TITLE: GCN CIRCULAR NUMBER: 9386 SUBJECT: GRB 090518: Swift detection of a burst DATE: 09/05/18 02:09:07 GMT FROM: Scott Barthelmy at NASA/GSFC P. A. Evans (U Leicester), S. D. Barthelmy (GSFC), D. N. Burrows (PSU), J. A. Kennea (PSU), F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester), D. M. Palmer (LANL), A. Rowlinson (U Leicester), M. H. Siegel (PSU), T. N. Ukwatta (GSFC/GWU) and L. Vetere (PSU) report on behalf of the Swift Team: At 01:54:44 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 090518 (trigger=352420). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 119.933, +0.768 which is RA(J2000) = 07h 59m 44s Dec(J2000) = +00d 46' 04" 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 5 sec. The peak count rate was ~4000 counts/sec (15-350 keV), at ~1 sec after the trigger. The XRT began observing the field at 01:55:57.8 UT, 73.5 seconds after the BAT trigger. Using promptly downlinked data we find a fading, uncatalogued X-ray source located at RA, Dec 119.95519, 0.75892 which is equivalent to: RA(J2000) = 07h 59m 49.25s Dec(J2000) = +00d 45' 32.1" with an uncertainty of 3.7 arcseconds (radius, 90% containment). This location is 86 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 (4.65e+20 cm^-2, Kalberla et al. 2005), with an excess column of 4.9 (+3.56/-2.94) x 10^21 cm^-2 (90% confidence). UVOT took a finding chart exposure of 150 seconds with the White filter starting 77 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 none of the XRT error circle. 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 0.04. 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.)