TITLE: GCN CIRCULAR NUMBER: 26678 SUBJECT: GRB 200109A: Swift detection of a burst DATE: 20/01/09 02:00:28 GMT FROM: David Palmer at LANL A. Melandri (INAF-OAB), M. G. Bernardini (INAF-OAB), P. D'Avanzo (INAF-OAB), J.D. Gropp (PSU), N. J. Klingler (PSU), S Laha (GSFC/UMBC/CRESST), D. M. Palmer (LANL), M. H. Siegel (PSU), K. K. Simpson (PSU) and A. Tohuvavohu (U Toronto) report on behalf of the Neil Gehrels Swift Observatory Team: At 01:46:16 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 200109A (trigger=948361). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 307.171, +53.003 which is RA(J2000) = 20h 28m 41s Dec(J2000) = +53d 00' 12" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a complex structure with a duration of 70 sec. The peak count rate was ~1600 counts/sec (15-350 keV), at ~2 sec after the trigger. The XRT began observing the field at 01:47:59.1 UT, 102.9 seconds after the BAT trigger. Using promptly downlinked data we find a bright, fading, uncatalogued X-ray source with an enhanced position: RA, Dec 307.1258, 52.9932 which is equivalent to: RA(J2000) = 20h 28m 30.19s Dec(J2000) = +52d 59' 35.5" with an uncertainty of 2.2 arcseconds (radius, 90% containment). This location is 104 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 3.98 x 10^21 cm^-2 (Willingale et al. 2013). The initial flux in the 2.5 s image was 1.12e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 109 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. Because of the density of catalogued stars, further analysis is required to report an upper limit for any afterglow in the sub-image. 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 mag. No correction has been made for the expected extinction corresponding to E(B-V) of 0.29. Burst Advocate for this burst is A. Melandri (andrea.melandri AT brera.inaf.it). 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/)