GCN Circular 37136

K. L. Page (U Leicester) and R. Gupta (NASA GSFC) report on behalf of
the Neil Gehrels Swift Observatory Team:

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/)