TITLE: GCN CIRCULAR NUMBER: 7598 SUBJECT: GRB 080413B: Swift detection of a possibly short burst with optical counterpart DATE: 08/04/13 09:20:28 GMT FROM: David Palmer at LANL M. Stamatikos (NASA/ORAU), A. P. Beardmore (U Leicester), N. Gehrels (NASA/GSFC), S. D. Hunsberger (PSU), C. B. Markwardt (CRESST/GSFC/UMD), J. P. Osborne (U Leicester), K. L. Page (U Leicester), D. M. Palmer (LANL), T. Sakamoto (NASA/UMBC), R. L. C. Starling (U Leicester) and E. Troja (U Leicester/INAF-IASFPA) report on behalf of the Swift Team: At 08:51:12 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 080413B (trigger=309111). Swift started slewing approximately 70 seconds after the burst. The BAT on-board calculated location is RA, Dec 326.134, -19.969 which is RA(J2000) = 21h 44m 32s Dec(J2000) = -19d 58' 08" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a single peaked structure with a duration of about 3 sec. The peak count rate was ~15,000 counts/sec (15-350 keV), at ~0.2 sec after the trigger. The XRT began observing the field at 08:53:23.8 UT, 131.2 seconds after the BAT trigger. Using promptly downlinked data we find a fading, uncatalogued X-ray source with an enhanced position: RA, Dec 326.14434, -19.98104 which is equivalent to: RA(J2000) = 21h 44m 34.64s Dec(J2000) = -19d 58' 51.7" with an uncertainty of 2.6 arcseconds (radius, 90% containment). This location is 55 arcseconds from the BAT onboard position, within the BAT error circle. A power-law fit to a spectrum formed from promptly downlinked event data does not constrain the column density, so we cannot provide limits on the redshift using spectroscopy and the relation from Grupe et al. (2007). A summary of the promptly downlinked data is given at http://www.swift.ac.uk/sper/309111/. UVOT took a finding chart exposure of 100 seconds with the White (160-650 nm) filter starting 134 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 21:44:34.68 = 326.1445 DEC(J2000) = -19:58:52.0 = -19.9811 with a 1-sigma error radius of about 0.6 arc sec. This position is 0.6 arc sec. from the XRT position. The estimated magnitude is 16.5 with a 1-sigma error of about 0.5 mag. No correction has been made for the expected extinction corresponding to E(B-V) of 0.04. 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.)