GRB 240811A: Swift detection of a burst

At 14:07:33 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 240811A (trigger=1248132). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 226.373, +28.854 which is RA(J2000) = 15h 05m 30s Dec(J2000) = +28d 51' 15" 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 50 sec. The peak count rate was ~3000 counts/sec (15-350 keV), at ~11 sec after the trigger. The XRT began observing the field at 14:09:08.3 UT, 95.1 seconds after the BAT trigger. Using promptly downlinked data we find a bright, fading, uncatalogued X-ray source with an enhanced position: RA, Dec 226.36218, 28.89573 which is equivalent to: RA(J2000) = 15h 05m 26.92s Dec(J2000) = +28d 53' 44.6" with an uncertainty of 1.9 arcseconds (radius, 90% containment). This location is 154 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 (1.98 x 10^20 cm^-2, Willingale et al. 2013), with an excess column of 3.5 (+3.65/-3.05) x 10^21 cm^-2 (90% confidence). The initial flux in the 2.5 s image was 7.46e-10 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 102 seconds after the BAT trigger. No credible afterglow candidate has been found in the initial data products. The 2.7'x2.7' sub-image covers 100% of the XRT error circle. The typical 3-sigma upper limit has been about 19.6 mag. The 8'x8' region for the list of sources generated on-board covers 100% of the XRT error circle. The list of sources is typically complete to about 18 mag. No correction has been made for the expected extinction corresponding to E(B-V) of 0.023. Burst Advocate for this burst is K. L. Page (klp5 AT leicester.ac.uk). 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/)