TITLE: GCN CIRCULAR NUMBER: 10775 SUBJECT: GRB 100518A: Swift target of opportunity observation of the INTEGRAL burst DATE: 10/05/18 14:35:35 GMT FROM: Scott Barthelmy at NASA/GSFC M. Stamatikos (OSU/NASA/GSFC), S. D. Barthelmy (GSFC), A. P. Beardmore (U Leicester), M. M. Chester (PSU), P. A. Evans (U Leicester), J. M. Gelbord (PSU), J. A. Kennea (PSU), C. Pagani (U Leicester), J. L. Racusin (NASA/GSFC) and L. Vetere (PSU) report on behalf of the Swift Team: Swift began a target of opportunity observation of GRB 100518A on May 18, 2010 at 14:10 UT, approximately 2.6 hours after the burst was detected by INTEGRAL. Swift data for these observations utilize Target ID 20138. The XRT began observing the field at 14:12:42.5 UT, 9.5 ks after the INTEGRAL trigger. Using promptly downlinked data we find an uncatalogued X-ray source located at RA, Dec 304.78875, -24.55459 which is equivalent to: RA(J2000) = 20h 19m 9.30s Dec(J2000) = -24d 33' 16.5" with an uncertainty of 4.1 arcseconds (radius, 90% containment). This location is 42 arcseconds from the INTEGRAL position, within the INTEGRAL error circle. This position may be improved as more data are received; the latest position is available at http://www.swift.ac.uk/sper. We cannot determine whether the source is fading at the present time. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density consistent with the Galactic value of 6.4e+20 cm^-2 (Kalberla et al. 2005). UVOT took a finding chart exposure of 150 seconds with the White filter starting 9551 seconds after the INTEGRAL 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 typical 3-sigma upper limit has been about 19.6 mag. The 8'x8' region for the list of sources generated on-board 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 M. Stamatikos (Michael.Stamatikos-1 AT 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.)