TITLE: GCN CIRCULAR NUMBER: 26895 SUBJECT: GRB 200127A: Swift detection of a burst DATE: 20/01/27 15:04:17 GMT FROM: Caryl Gronwall at PSU/Swift-UVOT M. J. Moss (GWU), A. P. Beardmore (U Leicester), P. A. Evans (U Leicester), C. Gronwall (PSU), J.D. Gropp (PSU), J. A. Kennea (PSU), N. J. Klingler (PSU), A. Y. Lien (GSFC/UMBC), F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester), M. H. Siegel (PSU) and A. Tohuvavohu (U Toronto) report on behalf of the Neil Gehrels Swift Observatory Team: At 14:44:31 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 200127A (trigger=952791). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 295.204, +44.910 which is RA(J2000) = 19h 40m 49s Dec(J2000) = +44d 54' 35" 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 14:46:52.1 UT, 140.5 seconds after the BAT trigger. Using promptly downlinked data we find a bright, fading, uncatalogued X-ray source located at RA, Dec 295.19447, 44.90030 which is equivalent to: RA(J2000) = 19h 40m 46.67s Dec(J2000) = +44d 54' 01.1" with an uncertainty of 3.5 arcseconds (radius, 90% containment). This location is 42 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 https://www.swift.ac.uk/sper. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density consistent with the Galactic value of 2.17 x 10^21 cm^-2 (Willingale et al. 2013). The initial flux in the 2.5 s image was 9.40e-10 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 250 seconds with the U filter starting 309 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 0.16. Burst Advocate for this burst is M. J. Moss (mikejmoss3 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/)