMT FROM: Scott Barthelmy at NASA/GSFC B. Sbarufatti (INAF-OAB/PSU), S. D. Barthelmy (GSFC), N. Gehrels (NASA/GSFC), E. A. Hoversten (PSU), N. P. M. Kuin (UCL-MSSL), F. E. Marshall (NASA/GSFC), D. M. Palmer (LANL), M. H. Siegel (PSU), M. Stamatikos (OSU/NASA/GSFC) and T. N. Ukwatta (MSU) report on behalf of the Swift Team: At 02:55:16 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 120327A (trigger=518731). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 246.828, -29.415, which is RA(J2000) = 16h 27m 19s Dec(J2000) = -29d 24' 54" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows 3 main peaks with a total duration of about 60 sec. The peak count rate was ~6500 counts/sec (15-350 keV), at ~38 sec after the trigger. The XRT began observing the field at 02:56:32.2 UT, 75.6 seconds after the BAT trigger. Using promptly downlinked data we find a bright, uncatalogued X-ray source located at RA, Dec 246.86308, -29.41514 which is equivalent to: RA(J2000) = 16h 27m 27.14s Dec(J2000) = -29d 24' 54.5" with an uncertainty of 3.6 arcseconds (radius, 90% containment). This location is 110 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 (1.40 x 10^21 cm^-2, Kalberla et al. 2005), with an excess column of 1.9 (+1.51/-1.37) x 10^21 cm^-2 (90% confidence). The initial flux in the 2.5 s image was 5.41e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 250 seconds with the U filter starting 140 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 16:27:27.47 = 246.86445 DEC(J2000) = -29:24:53.8 = -29.41494 with a 90%-confidence error radius of about 0.64 arc sec. This position is 3.6 arc sec. from the center of the XRT error circle. The estimated magnitude is 17.92 with a 1-sigma error of about 0.15. No correction has been made for the expected extinction corresponding to E(B-V) of 0.34. Burst Advocate for this burst is B. Sbarufatti (boris.sbarufatti AT brera.inaf.it). 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.)