GRB 210722A: Swift detection of a burst with a bright optical counterpart

At 20:54:42 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210722A (trigger=1061223). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 27.023, -6.354 which is RA(J2000) = 01h 48m 06s Dec(J2000) = -06d 21' 14" 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 30 sec. The peak count rate was ~1500 counts/sec (15-350 keV), at ~0 sec after the trigger. Using promptly downlinked XRT data we find an uncatalogued X-ray source located at RA, Dec 27.028, -6.345 which is equivalent to: RA(J2000) = +01h 48m 06.84 Dec(J2000) = -06d 20' 42.7" with an uncertainty of 5.0 arcseconds (radius, 90% containment). UVOT took a finding chart exposure of 150 seconds with the White filter starting 94 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 01:48:07.27 = 27.03028 DEC(J2000) = -06:20:49.8 = -6.34718 with a 90%-confidence error radius of about 0.74 arc sec. This position is 9.6 arc sec. from the center of the XRT error circle. The estimated magnitude is 14.37 with a 1-sigma error of about 0.14. No correction has been made for the expected extinction corresponding to E(B-V) of 0.027. Burst Advocate for this burst is P. D'Avanzo (paolo.davanzo 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/)