TITLE: GCN CIRCULAR NUMBER: 17510 SUBJECT: GRB 150301A: Swift detection of a short burst or new SGR DATE: 15/03/01 01:26:16 GMT FROM: Scott Barthelmy at NASA/GSFC A. Y. Lien (GSFC/UMBC), S. D. Barthelmy (GSFC), D. N. Burrows (PSU), J. R. Cummings (NASA/UMBC), M. De Pasquale (INAF-IASFPA), L. M. Z. Hagen (PSU), J. A. Kennea (PSU), C. B. Markwardt (NASA/GSFC), A. Maselli (INAF-IASFPA), D. M. Palmer (LANL) and M. H. Siegel (PSU) report on behalf of the Swift Team: At 01:04:28 UT, the Swift Burst Alert Telescope (BAT) triggered and located a source which could be a short GRB or a new SGR (trigger=632995). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 244.286, -48.697, which is RA(J2000) = 16h 17m 09s Dec(J2000) = -48d 41' 48" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows a single peak structure with a duration of about 0.1 sec. The peak count rate was ~8500 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 01:05:26.6 UT, 58.3 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec 244.3047, -48.7131 which is equivalent to: RA(J2000) = +16h 17m 13.13s Dec(J2000) = -48d 42' 47.2" with an uncertainty of 5.0 arcseconds (radius, 90% containment). This location is 73 arcseconds from the BAT onboard position, within the BAT error circle. No event data are yet available to determine the column density using X-ray spectroscopy. UVOT took a finding chart exposure of 113 seconds with the White filter starting 65 seconds after the BAT trigger. No credible afterglow candidate has been found in the initial data products. The 2.7'x2.7' sub-image covers none of the XRT error circle. The coverage of the XRT error circle by the 8'x8' region for the list of sources generated on-board is uncertain because the large number of sources filled the available telemetry. The list of sources is typically complete to about 18 mag. No correction has been made for the large, but uncertain extinction expected. We note that this source is near the Galactic Plane and Bulge (lat = 1.36, lon = 334) and so this might be a previously-unknown Soft Gamma Repeater. The burst has significant emission above 100 keV, which is unusual, but not unprecedented, for an SGR. Further analysis of the full data set and continued observation is required to determine the nature of this source. Burst Advocate for this burst is A. Y. Lien (amy.y.lien AT 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.)