TITLE: GCN CIRCULAR NUMBER: 21821 SUBJECT: GRB 170906A: Swift detection of a burst with an optical afterglow DATE: 17/09/06 00:55:00 GMT FROM: David Burrows at PSU/Swift M. H. Siegel (PSU), D. N. Burrows (PSU), J. R. Cummings (NASA/UMBC), C. Gronwall (PSU), J. A. Kennea (PSU), A. Y. Lien (GSFC/UMBC), F. E. Marshall (NASA/GSFC), D. M. Palmer (LANL), B. Sbarufatti (PSU) and A. Tohuvavohu (PSU) report on behalf of the Swift Team: At 00:43:11 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 170906A (trigger=770957). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 203.992, -47.125 which is RA(J2000) = 13h 35m 58s Dec(J2000) = -47d 07' 28" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a multiply-peaked structure with a duration of about 120 sec. The peak count rate was ~16000 counts/sec (15-350 keV), at ~30 sec after the trigger. The XRT began observing the field at 00:44:18.9 UT, 67.3 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec 203.9546, -47.1007 which is equivalent to: RA(J2000) = +13h 35m 49.10s Dec(J2000) = -47d 06' 02.5" with an uncertainty of 5.2 arcseconds (radius, 90% containment). This location is 126 arcseconds from the BAT onboard position, within the BAT error circle. No event data are yet available to determine the column density using X-ray spectroscopy. The initial flux in the 0.1 s image was 4.12e-08 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 77 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 13:35:49.19 = 203.95497 DEC(J2000) = -47:06:01.9 = -47.10054 with a 90%-confidence error radius of about 0.61 arc sec. This position is 1.1 arc sec. from the center of the XRT error circle. The estimated magnitude is 16.59 with a 1-sigma error of about 0.14. No correction has been made for the expected extinction corresponding to E(B-V) of 0.14. Burst Advocate for this burst is M. H. Siegel (siegel 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.)