TITLE: GCN CIRCULAR NUMBER: 23852 SUBJECT: GRB 190204A: Swift detection of a burst with a possible optical counterpart DATE: 19/02/04 06:12:47 GMT FROM: Aaron Tohuvavohu at PSU/Swift A. Y. Lien (GSFC/UMBC), M. G. Bernardini (INAF-OAB), P. D'Avanzo (INAF-OAB), N. J. Klingler (PSU), A. Melandri (INAF-OAB), D. M. Palmer (LANL) and A. Tohuvavohu (PSU) report on behalf of the Neil Gehrels Swift Observatory Team: At 05:46:01 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 190204A (trigger=887579 and 887580). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 351.514, +54.878 which is RA(J2000) = 23h 26m 03s Dec(J2000) = +54d 52' 42" 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 20 sec. The peak count rate was ~28000 counts/sec (15-350 keV), at ~5 sec after the trigger. The XRT began observing the field at 05:47:15.7 UT, 75.0 seconds after the BAT trigger. Using promptly downlinked data we find a bright, uncatalogued X-ray source located at RA, Dec 351.47755, 54.87623 which is equivalent to: RA(J2000) = 23h 25m 54.61s Dec(J2000) = +54d 52' 34.4" with an uncertainty of 3.6 arcseconds (radius, 90% containment). This location is 119 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. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density consistent with the Galactic value of 4.36 x 10^21 cm^-2 (Willingale et al. 2013). The initial flux in the 2.5 s image was 5.72e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 250 seconds with the U filter starting 295 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 23:25:54.58 = 351.47742 DEC(J2000) = +54:52:34.9 = 54.87637 with a 90%-confidence error radius of about 0.63 arc sec. This position is 4.2 arc sec. from the center of the XRT error circle. The estimated magnitude is 17.76 with a 1-sigma error of about 0.15. No correction has been made for the expected extinction corresponding to E(B-V) of 0.38. This GRB resulted in two BAT triggers in the on-board processing (#887579 at 05:46:01 and #887580 at 05:46:06). We adopt the first trigger number and time as T0. 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: /too.html.)