GRB 241127A: Swift detection of a burst with an optical counterpart

At 21:43:16 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 241127A (trigger=1270788). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 327.922, -56.685 which is RA(J2000) = 21h 51m 41s Dec(J2000) = -56d 41' 06" 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 10 sec. The peak count rate was ~2000 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 21:44:22.8 UT, 66.6 seconds after the BAT trigger. Using promptly downlinked data we find a bright, uncatalogued X-ray source located at RA, Dec 327.88355, -56.66459 which is equivalent to: RA(J2000) = 21h 51m 32.05s Dec(J2000) = -56d 39' 52.5" with an uncertainty of 4.0 arcseconds (radius, 90% containment). This location is 105 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. We cannot determine whether the source is fading at the present time. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density in excess of the Galactic value (2.68 x 10^20 cm^-2, Willingale et al. 2013), with an excess column of 6.1 (+6.36/-4.83) x 10^21 cm^-2 (90% confidence). The initial flux in the 2.5 s image was 2.12e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 484 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 21:51:32.32 = 327.88467 DEC(J2000) = -56:39:55.1 = -56.66531 with a 90%-confidence error radius of about 0.61 arc sec. This position is 2.1 arc sec. from the center of the XRT error circle. The estimated magnitude is 17.14 with a 1-sigma error of about 0.14. No correction has been made for the expected extinction corresponding to E(B-V) of 0.034. Burst Advocate for this burst is C. Salvaggio (chiara.salvaggio AT 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/)