TITLE: GCN CIRCULAR NUMBER: 9854 SUBJECT: GRB 090831C: Swift detection of a burst DATE: 09/08/31 21:42:30 GMT FROM: Scott Barthelmy at NASA/GSFC P. Schady (MSSL-UCL), S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC), A. P. Beardmore (U Leicester), J. R. Cummings (NASA/UMBC), P. A. Evans (U Leicester), C. Guidorzi (U Ferrara), S. T. Holland (CRESST/USRA/GSFC), J. A. Kennea (PSU), H. A. Krimm (CRESST/GSFC/USRA), C. B. Markwardt (CRESST/GSFC/UMD), F. E. Marshall (NASA/GSFC), C. Pagani (PSU), K. L. Page (U Leicester), D. M. Palmer (LANL), A. Rowlinson (U Leicester), M. H. Siegel (PSU), M. Stamatikos (OSU/NASA/GSFC) and M. A. Stark (PSU) report on behalf of the Swift Team: At 21:30:25 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 090831C (trigger=361489). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 108.278, -25.100 which is RA(J2000) = 07h 13m 07s Dec(J2000) = -25d 06' 00" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows a single peak with a duration of about 10 sec. The peak count rate was ~800 counts/sec (15-350 keV), at ~3 sec after the trigger. The XRT began observing the field at 21:32:23.5 UT, 117.5 seconds after the BAT trigger. Using promptly downlinked data we find a fading, uncatalogued X-ray source located at RA, Dec 108.29307, -25.11934 which is equivalent to: RA(J2000) = 07h 13m 10.34s Dec(J2000) = -25d 07' 09.6" with an uncertainty of 3.8 arcseconds (radius, 90% containment). This location is 85 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 3.24e+21 cm^-2 (Kalberla et al. 2005). UVOT took a finding chart exposure of 150 seconds with the White filter starting 120 seconds after the BAT trigger. No credible afterglow candidate has been found in the initial data products. Results from the 2.7'x2.7' sub-image are not available at this time. The coverage of the XRT error circle by the 8'x8' region for the list of sources generated on-board is uncertain because the large number of sources filled the available telemetry. No correction has been made for the expected extinction corresponding to E(B-V) of 0.38. Burst Advocate for this burst is P. Schady (ps AT mssl.ucl.ac.uk). 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.)