TITLE: GCN CIRCULAR NUMBER: 8948 SUBJECT: GRB 090308: Swift detection of a burst DATE: 09/03/08 18:21:15 GMT FROM: Scott Barthelmy at NASA/GSFC P. A. Evans (U Leicester), S. D. Barthelmy (GSFC), A. P. Beardmore (U Leicester), D. N. Burrows (PSU), N. Gehrels (NASA/GSFC), S. T. Holland (CRESST/USRA/GSFC), R. Margutti (Univ Bicocca&OAB), C. B. Markwardt (CRESST/GSFC/UMD), F. E. Marshall (NASA/GSFC), P. T. O'Brien (U Leicester), C. Pagani (PSU), K. L. Page (U Leicester), A. M. Parsons (GSFC), P. Romano (INAF-IASFPA), B. Sbarufatti (INAF-IASFPA), R. L. C. Starling (U Leicester), M. C. Stroh (PSU), T. N. Ukwatta (GSFC/GWU) and L. Vetere (PSU) report on behalf of the Swift Team: At 18:01:23 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 090308 (trigger=345777). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 183.526, -48.833 which is RA(J2000) = 12h 14m 06s Dec(J2000) = -48d 49' 57" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). This is an image trigger, but even so the BAT light curve shows a weak peak at T+20 to T+35 sec. The peak count rate was ~350 counts/sec (15-350 keV), at ~25 sec after the trigger. The XRT began observing the field at 18:03:30.4 UT, 126.9 seconds after the BAT trigger. Using promptly downlinked data we find a fading, uncatalogued X-ray source with an enhanced position: RA, Dec 183.5017, -48.8169 which is equivalent to: RA(J2000) = 12h 14m 0.41s Dec(J2000) = -48d 49' 00.9" with an uncertainty of 2.2 arcseconds (radius, 90% containment). This location is 81 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 in excess of the Galactic value (8.54e+20 cm^-2, Kalberla et al. 2005), with an excess column of 1.8 (+1.51/-1.36) x 10^21 cm^-2 (90% confidence). UVOT took a finding chart exposure of 150 seconds with the White filter starting 131 seconds after the BAT trigger. No credible afterglow candidate has been found in the initial data products. Because part of the 2.7'x2.7' sub-image was not received, there is no overlap with the XRT error circle. The 8'x8' region for the list of sources generated on-board covers the entire XRT error circle. No correction has been made for the expected extinction corresponding to E(B-V) of 0.11. Burst Advocate for this burst is P. A. Evans (pae9 AT star.le.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.)