TITLE: GCN CIRCULAR NUMBER: 11345 SUBJECT: GRB 101017A: Swift detection of a burst with optical counterpart DATE: 10/10/17 10:45:42 GMT FROM: David Palmer at LANL M. H. Siegel (PSU), J. M. Gelbord (PSU), D. Grupe (PSU), S. T. Holland (CRESST/USRA/GSFC), E. A. Hoversten (PSU), C. B. Markwardt (NASA/GSFC), P. T. O'Brien (U Leicester) and D. M. Palmer (LANL) report on behalf of the Swift Team: At 10:32:47 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 101017A (trigger=436429). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 291.378, -35.141 which is RA(J2000) = 19h 25m 31s Dec(J2000) = -35d 08' 26" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows a complex series of peaks in a FRED envelope about 40 seconds long, with possible further emission out to T+100 seconds. The peak count rate was ~12000 counts/sec (15-350 keV), at ~3 sec after the trigger. The XRT began observing the field at 10:34:08.4 UT, 81.0 seconds after the BAT trigger. Using promptly downlinked data we find a bright, fading, uncatalogued X-ray source located at RA, Dec 291.38546, -35.14513 which is equivalent to: RA(J2000) = 19h 25m 32.51s Dec(J2000) = -35d 08' 42.5" with an uncertainty of 3.9 arcseconds (radius, 90% containment). This location is 26 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 (7.69 x 10^20 cm^-2, Kalberla et al. 2005), with an excess column of 4.8 (+3.15/-2.65) x 10^21 cm^-2 (90% confidence). The initial flux in the 2.5 s image was 8.79e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 88 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 19:25:32.56 = 291.38565 DEC(J2000) = -35:08:41.4 = -35.14482 with a 90%-confidence error radius of about 0.72 arc sec. This position is 2.7 arc sec. from the center of the XRT error circle. The estimated magnitude is 19.98 with a 1-sigma error of about 0.19. No correction has been made for the expected extinction corresponding to E(B-V) of 0.17. Burst Advocate for this burst is M. H. Siegel (siegel AT astro.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.)