TITLE: GCN CIRCULAR NUMBER: 13203 SUBJECT: GRB 120403B: Swift detection of a burst DATE: 12/04/03 21:03:49 GMT FROM: Scott Barthelmy at NASA/GSFC V. D'Elia (ASDC), S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC), D. N. Burrows (PSU), S. Campana (INAF-OAB), M. M. Chester (PSU), J. A. Kennea (PSU), C. B. Markwardt (NASA/GSFC), A. Maselli (INAF-IASFPA), A. Melandri (INAF-OAB), P. T. O'Brien (U Leicester), T. Sakamoto (NASA/UMBC), C. J. Saxton (UCL-MSSL), B. Sbarufatti (INAF-OAB/PSU), M. H. Siegel (PSU), G. Stratta (ASDC), C. A. Swenson (PSU), T. N. Ukwatta (MSU) and B.-B. Zhang (PSU) report on behalf of the Swift Team: At 20:33:56 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 120403B (trigger=519256). Because of an observing constraint, the slew to the burst was delayed by 12.9 minutes. The BAT on-board calculated location is RA, Dec 54.809, -89.037, which is RA(J2000) = 03h 39m 14s Dec(J2000) = -89d 02' 12" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows several spikes with a total duration of about 10 sec. The peak count rate was ~2000 counts/sec (15-350 keV), at ~1 sec after the trigger. The XRT began observing the field at 20:49:04.0 UT, 907.5 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued X-ray source located at RA, Dec 55.33838, -89.00933 which is equivalent to: RA(J2000) = 03h 41m 21.21s Dec(J2000) = -89d 00' 33.6" with an uncertainty of 3.9 arcseconds (radius, 90% containment). This location is 104 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 consistent with the Galactic value of 9.52 x 10^20 cm^-2 (Kalberla et al. 2005). UVOT took a finding chart exposure of 150 seconds with the White filter starting 910 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 0.13. Burst Advocate for this burst is V. D'Elia (delia AT asdc.asi.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.)