TITLE: GCN CIRCULAR NUMBER: 12578 SUBJECT: GRB 111121A: Swift detection of a short hard burst DATE: 11/11/21 16:43:39 GMT FROM: Scott Barthelmy at NASA/GSFC V. D'Elia (ASDC), S. D. Barthelmy (GSFC), A. P. Beardmore (U Leicester), D. N. Burrows (PSU), S. Campana (INAF-OAB), M. M. Chester (PSU), N. Gehrels (NASA/GSFC), B. Gendre (ASDC), C. Guidorzi (U Ferrara), S. T. Holland (CRESST/USRA/GSFC), J. A. Kennea (PSU), H. A. Krimm (CRESST/GSFC/USRA), N. P. M. Kuin (UCL-MSSL), V. Mangano (INAF-IASFPA), C. B. Markwardt (NASA/GSFC), F. E. Marshall (NASA/GSFC), P. T. O'Brien (U Leicester), J. P. Osborne (U Leicester), K. L. Page (U Leicester), D. M. Palmer (LANL), P. Romano (INAF-IASFPA), T. Sakamoto (NASA/UMBC), B. Sbarufatti (INAF-OAB/PSU), M. H. Siegel (PSU), M. Stamatikos (OSU/NASA/GSFC), R. L. C. Starling (U Leicester), G. Stratta (ASDC), C. A. Swenson (PSU), E. Troja (NASA/GSFC/ORAU), T. N. Ukwatta (MSU) and B.-B. Zhang (PSU) report on behalf of the Swift Team: At 16:26:24 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 111121A (trigger=508161). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 154.751, -46.637 which is RA(J2000) = 10h 19m 00s Dec(J2000) = -46d 38' 13" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows a single spike with a duration of about 1 sec. The peak count rate was ~38000 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 16:27:41.3 UT, 76.9 seconds after the BAT trigger. Using promptly downlinked data we find a bright, fading, uncatalogued X-ray source with an enhanced position: RA, Dec 154.7612, -46.6709 which is equivalent to: RA(J2000) = 10h 19m 2.68s Dec(J2000) = -46d 40' 15.1" with an uncertainty of 2.2 arcseconds (radius, 90% containment). This location is 124 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 (1.20 x 10^21 cm^-2, Kalberla et al. 2005), with an excess column of 4 (+2.62/-2.25) x 10^21 cm^-2 (90% confidence). The initial flux in the 2.5 s image was 4.58e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 85 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 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 18.0 mag.No correction has been made for the expected extinction corresponding to E(B-V) of 0.26. We note that the XRT centroid is close to a known 14th magnitude star, which may complicate the analysis. 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.)