TITLE: GCN CIRCULAR NUMBER: 22442 SUBJECT: GRB 180224A: Swift detection of a burst with an optical counterpart DATE: 18/02/24 22:32:57 GMT FROM: Scott Barthelmy at NASA/GSFC A. Y. Lien (GSFC/UMBC), S. D. Barthelmy (GSFC), J.D. Gropp (PSU), H. A. Krimm (NSF/USRA), S. J. LaPorte (PSU), F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester), D. M. Palmer (LANL) and A. Tohuvavohu (PSU) report on behalf of the Neil Gehrels Swift Observatory Team: At 22:21:07 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180224A (trigger=811561). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 202.661, +38.088 which is RA(J2000) = 13h 30m 39s Dec(J2000) = +38d 05' 17" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows a single FRED peak with a duration of about 15 sec. The peak count rate was ~2200 counts/sec (15-350 keV), at ~1 sec after the trigger. The XRT began observing the field at 22:22:28.1 UT, 80.5 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued X-ray source located at RA, Dec 202.68324, 38.07881 which is equivalent to: RA(J2000) = 13h 30m 43.98s Dec(J2000) = +38d 04' 43.7" with an uncertainty of 3.8 arcseconds (radius, 90% containment). This location is 71 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. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density consistent with the Galactic value of 8.24 x 10^19 cm^-2 (Willingale et al. 2013). UVOT took a finding chart exposure of 150 seconds with the White filter starting 85 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 13:30:44.09 = 202.68370 DEC(J2000) = +38:04:44.6 = 38.07906 with a 90%-confidence error radius of about 0.63 arc sec. This position is 0.9 arc sec. from the center of the XRT error circle. The estimated magnitude is 18.94 with a 1-sigma error of about 0.15. No correction has been made for the expected extinction corresponding to E(B-V) of 0.01. Burst Advocate for this burst is A. Y. Lien (amy.y.lien 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.)