TITLE: GCN CIRCULAR NUMBER: 30032 SUBJECT: GRB 210517A: Swift detection of a burst DATE: 21/05/17 05:52:10 GMT FROM: David Palmer at LANL V. D'Elia (SSDC & INAF-OAR), E. Ambrosi (INAF-IASFPA), A. D'Ai (INAF-IASFPA), J.D. Gropp (PSU), H. A. Krimm (NSF), A. Y. Lien (GSFC/UMBC), K. L. Page (U Leicester), D. M. Palmer (LANL), B. Sbarufatti (PSU), M. H. Siegel (PSU) and A. Tohuvavohu (U Toronto) report on behalf of the Neil Gehrels Swift Observatory Team: At 05:28:01 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210517A (trigger=1048783). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 358.174, -39.105 which is RA(J2000) = 23h 52m 42s Dec(J2000) = -39d 06' 17" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT lightcurve shows a peak at least 2 seconds long and a maximum count rate of ~2200 counts/s (15-350 keV). The XRT began observing the field at 05:29:08.6 UT, 67.0 seconds after the BAT trigger. Using promptly downlinked data we find a fading, uncatalogued X-ray source with an enhanced position: RA, Dec 358.2245, -39.1020 which is equivalent to: RA(J2000) = 23h 52m 53.88s Dec(J2000) = -39d 06' 07.2" with an uncertainty of 2.1 arcseconds (radius, 90% containment). This location is 141 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 https://www.swift.ac.uk/sper. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density in excess of the Galactic value (1.30 x 10^20 cm^-2, Willingale et al. 2013), with an excess column of 5.2 (+2.58/-2.26) x 10^21 cm^-2 (90% confidence). UVOT took a finding chart exposure of 150 seconds with the White filter starting 69 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 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 expected extinction corresponding to E(B-V) of 0.011. Burst Advocate for this burst is V. D'Elia (valerio.delia AT ssdc.asi.it). 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/)