TITLE: GCN CIRCULAR NUMBER: 23845 SUBJECT: GRB 190203A: Swift detection of a burst with an optical counterpart DATE: 19/02/03 15:57:07 GMT FROM: Aaron Tohuvavohu at PSU/Swift A. Y. Lien (GSFC/UMBC), S. D. Barthelmy (GSFC), P. D'Avanzo (INAF-OAB), J. A. Kennea (PSU), N. J. Klingler (PSU), M. H. Siegel (PSU) and A. Tohuvavohu (PSU) report on behalf of the Neil Gehrels Swift Observatory Team: At 15:44:08 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 190203A (trigger=887512). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 22.577, +55.738 which is RA(J2000) = 01h 30m 18s Dec(J2000) = +55d 44' 18" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a multi-peaked structure with a duration of about 55 sec. The peak count rate was ~2000 counts/sec (15-350 keV), at ~2 sec after the trigger. The XRT began observing the field at 15:46:04.0 UT, 115.7 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec 22.5984, 55.7211 which is equivalent to: RA(J2000) = 01h 30m 23.62s Dec(J2000) = +55d 43' 16.0" with an uncertainty of 4.8 arcseconds (radius, 90% containment). This location is 74 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 4.46e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 120 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 01:30:23.19 = 22.59662 DEC(J2000) = +55:43:20.7 = 55.72241 with a 90%-confidence error radius of about 0.63 arc sec. This position is 5.9 arc sec. from the center of the XRT error circle. The estimated magnitude is 18.58 with a 1-sigma error of about 0.15. No correction has been made for the expected extinction corresponding to E(B-V) of 0.64. 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.)