TITLE: GCN CIRCULAR NUMBER: 12279 SUBJECT: GRB 110818A: Swift detection of a burst DATE: 11/08/18 21:00:09 GMT FROM: Scott Barthelmy at NASA/GSFC C. B. Markwardt (NASA/GSFC), S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC), A. P. Beardmore (U Leicester), D. N. Burrows (PSU), J. R. Cummings (NASA/UMBC), S. T. Holland (CRESST/USRA/GSFC), E. A. Hoversten (PSU), J. A. Kennea (PSU), H. A. Krimm (CRESST/GSFC/USRA), N. P. M. Kuin (UCL-MSSL), O. M. Littlejohns (U Leicester), F. E. Marshall (NASA/GSFC), A. Melandri (INAF-OAB), P. T. O'Brien (U Leicester), K. L. Page (U Leicester), T. A. Pritchard (PSU), A. Rowlinson (U Leicester), T. Sakamoto (NASA/UMBC), C. J. Saxton (UCL-MSSL), M. H. Siegel (PSU) and R. L. C. Starling (U Leicester) report on behalf of the Swift Team: At 20:37:49 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 110818A (trigger=500914). Because of an Earth-limb observing constraint, the slew to the burst was delayed by ~4 min. The BAT on-board calculated location is RA, Dec 317.384, -63.970 which is RA(J2000) = 21h 09m 32s Dec(J2000) = -63d 58' 10" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows several peaks with a total duration of about 60 sec. The peak count rate was ~2000 counts/sec (15-350 keV), at ~7 sec after the trigger. The XRT began observing the field at 20:44:09.7 UT, 380.4 seconds after the BAT trigger. Using promptly downlinked data we find a fading, uncatalogued X-ray source with an enhanced position: RA, Dec 317.3382, -63.9814 which is equivalent to: RA(J2000) = 21h 09m 21.17s Dec(J2000) = -63d 58' 53.1" with an uncertainty of 2.1 arcseconds (radius, 90% containment). This location is 83 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 (2.62 x 10^20 cm^-2, Kalberla et al. 2005), with an excess column of 2 (+1.14/-1.05) x 10^21 cm^-2 (90% confidence). UVOT took a finding chart exposure of 150 seconds with the U filter starting 856 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 typical 3-sigma upper limit has been about 19.2 mag. The 8'x8' region for the list of sources generated on-board covers 100% of the XRT error circle. No correction has been made for the expected extinction corresponding to E(B-V) of 0.04. Burst Advocate for this burst is C. B. Markwardt (Craig.Markwardt 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.)