TITLE: GCN CIRCULAR NUMBER: 18373 SUBJECT: GRB 151001A: Swift detection of a burst with an optical counterpart DATE: 15/10/01 16:38:56 GMT FROM: Jamie A. Kennea at PSU/Swift-XRT M. Stamatikos (OSU/NASA/GSFC), S. D. Barthelmy (GSFC), P. A. Evans (U Leicester), N. Gehrels (NASA/GSFC), S. L. Gibson (U Leicester), J. A. Kennea (PSU), H. A. Krimm (CRESST/GSFC/USRA), N. P. M. Kuin (UCL-MSSL), A. Y. Lien (GSFC/UMBC), F. E. Marshall (NASA/GSFC), C. Pagani (U Leicester), B. Sbarufatti (INAF-OAB/PSU) and M. H. Siegel (PSU) report on behalf of the Swift Team: At 15:04:22 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 151001A (trigger=657286). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 233.731, +10.971 which is RA(J2000) = 15h 34m 55s Dec(J2000) = +10d 58' 16" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows a couple overlapping peaks with a duration of about 100 sec. The peak count rate was ~1000 counts/sec (15-350 keV), at ~10 sec after the trigger. The XRT began observing the field at 15:05:26.9 UT, 64.0 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued X-ray source with an enhanced position: RA, Dec 233.7289, 10.9669 which is equivalent to: RA(J2000) = 15h 34m 54.94s Dec(J2000) = +10d 58' 00.8" with an uncertainty of 2.0 arcseconds (radius, 90% containment). This location is 16 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 (3.53 x 10^20 cm^-2, Willingale et al. 2013), with an excess column of 1.3 (+1.39/-1.27) x 10^21 cm^-2 (90% confidence). UVOT took a finding chart exposure of 150 seconds with the White filter starting 67 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 15:34:54.90 = 233.72874 DEC(J2000) = +10:58:02.1 = 10.96724 with a 90%-confidence error radius of about 0.62 arc sec. This position is 3.8 arc sec. from the center of the XRT error circle. The estimated magnitude is 17.94 with a 1-sigma error of about 0.14. No correction has been made for the expected extinction corresponding to E(B-V) of 0.04. Burst Advocate for this burst is M. Stamatikos (Michael.Stamatikos-1 AT 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.)