TITLE: GCN CIRCULAR NUMBER: 8648 SUBJECT: GRB 081210: Swift detection of a burst with optical afterglow DATE: 08/12/10 20:37:36 GMT FROM: David Palmer at LANL H. A. Krimm (CRESST/GSFC/USRA), A. P. Beardmore (U Leicester), P. J. Brown (PSU), D. N. Burrows (PSU), J. R. Cummings (NASA/UMBC), P. A. Evans (U Leicester), C. Guidorzi (UniFE), S. T. Holland (CRESST/USRA/GSFC), S. D. Hunsberger (PSU), S. Immler (CRESST/GSFC/UMD), J. A. Kennea (PSU), W.B Landsman (GSFC), F. E. Marshall (NASA/GSFC), J. P. Osborne (U Leicester), C. Pagani (PSU), D. M. Palmer (LANL), A. M. Parsons (GSFC), M. Stamatikos (NASA/ORAU) and E. Troja (INAF-IASFPA) report on behalf of the Swift Team: At 20:19:34 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 081210 (trigger=337073). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 70.458, -11.266 which is RA(J2000) = 04h 41m 50s Dec(J2000) = -11d 15' 55" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a double-peaked structure with a duration of about 30 sec, followed by an additional peak at T+140 s. The peak count rate was ~5000 counts/sec (15-350 keV), at ~18 sec after the trigger. The XRT began observing the field at 20:20:46.1 UT, 71.9 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec 70.4853, -11.2568 which is equivalent to: RA(J2000) = 04h 41m 56.47s Dec(J2000) = -11d 15' 24.4" with an uncertainty of 5.4 arcseconds (radius, 90% containment). This location is 101 arcseconds from the BAT onboard position, within the BAT error circle. The full field-of-view light curve shows a strong flare at T+140 s, coincident with a flare seen in the BAT. No event data are yet available to determine the column density using X-ray spectroscopy. The initial flux in the 2.5 s image was 1.35e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 80 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 04:41:56.20 = 70.48417 DEC(J2000) = -11:15:26.7 = -11.25741 with a 90%-confidence error radius of about 0.64 arc sec. This position is 4.6 arc sec. from the center of the XRT error circle. The estimated magnitude is 19.12 with a 1-sigma error of about 0.15. No correction has been made for the expected extinction corresponding to E(B-V) of 0.08. The afterglow is detected in the UVOT u filter, so the redshift must be less than approximately 3.3. Burst Advocate for this burst is H. A. Krimm (krimm AT milkyway.gsfc.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.)