TITLE: GCN CIRCULAR NUMBER: 8021 SUBJECT: GRB 080727: Swift detection of a burst DATE: 08/07/27 06:16:46 GMT FROM: David Palmer at LANL S. Immler (CRESST/GSFC/UMD), W. H. Baumgartner (GSFC/UMBC), N. Gehrels (NASA/GSFC), S. D. Hunsberger (PSU), W.B Landsman (GSFC), C. B. Markwardt (CRESST/GSFC/UMD), K. L. Page (U Leicester), D. M. Palmer (LANL), A. M. Parsons (GSFC), D. Perez (U Leicester), T. Sakamoto (NASA/UMBC) and T. N. Ukwatta (GSFC/GWU) report on behalf of the Swift Team: At 05:57:39 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 080727 (trigger=318094). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 208.389, -18.549 which is RA(J2000) = 13h 53m 33s Dec(J2000) = -18d 32' 55" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a single peak structure with a duration of about 5 sec. The peak count rate was ~700 counts/sec (15-350 keV), at ~2 sec after the trigger. The XRT began observing the field at 05:59:28.5 UT, 109.2 seconds after the BAT trigger. Using promptly downlinked data we find a fading, uncatalogued X-ray source with an enhanced position: RA, Dec 208.39099, -18.54505 which is equivalent to: RA(J2000) = 13h 53m 33.84s Dec(J2000) = -18d 32' 42.2" with an uncertainty of 2.7 arcseconds (radius, 90% containment). This location is 15 arcseconds from the BAT onboard position, within the BAT error circle. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density consistent with the Galactic value of 6.75e+20 cm^-2 (Kalberla et al. 2005). UVOT took a finding chart exposure of 100 seconds with the White (160-650 nm) filter starting 114 seconds after the BAT trigger. No afterglow candidate has been found in the initial data products. The 2.7'x2.7' sub-image covers 25% of the BAT error circle and 100% of the XRT error circle. The typical 3-sigma upper limit has been about 18.5 mag. The list of sources is typically complete to about 18 mag. No correction has been made for the expected extinction corresponding to E(B-V) of 0.07. Burst Advocate for this burst is S. Immler (immler 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.)