TITLE: GCN CIRCULAR NUMBER: 19365 SUBJECT: GRB 160501A: Swift detection of a burst DATE: 16/05/01 00:58:55 GMT FROM: David Palmer at LANL J. R. Cummings (NASA/UMBC), A. P. Beardmore (U Leicester), J. A. Kennea (PSU), F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester), D. M. Palmer (LANL), T. Sakamoto (AGU), M. H. Siegel (PSU) and T. N. Ukwatta (LANL) report on behalf of the Swift Team: At 00:40:31 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 160501A (trigger=684679). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 286.377, -17.240 which is RA(J2000) = 19h 05m 30s Dec(J2000) = -17d 14' 22" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). As is usual for an image trigger, there is no clear variation visible in the immediately-available lightcurve data. The XRT began observing the field at 00:42:39.8 UT, 128.1 seconds after the BAT trigger. Using promptly downlinked data we find a bright, fading, uncatalogued X-ray source located at RA, Dec 286.38344, -17.24088 which is equivalent to: RA(J2000) = 19h 05m 32.03s Dec(J2000) = -17d 14' 27.2" with an uncertainty of 3.7 arcseconds (radius, 90% containment). This location is 22 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 (1.62 x 10^21 cm^-2, Willingale et al. 2013), with an excess column of 2.7 (+3.06/-2.65) x 10^21 cm^-2 (90% confidence). The initial flux in the 2.5 s image was 1.08e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 136 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 upper limit is 19.6 magnitude. The 8'x8' region for the list of sources generated on-board covers 100% of the XRT error circle. The typical upper limit is 18.0. No correction has been made for the expected extinction corresponding to E(B-V) of 0.18. 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.)