TITLE: GCN CIRCULAR NUMBER: 8939 SUBJECT: GRB 090306: Swift detection of a possible burst DATE: 09/03/06 15:21:00 GMT FROM: Scott Barthelmy at NASA/GSFC O. Godet (U Leicester), S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC), A. P. Beardmore (U Leicester), S. Campana (INAF-OAB), P. A. Evans (U Leicester), J. A. Kennea (PSU), R. Margutti (Univ Bicocca&OAB), J. P. Osborne (U Leicester), C. Pagani (PSU), K. L. Page (U Leicester), D. M. Palmer (LANL), M. Perri (ASDC), P. Romano (INAF-IASFPA), B. A. Rowlinson (U Leicester), G. Stratta (ASDC), M. C. Stroh (PSU), G. Tagliaferri (INAF-OAB) and L. Vetere (PSU) report on behalf of the Swift Team: At 13:54:24 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 090306 (trigger=345396). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 16.740, -50.065 which is RA(J2000) = 01h 06m 58s Dec(J2000) = -50d 03' 54" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a short, hard structure with a duration less than a single 64 msec timebin. The peak count rate was ~2000 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 13:55:57.4 UT, 95.5 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued X-ray source located at RA, Dec 16.74294, -50.00206 which is equivalent to: RA(J2000) = 01h 06m 58.31s Dec(J2000) = -50d 00' 07.4" with an uncertainty of 4.9 arcseconds (radius, 90% containment). 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 1.37e+20 cm^-2 (Kalberla et al. 2005). At this time we do not have UVOT data. This trigger was the first of a new triggering function within BAT (the so called Subthreshold trigger). While there is a real source in the XRT initial image, this would say the trigger is a real GRB. However because we did not receive the usual BAT Position message (for reasons yet unknown), we caution that we can not say with confidence that this is a real burst. We will not have the full data downlinked until ~17:00 UT. Burst Advocate for this burst is O. Godet (og19 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.)