TITLE: GCN CIRCULAR NUMBER: 9746 SUBJECT: GRB 090807: Swift detection of a burst DATE: 09/08/07 15:18:32 GMT FROM: Scott Barthelmy at NASA/GSFC J. K. Cannizzo (NASA/UMBC), S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC), D. N. Burrows (PSU), P.A. Curran (MSSL-UCL), A. de Ugarte Postigo (INAF-OAB), P. A. Evans (U Leicester), N. Gehrels (NASA/GSFC), C. Guidorzi (U Ferrara), J. A. Kennea (PSU), H. A. Krimm (CRESST/GSFC/USRA), N. P. M. Kuin (MSSL), J. Mao (INAF-OAB), R. Margutti (Univ Bicocca&OAB), K. L. Page (U Leicester), T. Sakamoto (NASA/UMBC), M. H. Siegel (PSU), M. Stamatikos (NASA/ORAU), T. N. Ukwatta (GSFC/GWU) and L. Vetere (PSU) report on behalf of the Swift Team: At 15:00:27 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 090807 (trigger=359378). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 273.744, +10.281 which is RA(J2000) = 18h 14m 59s Dec(J2000) = +10d 16' 51" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT TDRSS light curve does not show anything as is typical for image triggers. The XRT began observing the field at 15:03:00.5 UT, 153.5 seconds after the BAT trigger. Using promptly downlinked data we find a bright, fading, uncatalogued X-ray source with an enhanced position: RA, Dec 273.7438, 10.2665 which is equivalent to: RA(J2000) = 18h 14m 58.51s Dec(J2000) = +10d 15' 59.3" with an uncertainty of 2.4 arcseconds (radius, 90% containment). This location is 52 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 (8.6e+20 cm^-2, Kalberla et al. 2005), with an excess column of 2 (+1.68/-1.50) x 10^21 cm^-2 (90% confidence). The initial flux in the 2.5 s image was 8.55e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of nominal 150 seconds with the White filter starting 163 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 3-sigma upper limit is 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 0.18. Burst Advocate for this burst is J. K. Cannizzo (cannizzo 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.)