TITLE: GCN CIRCULAR NUMBER: 8687 SUBJECT: GRB 081221: Swift detection of a burst DATE: 08/12/21 16:42:05 GMT FROM: David Burrows at PSU/Swift E. A. Hoversten (PSU), W. H. Baumgartner (GSFC/UMBC), D. N. Burrows (PSU), M. M. Chester (PSU), J. R. Cummings (NASA/UMBC), J. A. Kennea (PSU), V. Mangano (INAF-IASFPA), C. B. Markwardt (CRESST/GSFC/UMD), F. E. Marshall (NASA/GSFC), P. T. O'Brien (U Leicester), D. M. Palmer (LANL), J. L. Racusin (PSU), P. Romano (INAF-IASFPA), B. A. Rowlinson (U Leicester), M. C. Stroh (PSU), G. Tagliaferri (INAF-OAB), T. N. Ukwatta (GSFC/GWU) and L. Vetere (PSU) report on behalf of the Swift Team: At 16:21:11 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 081221 (trigger=337889). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 15.805, -24.526 which is RA(J2000) = 01h 03m 13s Dec(J2000) = -24d 31' 32" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed two peaks, at T+0 to T+10 seconds and a much larger one at T+15 to T+40 seconds. The peak count rate was ~20000 counts/sec (15-350 keV), at 22 sec after the trigger. The XRT began observing the field at 16:22:20.2 UT, 68.4 seconds after the BAT trigger. XRT found a bright, fading, uncatalogued X-ray source located at RA, Dec 15.7934, -24.5484 which is equivalent to: RA(J2000) = 01h 03m 10.41s Dec(J2000) = -24d 32' 54.2" with an uncertainty of 5.0 arcseconds (radius, 90% containment). This location is 89 arcseconds from the BAT onboard position, within the BAT error circle. No event data are yet available to determine the column density using X-ray spectroscopy. The initial flux in the 0.1 s image was 7.36e-08 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 78 seconds after the BAT trigger, and a finding chart exposure of 250 seconds with the u filter starting 290 seconds after the 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 3-sigma upper limit at the XRT position is 19.9 mag in White, 19.8 mag in u. No correction has been made for the expected extinction corresponding to E(B-V) of 0.02. We note that this burst is coincident with a Fermi GBM burst (trigger 251569273). Burst Advocate for this burst is E. A. Hoversten (hoversten 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.)