TITLE: GCN CIRCULAR NUMBER: 20530 SUBJECT: GRB 170126A: Swift detection of a burst with an optical counterpart DATE: 17/01/26 11:44:29 GMT FROM: Scott Barthelmy at NASA/GSFC D. Kocevski (NASA/MSFC), S. D. Barthelmy (GSFC), A. A. Breeveld (UCL-MSSL), D. N. Burrows (PSU), A. D'Ai (INAF-IASFPA), S. W. K Emery (UCL-MSSL), J. A. Kennea (PSU), F. E. Marshall (NASA/GSFC) and M. H. Siegel (PSU) report on behalf of the Swift Team: At 11:30:42 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 170126A (trigger=735188). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 263.579, -64.757 which is RA(J2000) = 17h 34m 19s Dec(J2000) = -64d 45' 26" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows a single FRED-like peak structure with a duration of about 15 sec. The peak count rate was ~3000 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 11:32:09.4 UT, 87.1 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued X-ray source located at RA, Dec 263.63341, -64.76046 which is equivalent to: RA(J2000) = 17h 34m 32.02s Dec(J2000) = -64d 45' 37.7" with an uncertainty of 4.2 arcseconds (radius, 90% containment). This location is 84 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. We cannot determine whether the source is fading at the present time. No spectrum from the promptly downlinked event data is yet available to determine the column density. UVOT took a finding chart exposure of 131 seconds with the White filter starting 90 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 17:34:31.95 = 263.63311 DEC(J2000) = -64:45:37.0 = -64.76027 with a 90%-confidence error radius of about 0.62 arc sec. This position is 0.9 arc sec. from the center of the XRT error circle. The estimated magnitude is 18.12 with a 1-sigma error of about 0.15. No correction has been made for the expected extinction corresponding to E(B-V) of 0.10. Burst Advocate for this burst is D. Kocevski (dankocevski 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.)