TITLE: GCN CIRCULAR NUMBER: 18215 SUBJECT: GRB 150831B: Swift detection of a probable burst DATE: 15/08/31 22:34:04 GMT FROM: Scott Barthelmy at NASA/GSFC F. E. Marshall (NASA/GSFC), S. D. Barthelmy (GSFC), A. Y. Lien (GSFC/UMBC), D. M. Palmer (LANL) and T. Sakamoto (AGU) report on behalf of the Swift Team: At 22:19:27 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 150831B (trigger=653978). Swift did not slew due to observing constraint. The BAT on-board calculated location is RA, Dec 271.021, -27.252, which is RA(J2000) = 18h 04m 05s Dec(J2000) = -27d 15' 07" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows a couple of overlapping peaks structure with a total duration of about 3 sec. The peak count rate was ~2000 counts/sec (15-350 keV), at ~0 sec after the trigger. Due to an observing constraint, Swift will not slew until T0+49.6 minutes. There will be no XRT or UVOT data until this time. We note that this source is in the projected direction of the Galactic bulge (lon,lat = 3.51,-2.66) and for that reason there is a strong possibility that this is a Galactic source. However, its light curve and the presence of flux above 100 keV are more consistent with a cosmological GRB. Determination of the nature of this event will require XRT observations and the full downlinked dataset. Burst Advocate for this burst is F. E. Marshall (marshall 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.)