TITLE: GCN CIRCULAR NUMBER: 8398 SUBJECT: GRB 081024: Swift detection of a burst DATE: 08/10/24 06:18:55 GMT FROM: Neil Gehrels at GSFC G. Stratta (ASDC), W. H. Baumgartner (GSFC/UMBC), N. Gehrels (NASA/GSFC), E. A. Hoversten (PSU), C. B. Markwardt (CRESST/GSFC/UMD), M. Perri (ASDC) and L. Vetere (PSU) report on behalf of the Swift Team: At 05:53:08 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 081024 (trigger=332516). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 27.898, +61.346 which is RA(J2000) = 01h 51m 36s Dec(J2000) = +61d 20' 47" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a short hard peak with a duration of about 0.3 sec at T+0 with a short precursor peak at T-2.5 seconds. The peak count rate was ~6,000 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 05:54:21.1 UT, 72.3 seconds after the BAT trigger. Using promptly downlinked data we find a fading, uncatalogued X-ray source with an enhanced position: RA, Dec 27.8745, 61.3317 which is equivalent to: RA(J2000) = 01h 51m 29.88s Dec(J2000) = +61d 19' 54.1" with an uncertainty of 2.3 arcseconds (radius, 90% containment). This location is 65 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 7.71e+21 cm^-2 (Kalberla et al. 2005). UVOT took a finding chart exposure of nominal 150 seconds with the White (160-650 nm) filter starting 76 seconds after the BAT trigger. No afterglow candidate has been found in the initial data products. Image catalog data are not available at this time. The 8'x8' region for the list of sources generated on-board covers 100% of the BAT error circle. The list of sources is typically complete to about 18 mag. No correction has been made for the large, but uncertain extinction expected. This appears to be a short hard GRB with a precursor and a fading X-ray afterglow. We note that the position in galactic coordinates is l=130deg, b=-0.7 deg. It is not impossible that this is a galactic transient or even an SGR. Burst Advocate for this burst is G. Stratta (giulia.stratta AT asdc.asi.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.)