TITLE: GCN CIRCULAR NUMBER: 19394 SUBJECT: GRB 160506A: Swift detection of a burst DATE: 16/05/06 03:50:19 GMT FROM: Scott Barthelmy at NASA/GSFC L. M. Z. Hagen (PSU), S. D. Barthelmy (GSFC), H. A. Krimm (CRESST/GSFC/USRA), F. E. Marshall (NASA/GSFC), D. M. Palmer (LANL) and M. H. Siegel (PSU) report on behalf of the Swift Team: At 03:29:15 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 160506A (trigger=685245). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 265.774, -46.117 which is RA(J2000) = 17h 43m 06s Dec(J2000) = -46d 07' 00" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows broad peak with a total duration of about 50 sec. The peak count rate was ~1200 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 03:30:50.6 UT, 94.8 seconds after the BAT trigger. Using promptly downlinked data we find a bright, fading, uncatalogued X-ray source with an enhanced position: RA, Dec 265.8484, -46.1340 which is equivalent to: RA(J2000) = 17h 43m 23.60s Dec(J2000) = -46d 08' 02.3" with an uncertainty of 2.0 arcseconds (radius, 90% containment). This location is 195 arcseconds from the BAT onboard position, outside 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 (2.67 x 10^21 cm^-2, Willingale et al. 2013), with an excess column of 4.8 (+2.85/-2.47) x 10^21 cm^-2 (90% confidence). The initial flux in the 2.5 s image was 2.68e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 250 seconds with the U filter starting 101 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 has been 19.6 mag. The 8'x8' region for the list of sources generated on-board covers 100% of the XRT error circle. The typical upper limit has been 18.0 mag. No correction has been made for the expected extinction corresponding to E(B-V) of 0.26. Burst Advocate for this burst is L. M. Z. Hagen (lea.zernow.hagen AT gmail.com). 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.)