TITLE: GCN CIRCULAR NUMBER: 9768 SUBJECT: GRB 090812: Swift detection of a burst with an optical counterpart DATE: 09/08/12 06:20:46 GMT FROM: David Palmer at LANL M. Stamatikos (NASA/ORAU), J. R. Cummings (NASA/UMBC), P. A. Evans (U Leicester), C. Gronwall (PSU), C. Guidorzi (U Ferrara), C. B. Markwardt (CRESST/GSFC/UMD), P. T. O'Brien (U Leicester), K. L. Page (U Leicester), D. M. Palmer (LANL) and R. L. C. Starling (U Leicester) report on behalf of the Swift Team: At 06:02:08 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 090812 (trigger=359711). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 353.194, -10.595 which is RA(J2000) = 23h 32m 47s Dec(J2000) = -10d 35' 39" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a triple-peaked structure with a duration of about 70 sec. The peak count rate was ~6500 counts/sec (15-350 keV), at ~25 sec after the trigger. The XRT began observing the field at 06:03:25.7 UT, 76.8 seconds after the BAT trigger. Using promptly downlinked data we find a bright, uncatalogued X-ray source located at RA, Dec 353.20133, -10.60457 which is equivalent to: RA(J2000) = 23h 32m 48.32s Dec(J2000) = -10d 36' 16.5" with an uncertainty of 3.7 arcseconds (radius, 90% containment). This location is 43 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 consistent with the Galactic value of 2.26e+20 cm^-2 (Kalberla et al. 2005). The initial flux in the 2.5 s image was 4.95e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 250 seconds with the U filter starting 298 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 23:32:48.53 = 353.20222 DEC(J2000) = -10:36:17.3 = -10.60481 with a 90%-confidence error radius of about 0.78 arc sec. This position is 5.1 arc sec. from the center of the XRT error circle. The estimated magnitude is 18.62 with a 1-sigma error of about 0.16. No correction has been made for the expected extinction corresponding to E(B-V) of 0.02. Burst Advocate for this burst is M. Stamatikos (michael 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.)