TITLE: GCN CIRCULAR NUMBER: 27173 SUBJECT: GRB 200224A: Swift detection of a burst DATE: 20/02/24 03:41:23 GMT FROM: David Palmer at LANL T. N. Ukwatta (LANL), E. Ambrosi (INAF-IASFPA), A. D'Ai (INAF-IASFPA), V. D'Elia (SSDC), J. A. Kennea (PSU), N. J. Klingler (PSU), S. Laha (GSFC/UMBC/CRESST), A. Y. Lien (GSFC/UMBC) and D. M. Palmer (LANL) report on behalf of the Neil Gehrels Swift Observatory Team: At 03:24:49 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 200224A (trigger=958141). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 248.794, +41.675 which is RA(J2000) = 16h 35m 11s Dec(J2000) = +41d 40' 31" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a complex structure with a duration of about 30 sec. The peak count rate was ~1000 counts/sec (15-350 keV), at ~3 sec after the trigger. The XRT began observing the field at 03:26:26.9 UT, 97.7 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued X-ray source with an enhanced position: RA, Dec 248.7668, 41.6123 which is equivalent to: RA(J2000) = 16h 35m 04.04s Dec(J2000) = +41d 36' 44.1" with an uncertainty of 2.9 arcseconds (radius, 90% containment). This location is 237 arcseconds from the BAT onboard position. This position may be improved as more data are received; the latest position is available at https://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 8.91 x 10^19 cm^-2 (Willingale et al. 2013). UVOT results will be reported later. Burst Advocate for this burst is T. N. Ukwatta (tilan.ukwatta 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/) [GCN OPS NOTE(24feb20): This is the second Circular in the sequence of 8 circulars which were all labeled A because of the sequence assigner being offline. This circular was correctly labelled A.]