TITLE: GCN CIRCULAR NUMBER: 9718 SUBJECT: GRB 090727: Swift detection of a burst DATE: 09/07/27 22:59:22 GMT FROM: Scott Barthelmy at NASA/GSFC P. A. Evans (U Leicester), S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC), A. P. Beardmore (U Leicester), M. M. Chester (PSU), S. T. Holland (CRESST/USRA/GSFC), N. P. M. Kuin (MSSL), C. B. Markwardt (CRESST/GSFC/UMD), K. L. Page (U Leicester), A. Rowlinson (U Leicester), M. H. Siegel (PSU), M. A. Stark (PSU) and T. N. Ukwatta (GSFC/GWU) report on behalf of the Swift Team: At 22:42:18 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 090727 (trigger=358520). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 315.972, +64.916 which is RA(J2000) = 21h 03m 53s Dec(J2000) = +64d 54' 58" 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 25 sec. The peak count rate was ~1000 counts/sec (15-350 keV), at ~1 sec after the trigger. The XRT began observing the field at 22:43:46.0 UT, 87.9 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued X-ray source located at RA, Dec 315.96139, 64.92438 which is equivalent to: RA(J2000) = 21h 03m 50.73s Dec(J2000) = +64d 55' 27.8" with an uncertainty of 4.0 arcseconds (radius, 90% containment). This location is 34 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 2.48e+21 cm^-2 (Kalberla et al. 2005). UVOT took a finding chart exposure of nominal 150 seconds with the White filter starting 91 seconds after the BAT trigger. No credible afterglow candidate has been found in the initial data products. Data from the 2.7'x2.7' sub-image are not available at this time. 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 mag. No correction has been made for the expected extinction corresponding to E(B-V) of 0.54. Burst Advocate for this burst is P. A. Evans (pae9 AT star.le.ac.uk). 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.)