TITLE: GCN CIRCULAR NUMBER: 21788 SUBJECT: GRB 170902A: Swift detection of a burst or possible Galactic transient DATE: 17/09/02 20:41:05 GMT FROM: Scott Barthelmy at NASA/GSFC C. B. Markwardt (NASA/GSFC), D. N. Burrows (PSU), J. R. Cummings (NASA/UMBC), J. A. Kennea (PSU), F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester), D. M. Palmer (LANL) and M. H. Siegel (PSU) report on behalf of the Swift Team: At 20:00:36 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 170902A (trigger=770431). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 233.895, -57.213, which is RA(J2000) = 15h 35m 35s Dec(J2000) = -57d 12' 44" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). As is typical for image triggers, there is nothing significant in the real-time data. The XRT began observing the field at 20:07:20.4 UT, 404.1 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec 233.8330, -57.2313 which is equivalent to: RA(J2000) = +15h 35m 19.92s Dec(J2000) = -57d 13' 52.7" with an uncertainty of 5.8 arcseconds (radius, 90% containment). This location is 137 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 128 seconds with the White filter starting 413 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. Because this is a long event (336 second image trigger) that is close to the Galactic Plane (lat=-1.13 deg) there is a possibility that this is a Galactic transient rather than a cosmic GRB. Determination of the nature of this object will require the full downlinked dataset and further observations. Burst Advocate for this burst is C. B. Markwardt (Craig.Markwardt 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.)