TITLE: GCN CIRCULAR NUMBER: 9461 SUBJECT: GRB 090531B: Swift detection of a burst DATE: 09/05/31 18:44:29 GMT FROM: Scott Barthelmy at NASA/GSFC J. R. Cummings (NASA/UMBC), S. D. Barthelmy (GSFC), N. Gehrels (NASA/GSFC), C. Gronwall (PSU), S. T. Holland (CRESST/USRA/GSFC), J. A. Kennea (PSU), V. Mangano (INAF-IASFPA), C. B. Markwardt (CRESST/GSFC/UMD), P. T. O'Brien (U Leicester), K. L. Page (U Leicester), D. M. Palmer (LANL), T. Sakamoto (NASA/UMBC) and B. Sbarufatti (INAF-IASFPA) report on behalf of the Swift Team: At 18:35:56 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 090531B (trigger=353728). Swift will execute a delayed slew to the burst. The BAT on-board calculated location is RA, Dec 252.064, -36.025 which is RA(J2000) = 16h 48m 15s Dec(J2000) = -36d 01' 27" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows an initial bright spike at T_zero, then two much smaller spikes at T+21 sec and T+35 sec. The peak count rate was ~7000 counts/sec (15-350 keV), at ~0 sec after the trigger. Due to an observing constraint, Swift will not slew until T0+50.1 minutes. There will be no XRT or UVOT data until this time. 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.)