TITLE: GCN CIRCULAR NUMBER: 12288 SUBJECT: GRB 110820A: Swift detection of a burst DATE: 11/08/20 17:55:18 GMT FROM: Scott Barthelmy at NASA/GSFC E. Troja (NASA/GSFC/ORAU), S. D. Barthelmy (GSFC), J. R. Cummings (NASA/UMBC), V. D'Elia (ASDC), P. A. Evans (U Leicester), N. Gehrels (NASA/GSFC), S. T. Holland (CRESST/USRA/GSFC), J. A. Kennea (PSU), N. P. M. Kuin (UCL-MSSL), C. B. Markwardt (NASA/GSFC), F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester), B. Sbarufatti (INAF-OAB/IASFPA), M. H. Siegel (PSU), E. Sonbas (GSFC/USRA/Adiyaman Univ.) and T. N. Ukwatta (MSU) report on behalf of the Swift Team: At 17:38:27 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 110820A (trigger=501095). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 343.335, +70.305 which is RA(J2000) = 22h 53m 20s Dec(J2000) = +70d 18' 20" 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 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 17:39:46.8 UT, 79.0 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued X-ray source with an enhanced position: RA, Dec 343.1911, 70.2974 which is equivalent to: RA(J2000) = 22h 52m 45.86s Dec(J2000) = +70d 17' 50.5" with an uncertainty of 2.3 arcseconds (radius, 90% containment). This location is 177 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 3.34 x 10^21 cm^-2 (Kalberla et al. 2005). UVOT took a finding chart exposure of 250 seconds with the U filter starting 139 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. Data from the list of sources generated on-board are not available at this time. No correction has been made for the expected extinction corresponding to E(B-V) of 0.55. Burst Advocate for this burst is E. Troja (eleonora.troja AT 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.)