TITLE: GCN CIRCULAR NUMBER: 9781 SUBJECT: GRB 090813: Swift detection of a burst DATE: 09/08/13 04:26:12 GMT FROM: Scott Barthelmy at NASA/GSFC J. R. Cummings (NASA/UMBC), S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC), A. P. Beardmore (U Leicester), C. Gronwall (PSU), F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester), D. M. Palmer (LANL), M. Stamatikos (OSU/NASA/GSFC) and T. N. Ukwatta (GSFC/GWU) report on behalf of the Swift Team: At 04:10:43 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 090813 (trigger=359884). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 227.017, +88.573 which is RA(J2000) = 15h 08m 04s Dec(J2000) = +88d 34' 21" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows two peaks with a duration of about 8 sec. The peak count rate was ~9000 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 04:12:01.7 UT, 78.7 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec 225.8016, +88.5687 which is equivalent to: RA(J2000) = 15h 03m 12.38s Dec(J2000) = +88d 34' 07.3" with an uncertainty of 6.1 arcseconds (radius, 90% containment). This location is 110 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 2.5 s image was 6.26e-10 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 87 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 0.17. Burst Advocate for this burst is J. R. Cummings (jayc 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.)