TITLE: GCN CIRCULAR NUMBER: 20846 SUBJECT: GRB 170311A: Swift detection of a burst DATE: 17/03/11 08:25:14 GMT FROM: David Palmer at LANL J. R. Cummings (NASA/UMBC), K. L. Page (U Leicester), D. M. Palmer (LANL) and B. Sbarufatti (INAF-OAB/PSU) report on behalf of the Swift Team: At 08:08:42 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 170311A (trigger=741965). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 280.563, -30.063 which is RA(J2000) = 18h 42m 15s Dec(J2000) = -30d 03' 45" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a complex structure with a duration of about 15 sec. The peak count rate was ~1200 counts/sec (15-350 keV), at ~2 sec after the trigger. The XRT began observing the field at 08:10:28.0 UT, 105.1 seconds after the BAT trigger. Using promptly downlinked data we find a fading, uncatalogued X-ray source with an enhanced position: RA, Dec 280.5893, -30.0458 which is equivalent to: RA(J2000) = 18h 42m 21.43s Dec(J2000) = -30d 02' 44.8" with an uncertainty of 2.1 arcseconds (radius, 90% containment). This location is 102 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 consistent with the Galactic value of 1.30 x 10^21 cm^-2 (Willingale et al. 2013). UVOT took a finding chart exposure of 150 seconds with the White filter starting 109 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. Because of the density of catalogued stars, further analysis is required to report an upper limit for any afterglow in the region. No correction has been made for the expected extinction corresponding to E(B-V) of 0.16. Burst Advocate for this burst is J. R. Cummings (jayc 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.)