TITLE: GCN CIRCULAR NUMBER: 21135 SUBJECT: GRB 170524A: Swift detection of a burst (or possible new SGR) DATE: 17/05/24 20:06:28 GMT FROM: Scott Barthelmy at NASA/GSFC J.G. Gropp (PSU), S. D. Barthelmy (GSFC), C. Gronwall (PSU), J. A. Kennea (PSU), H. A. Krimm (NSF/USRA), N. P. M. Kuin (UCL-MSSL), A. Y. Lien (GSFC/UMBC) and D. M. Palmer (LANL) report on behalf of the Swift Team: At 19:49:15 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 170524A (trigger=754322). Swift slewed immediately to the source. The BAT on-board calculated location is RA, Dec 319.456, +48.587, which is RA(J2000) = 21h 17m 49s Dec(J2000) = +48d 35' 15" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows a single peak with a duration of about 0.25 sec. The peak count rate was ~3000 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 19:50:16.0 UT, 60.7 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued X-ray source with an enhanced position: RA, Dec 319.4869, 48.6065 which is equivalent to: RA(J2000) = 21h 17m 56.86s Dec(J2000) = +48d 36' 23.4" with an uncertainty of 4.1 arcseconds (radius, 90% containment). This location is 101 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. We cannot determine whether the source is fading at the present time. A power-law fit to a spectrum formed from promptly downlinked event data does not constrain the column density. UVOT took a finding chart exposure of 150 seconds with the White filter starting 67 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 100% of the XRT error circle. The typical 3-sigma upper limit has been about 19.6 mag. The 8'x8' region for the list of sources generated on-board covers 100% of the XRT error circle. 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 is a short burst, close to the Galactic plane (lat = -0.5 degrees), although far from the Galactic center (lon = 91 degrees). Thus there is a possibility that this is a previously unknown Soft Gamma Repeater (SGR). However, the preliminary 4-energy lightcurve shows more flux above 100 keV than is typical for an SGR burst. Burst Advocate for this burst is J.G. Gropp (jdg44 AT psu.edu). 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.)