TITLE: GCN CIRCULAR NUMBER: 18611 SUBJECT: GRB 151118A: Swift detection of a burst DATE: 15/11/18 03:24:39 GMT FROM: Scott Barthelmy at NASA/GSFC A. Amaral-Rogers (U Leicester), S. D. Barthelmy (GSFC), J. A. Kennea (PSU), F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester) and M. H. Siegel (PSU) report on behalf of the Swift Team: At 03:06:30 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 151118A (trigger=664078). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 57.158, +65.869, which is RA(J2000) = 03h 48m 38s Dec(J2000) = +65d 52' 07" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows a few peaks with a duration of about 25 sec. The peak count rate was ~2600 counts/sec (15-350 keV), at ~6 sec after the trigger. The XRT began observing the field at 03:07:36.2 UT, 66.2 seconds after the BAT trigger. Using promptly downlinked data we find a fading, uncatalogued X-ray source with an enhanced position: RA, Dec 57.1719, 65.9012 which is equivalent to: RA(J2000) = 03h 48m 41.25s Dec(J2000) = +65d 54' 04.4" with an uncertainty of 2.4 arcseconds (radius, 90% containment). This location is 117 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 http://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 (4.54 x 10^21 cm^-2, Willingale et al. 2013), with an excess column of 5.9 (+4.29/-3.55) x 10^21 cm^-2 (90% confidence). UVOT took a finding chart exposure of 250 seconds with the U filter starting 281 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 18.0 mag. No correction has been made for the expected extinction corresponding to E(B-V) of 1.10. Burst Advocate for this burst is A. Amaral-Rogers (aar14 AT le.ac.uk). 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.)