TITLE: GCN CIRCULAR NUMBER: 8169 SUBJECT: GRB 080903: Swift detection of a burst DATE: 08/09/03 01:38:31 GMT FROM: David Burrows at PSU/Swift D. Grupe (PSU), W. H. Baumgartner (GSFC/UMBC), D. N. Burrows (PSU), M. M. Chester (PSU), J. R. Cummings (NASA/UMBC), S. T. Holland (CRESST/USRA/GSFC), J. A. Kennea (PSU), C. B. Markwardt (CRESST/GSFC/UMD), F. E. Marshall (NASA/GSFC), D. M. Palmer (LANL), T. N. Ukwatta (GSFC/GWU) and L. Vetere (PSU) report on behalf of the Swift Team: At 01:12:23 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 080903 (trigger=323542). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 86.797, +51.326 which is RA(J2000) = 05h 47m 11s Dec(J2000) = +51d 19' 35" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a broad single-peaked structure with a duration of about 50 sec. The peak count rate was 1250 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 01:13:21.8 UT, 58.5 seconds after the BAT trigger. Using promptly downlinked data we find a bright, fading, uncatalogued X-ray source located at RA, Dec 86.79071, 51.26363 which is equivalent to: RA(J2000) = 05h 47m 9.77s Dec(J2000) = +51d 15' 49.1" with an uncertainty of 3.0 arcseconds (radius, 90% containment). This location is 224 arcseconds from the BAT onboard position, outside the on-board BAT error circle, but within the BAT ground-calculated error circle. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density consistent with the Galactic value of 1.67e+21 cm^-2 (Kalberla et al. 2005). The initial flux in the 2.5 s image was 1.80e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 400 seconds with the V filter starting 75 seconds after the BAT trigger. No 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 location of the XRT candidate is 19.4 mag. No correction has been made for the expected extinction of about 0.7 magnitudes. Burst Advocate for this burst is D. Grupe (grupe 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.)