TITLE: GCN CIRCULAR NUMBER: 10317 SUBJECT: GRB 100111A: Swift detection of a burst with an optical afterglow DATE: 10/01/11 04:24:29 GMT FROM: David Palmer at LANL L. Vetere (PSU), S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC), D. N. Burrows (PSU), C. Gronwall (PSU), D. Grupe (PSU), S. T. Holland (CRESST/USRA/GSFC), E. A. Hoversten (PSU), C. B. Markwardt (CRESST/GSFC/UMD), C. Pagani (PSU), D. M. Palmer (LANL) and M. H. Siegel (PSU) report on behalf of the Swift Team: At 04:12:49 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 100111A (trigger=382399). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 247.038, +15.557 which is RA(J2000) = 16h 28m 09s Dec(J2000) = +15d 33' 25" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a small precursor peak followed by a main FRED peak for a total duration of about 30 sec. The peak count rate was ~3800 counts/sec (15-350 keV), at ~1 sec after the trigger. The XRT began observing the field at 04:13:52.1 UT, 63.0 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued X-ray source located at RA, Dec 247.04766, 15.55090 which is equivalent to: RA(J2000) = 16h 28m 11.44s Dec(J2000) = +15d 33' 03.2" with an uncertainty of 3.8 arcseconds (radius, 90% containment). This location is 40 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 in excess of the Galactic value (4.09e+20 cm^-2, Kalberla et al. 2005), with an excess column of 2 (+2.19/-1.28) x 10^21 cm^-2 (90% confidence). UVOT took a finding chart exposure of 150 seconds with the White filter starting 67 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 16:28:11.60 = 247.04833 DEC(J2000) = +15:33:02.3 = 15.55064 with a 90%-confidence error radius of about 0.62 arc sec. This position is 3.4 arc sec. from the center of the XRT error circle. The estimated magnitude is 17.58 with a 1-sigma error of about 0.14. No correction has been made for the expected extinction corresponding to E(B-V) of 0.05. Burst Advocate for this burst is L. Vetere (vetere AT astro.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.)