TITLE: GCN CIRCULAR NUMBER: 21861 SUBJECT: GRB 170912A: Swift detection of a burst DATE: 17/09/12 01:45:00 GMT FROM: David Palmer at LANL A. Tohuvavohu (PSU), D. N. Burrows (PSU), C. Gronwall (PSU), A. Y. Lien (GSFC/UMBC), D. M. Palmer (LANL), B. Sbarufatti (PSU) and M. H. Siegel (PSU) report on behalf of the Swift Team: At 01:34:16 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 170912A (trigger=772006). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 167.372, -54.360 which is RA(J2000) = 11h 09m 29s Dec(J2000) = -54d 21' 34" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a single-peak structure with a duration of about 20 sec. The peak count rate was ~1200 counts/sec (15-350 keV), at ~1 sec after the trigger. The XRT began observing the field at 01:36:15.6 UT, 118.7 seconds after the BAT trigger. Using promptly downlinked data we find a fading, uncatalogued X-ray source located at RA, Dec 167.37341, -54.32690 which is equivalent to: RA(J2000) = 11h 09m 29.62s Dec(J2000) = -54d 19' 36.8" with an uncertainty of 3.8 arcseconds (radius, 90% containment). This location is 119 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 3.14 x 10^21 cm^-2 (Willingale et al. 2013). UVOT took a finding chart exposure of 150 seconds with the White filter starting 122 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 expected extinction corresponding to E(B-V) of 0.46. Burst Advocate for this burst is A. Tohuvavohu (aaronb AT swift.psu.edu). 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.)