TITLE: GCN CIRCULAR NUMBER: 22265 SUBJECT: GRB 171222A: Swift detection of a burst DATE: 17/12/22 16:42:04 GMT FROM: Judith Racusin at GSFC J. L. Racusin (NASA/GSFC), A. Deich (PSU), S. W. K Emery (UCL-MSSL), J.D. Gropp (PSU), J. A. Kennea (PSU), H. A. Krimm (NSF/USRA), N. P. M. Kuin (UCL-MSSL), A. Y. Lien (GSFC/UMBC), F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester), B. Sbarufatti (PSU) and A. Tohuvavohu (PSU) report on behalf of the Swift Team: At 16:24:59 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 171222A (trigger=799669). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 148.293, +35.622 which is RA(J2000) = 09h 53m 10s Dec(J2000) = +35d 37' 18" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). As is usual with an image trigger, the BAT light curve does not show any significant features. The XRT began observing the field at 16:27:49.5 UT, 169.7 seconds after the BAT trigger. Using promptly downlinked data we find a bright, uncatalogued X-ray source located at RA, Dec 148.27784, 35.62759 which is equivalent to: RA(J2000) = 09h 53m 06.68s Dec(J2000) = +35d 37' 39.3" with an uncertainty of 4.0 arcseconds (radius, 90% containment). This location is 48 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. The initial flux in the 2.5 s image was 6.69e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 250 seconds with the U filter starting 175 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 3-sigma upper limit has been about 19.2 mag. 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 18.0 mag. No correction has been made for the expected extinction corresponding to E(B-V) of 0.01. Burst Advocate for this burst is J. L. Racusin (judith.racusin 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.)