TITLE: GCN CIRCULAR NUMBER: 17950 SUBJECT: Swift Trigger 645340: a possible GRB DATE: 15/06/21 12:01:59 GMT FROM: Frank Marshall at GSFC F. E. Marshall (NASA/GSFC), A. Y. Lien (GSFC/UMBC), D. Malesani (DARK/NBI) and K. L. Page (U Leicester) report on behalf of the Swift Team: At 11:28:58 UT, the Swift Burst Alert Telescope (BAT) triggered and located either Sco X-1 or a source near Sco X-1. Swift slewed immediately to the source. The BAT on-board calculated location is RA, Dec 245.071, -15.691 which is RA(J2000) = 16h 20m 17s Dec(J2000) = -15d 41' 26" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT position is 6.1 arcmin from Sco X-1. This was a 64 s image trigger with significance of 17.7 sigma. As is usual with an image trigger, the available BAT light curve shows no significant structure. The source centroided on by the XRT is Sco X-1. This is, however, 363 arcsec from the BAT position, and thus outside the BAT error circle. We cannot determine at this time whether there is a new X-ray source within the BAT error circle. UVOT took a finding chart exposure of 150 seconds with the White filter starting 185 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 BAT error circle. The 8'x8' region for the list of sources generated on-board covers 100% of the BAT error circle. No correction has been made for the expected extinction corresponding to E(B-V) of 0.38. We await the full downlinked dataset to make a more definitive statement on the nature of this trigger. 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.)