TITLE: GCN CIRCULAR NUMBER: 22468 SUBJECT: GRB 180311A: Swift detection of a burst DATE: 18/03/11 18:18:00 GMT FROM: David Palmer at LANL K. L. Page (U Leicester), A. Deich (PSU), S. W. K Emery (UCL-MSSL), J. A. Kennea (PSU), F. E. Marshall (NASA/GSFC), D. M. Palmer (LANL), M. H. Siegel (PSU) and A. Tohuvavohu (PSU) report on behalf of the Neil Gehrels Swift Observatory Team: At 18:03:49 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180311A (trigger=813721). Swift slewed to the burst after a brief delay to clear the Earth limb observing contraint. The BAT on-board calculated location is RA, Dec 3.433, -54.479 which is RA(J2000) = 00h 13m 44s Dec(J2000) = -54d 28' 43" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a complex peak structure with a duration of about 30 sec. The peak count rate was ~1000 counts/sec (15-350 keV), at ~4 sec after the trigger. The XRT began observing the field at 18:08:24.5 UT, 275.3 seconds after the BAT trigger. Using promptly downlinked data we find a fading, uncatalogued X-ray source located at RA, Dec 3.39027, -54.49162 which is equivalent to: RA(J2000) = 00h 13m 33.66s Dec(J2000) = -54d 29' 29.8" with an uncertainty of 3.7 arcseconds (radius, 90% containment). This location is 100 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 (1.59 x 10^20 cm^-2, Willingale et al. 2013), with an excess column of 4.9 (+3.32/-2.86) x 10^21 cm^-2 (90% confidence). UVOT took a finding chart exposure of 150 seconds with the White filter starting 279 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 26% 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 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 0.01. Burst Advocate for this burst is K. L. Page (klp5 AT leicester.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.)