TITLE: GCN CIRCULAR NUMBER: 11102 SUBJECT: GRB 100816A: Swift detection of a possibly short burst with optical afterglow DATE: 10/08/16 01:17:12 GMT FROM: David Palmer at LANL S. R. Oates (UCL-MSSL), S. D. Barthelmy (GSFC), A. P. Beardmore (U Leicester), J. R. Cummings (NASA/UMBC), N. Gehrels (NASA/GSFC), J. M. Gelbord (PSU), J. A. Kennea (PSU), N. P. M. Kuin (UCL-MSSL), O. M. Littlejohns (U Leicester), C. B. Markwardt (NASA/GSFC), F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester), D. M. Palmer (LANL), C. J. Saxton (UCL-MSSL), M. Stamatikos (OSU/NASA/GSFC) and R. L. C. Starling (U Leicester) report on behalf of the Swift Team: At 00:37:51 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 100816A (trigger=431764). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 351.740, +26.561 which is RA(J2000) = 23h 26m 58s Dec(J2000) = +26d 33' 38" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a single symmetric peak structure with a duration of about 2 sec. There is possible further activity at around T+80s. The peak count rate was ~13000 counts/sec (15-350 keV), at ~0.5 sec after the trigger. The XRT began observing the field at 00:39:14.2 UT, 82.8 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec 351.7381, +26.5785 which is equivalent to: RA(J2000) = 23h 26m 57.15s Dec(J2000) = +26d 34' 42.6" with an uncertainty of 5.5 arcseconds (radius, 90% containment). This location is 63 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 91 seconds after the BAT trigger. The refined UVOT position is RA (J2000) 23:26:57.56 Dec (J2000) 26:34:42.9 with an estimated uncertainty of 0.5 arcsec (radius, 90% confidence). This position is 5.5 arc sec. from the center of the XRT error circle. The estimated magnitude is 17.00 with a 1-sigma error of about 0.03. No correction has been made for the expected extinction corresponding to E(B-V) of 0.09. There were no real-time GCN Notices on this burst because the block2world filter was still in place due to the earlier episode of the startracker loss-of-lock problem. The MOC had just gotten the startracker back to lock and BAT triggers restored when the trigger occurred. But the block2world had not yet been removed. BAT_POSITION and XRT_POSITION notices were manually generated and distributed. Burst Advocate for this burst is S. R. Oates (sro AT mssl.ucl.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.)