TITLE: GCN CIRCULAR NUMBER: 19211 SUBJECT: GRB 160321A: Swift detection of a probable burst DATE: 16/03/21 16:11:48 GMT FROM: Scott Barthelmy at NASA/GSFC M. Stamatikos (OSU/NASA/GSFC), S. D. Barthelmy (GSFC), A. A. Breeveld (UCL-MSSL), D. N. Burrows (PSU), S. B. Cenko (GSFC), P. D'Avanzo (INAF-OAB), P.A. Evans (U Leicester), J. A. Kennea (PSU), H. A. Krimm (CRESST/GSFC/USRA), N. P. M. Kuin (UCL-MSSL), L. M. McCauley (PSU), A. Melandri (INAF-OAB), B. Mingo (U Leicester), K. L. Page (U Leicester), T. G. R. Roegiers (PSU), T. Sakamoto (AGU), B. Sbarufatti (INAF-OAB/PSU) and M. H. Siegel (PSU) report on behalf of the Swift Team: At 15:55:27 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 160321A (trigger=680017). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 99.414, +5.715, which is RA(J2000) = 06h 37m 39s Dec(J2000) = +05d 42' 53" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows a few weak peaks with a total duration of about 35 sec. The peak count rate was ~700 counts/sec (15-350 keV), at ~1 sec after the trigger. The XRT began observing the field at 15:57:18.0 UT, 110.9 seconds after the BAT trigger. Using promptly downlinked data we find a fading, uncatalogued X-ray source with an enhanced position: RA, Dec 99.4192, 5.7474 which is equivalent to: RA(J2000) = 06h 37m 40.60s Dec(J2000) = +05d 44' 50.8" with an uncertainty of 2.3 arcseconds (radius, 90% containment). This location is 118 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 7.16 x 10^21 cm^-2 (Willingale et al. 2013). UVOT took a finding chart exposure of 150 seconds with the White filter starting 114 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. The list of sources is typically complete to about 18 mag. No correction has been made for the large, but uncertain extinction expected. Due to the proximity of the source to the Galactic plane (lat = -0.43 degrees) we cannot determine whether this is a Galactic source or a GRB, based on the immediately available data. Burst Advocate for this burst is M. Stamatikos (Michael.Stamatikos-1 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.)