TITLE: GCN CIRCULAR NUMBER: 21011 SUBJECT: Swift Trigger 748858: a possible GRB DATE: 17/04/19 08:07:46 GMT FROM: Kim Page at U.of Leicester M. H. Siegel (PSU), C. Gronwall (PSU), A. Y. Lien (GSFC/UMBC), K. L. Page (U Leicester) and B. Sbarufatti (INAF-OAB/PSU) report on behalf of the Swift Team: At 07:43:33 UT, the Swift Burst Alert Telescope (BAT) triggered and located a possible GRB (trigger=748858). Swift slewed immediately to the source. The BAT on-board calculated location is RA, Dec 199.853, +15.471 which is RA(J2000) = 13h 19m 25s Dec(J2000) = +15d 28' 16" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). As is usual with an image trigger, the available BAT light curve shows no significant structure. The XRT began observing the field at 07:45:58.1 UT, 145.1 seconds after the BAT trigger. No source was detected in 844 s of promptly downlinked data. We are waiting for the full dataset to detect and localise any XRT counterpart. UVOT took a finding chart exposure of 150 seconds with the White filter starting 146 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 25% of the BAT 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 BAT error circle. The list of sources is typically complete to about 18 mag. No correction has been made for the expected extinction corresponding to E(B-V) of 0.03. Given the relatively low significance of the image detection (7.1 sigma) and the lack of detection in the initial XRT data, we cannot confirm that this is a true astrophysical event. A final determination of the nature of this trigger will require the full downlinked dataset. Burst Advocate for this burst is M. H. Siegel (siegel AT swift.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.)