TITLE: GCN CIRCULAR NUMBER: 12373 SUBJECT: GRB 110921A: Swift detection of a burst DATE: 11/09/21 14:01:54 GMT FROM: David Burrows at PSU/Swift W. H. Baumgartner (GSFC/UMBC), A. P. Beardmore (U Leicester), D. N. Burrows (PSU), S. Campana (INAF-OAB), J. R. Cummings (NASA/UMBC), V. D'Elia (ASDC), P. A. Evans (U Leicester), C. Guidorzi (U Ferrara), S. T. Holland (CRESST/USRA/GSFC), E. A. Hoversten (PSU), J. A. Kennea (PSU), H. A. Krimm (CRESST/GSFC/USRA), O. M. Littlejohns (U Leicester), V. Mangano (INAF-IASFPA), F. E. Marshall (NASA/GSFC), A. Melandri (INAF-OAB), P. T. O'Brien (U Leicester), C. Pagani (U Leicester), K. L. Page (U Leicester), J. L. Racusin (NASA/GSFC), T. Sakamoto (NASA/UMBC), B. Sbarufatti (INAF-OAB/IASFPA), M. H. Siegel (PSU), E. Sonbas (GSFC/USRA/Adiyaman Univ.), R. L. C. Starling (U Leicester) and B.-B. Zhang (PSU) report on behalf of the Swift Team: At 13:51:20 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 110921A (trigger=503652). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 294.078, +36.377 which is RA(J2000) = 19h 36m 19s Dec(J2000) = +36d 22' 36" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a ragged, single-peaked structure with a duration of about 10 sec. The peak count rate was ~1000 counts/sec (15-350 keV), at ~4 sec after the trigger. The XRT began observing the field at 13:53:22.5 UT, 122.4 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued X-ray source located at RA, Dec 294.09838, 36.32882 which is equivalent to: RA(J2000) = 19h 36m 23.61s Dec(J2000) = +36d 19' 43.8" with an uncertainty of 3.8 arcseconds (radius, 90% containment). This location is 182 arcseconds from the BAT onboard position, outside 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 (1.43 x 10^21 cm^-2, Kalberla et al. 2005), with an excess column of 3.1 (+2.28/-1.97) x 10^21 cm^-2 (90% confidence). UVOT took a finding chart exposure of 150 seconds with the White filter starting 125 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. 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 0.14. Burst Advocate for this burst is W. H. Baumgartner (wayne AT milkyway.gsfc.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.)