GRB 220118A: Swift detection of a burst with an optical counterpart

At 18:20:38 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 220118A (trigger=1093742). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 192.274, +22.909 which is RA(J2000) = 12h 49m 06s Dec(J2000) = +22d 54' 33" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a single-peaked structure with a duration of about 20 sec. The peak count rate was ~1400 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 18:22:08.3 UT, 90.3 seconds after the BAT trigger. Using promptly downlinked data we find a bright, fading, uncatalogued X-ray source located at RA, Dec 192.26936, 22.91600 which is equivalent to: RA(J2000) = 12h 49m 04.65s Dec(J2000) = +22d 54' 57.6" with an uncertainty of 3.5 arcseconds (radius, 90% containment). This location is 29 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 in excess of the Galactic value (2.25 x 10^20 cm^-2, Willingale et al. 2013), with an excess column of 3.4 (+2.60/-2.24) x 10^21 cm^-2 (90% confidence). The initial flux in the 2.5 s image was 4.57e-10 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 99 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 12:49:04.53 = 192.26888 DEC(J2000) = +22:54:56.2 = 22.91562 with a 90%-confidence error radius of about 0.75 arc sec. This position is 1.8 arc sec. from the center of the XRT error circle. The estimated magnitude is 17.83 with a 1-sigma error of about 0.15. No correction has been made for the expected extinction corresponding to E(B-V) of 0.025. Burst Advocate for this burst is N. J. Klingler (noelklin AT umbc.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/)