TITLE: GCN CIRCULAR NUMBER: 45974 SUBJECT: GRB 261017A: Swift detection of a burst DATE: 26/10/17 16:18:01 GMT FROM: Jimmy DeLaunay at PSU M. H. Siegel (PSU), S. D. Barthelmy (GSFC), A. P. Beardmore (U Leicester), S. Dichiara (PSU), R. Gupta (NASA GSFC), N. P. M. Kuin (UCL-MSSL), A. Y. Lien (U Tampa), M. J. Moss (GSFC), K. L. Page (U Leicester) and D. M. Palmer (LANL) report on behalf of the Neil Gehrels Swift Observatory Team: At 16:05:05 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 261017A (trigger=1519536). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 139.900, +12.644 which is RA(J2000) = 09h 19m 36s Dec(J2000) = +12d 38' 38" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a complex structure with a duration of about 11 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 16:06:26.4 UT, 81.0 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec 139.9021, 12.6394 which is equivalent to: RA(J2000) = 09h 19m 36.50s Dec(J2000) = +12d 38' 21.8" with an uncertainty of 4.8 arcseconds (radius, 90% containment). This location is 18 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 150 seconds with the White filter starting 89 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 extinction. 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/)