TITLE: GCN CIRCULAR NUMBER: 28337 SUBJECT: GRB 200901B: Swift detection of a burst DATE: 20/09/01 07:25:54 GMT FROM: Boris Sbarufatti at PSU B. Sbarufatti (PSU), P. A. Evans (U Leicester), A. Y. Lien (GSFC/UMBC), K. L. Page (U Leicester) and A. Tohuvavohu (U Toronto) report on behalf of the Neil Gehrels Swift Observatory Team: At 07:09:26 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 200901B (trigger=994165). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 217.879, -38.716 which is RA(J2000) = 14h 31m 31s Dec(J2000) = -38d 42' 58" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). As is usual with an image trigger, the available BAT light curve shows no significant structure. The XRT began observing the field at 07:12:16.1 UT, 169.7 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec 217.8572, -38.7117 which is equivalent to: RA(J2000) = 14h 31m 25.73s Dec(J2000) = -38d 42' 42.1" with an uncertainty of 5.5 arcseconds (radius, 90% containment). This location is 63 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 2.5 s image was 1.11e-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 0.09. Burst Advocate for this burst is B. Sbarufatti (bxs60 AT 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/)