TITLE: GCN CIRCULAR NUMBER: 8560 SUBJECT: GRB 081127: Swift detection of a burst DATE: 08/11/27 07:25:04 GMT FROM: David Palmer at LANL J. Mao (INAF-OAB), W. H. Baumgartner (GSFC/UMBC), P. A. Evans (U Leicester), C. Guidorzi (INAF-OAB), S. T. Holland (CRESST/USRA/GSFC), J. A. Kennea (PSU), R. Margutti (Univ Bicocca&OAB), C. Pagani (PSU), K. L. Page (U Leicester), D. M. Palmer (LANL), T. Sakamoto (NASA/UMBC) and L. Vetere (PSU) report on behalf of the Swift Team: At 07:05:08 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 081127 (trigger=335715). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 332.093, +6.859 which is RA(J2000) = 22h 08m 22s Dec(J2000) = +06d 51' 33" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a single-peak of 2 seconds on a broader plateau with a total duration of about 30 sec. The peak count rate was ~900 counts/sec (15-350 keV), at ~7 sec after the trigger. The XRT began observing the field at 07:07:10.8 UT, 121.9 seconds after the BAT trigger. Using promptly downlinked data we find a bright, fading, uncatalogued X-ray source with an enhanced position: RA, Dec 332.0646, 6.8504 which is equivalent to: RA(J2000) = 22h 08m 15.49s Dec(J2000) = +06d 51' 01.6" with an uncertainty of 2.4 arcseconds (radius, 90% containment). This location is 106 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 consistent with the Galactic value of 6.86e+20 cm^-2 (Kalberla et al. 2005). The initial flux in the 2.5 s image was 9.56e-10 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 130 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 typical 3-sigma upper limit has been about 19.6 mag. 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 0.09. The XRT position is 9.8 arcseconds from the B = 13 USNO-B1.0 star 0968-0627697. Burst Advocate for this burst is J. Mao (jirong.mao AT brera.inaf.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.)