TITLE: GCN CIRCULAR NUMBER: 8753 SUBJECT: GRB 081230: Swift detection of a burst DATE: 08/12/30 21:02:09 GMT FROM: Scott Barthelmy at NASA/GSFC V. La Parola (INAF-IASFPA), S. D. Barthelmy (GSFC), P. J. Brown (PSU), J. R. Cummings (NASA/UMBC), N. Gehrels (NASA/GSFC), S. T. Holland (CRESST/USRA/GSFC), E. A. Hoversten (PSU), J. A. Kennea (PSU), W.B Landsman (GSFC), V. Mangano (INAF-IASFPA), J. P. Osborne (U Leicester), K. L. Page (U Leicester), B. Sbarufatti (INAF-IASFPA) and T. N. Ukwatta (GSFC/GWU) report on behalf of the Swift Team: At 20:36:12 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 081230 (trigger=338633). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 37.315, -25.142 which is RA(J2000) = 02h 29m 16s Dec(J2000) = -25d 08' 29" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows a single peak with a duration of about 50 sec. The peak count rate was ~600 counts/sec (15-350 keV), at ~50 sec after the trigger. The XRT began observing the field at 20:38:16.7 UT, 124.1 seconds after the BAT trigger. Using promptly downlinked data we find a bright, fading, uncatalogued X-ray source located at RA, Dec 37.33148, -25.14645 which is equivalent to: RA(J2000) = 02h 29m 19.56s Dec(J2000) = -25d 08' 47.2" with an uncertainty of 3.9 arcseconds (radius, 90% containment). This location is 56 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 (1.68e+20 cm^-2, Kalberla et al. 2005), with an excess column of 3.2 (+2.81/-2.34) x 10^21 cm^-2 (90% confidence). UVOT took a finding chart exposure of 250 seconds with the U filter starting 132 seconds after the BAT trigger. No credible afterglow candidate has been found in the initial data products. The 2.7'x2.7' sub-image covers 100% of the XRT error circle. The typical 3-sigma upper limit has been about 19.2 mag. The 8'x8' region for the list of sources generated on-board covers 100% of the XRT error circle. The list of sources is typically complete to about 18.0 mag. No correction has been made for the expected extinction corresponding to E(B-V) of 0.01. Burst Advocate for this burst is V. La Parola (laparola AT ifc.inaf.it). 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.)