GCN Circular 18119

P. A. Evans (U Leicester), A. Y. Lien (GSFC/UMBC) and
K. L. Page (U Leicester) report on behalf of the Swift Team:

At 04:06:09 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 150811A (trigger=651882).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 291.371, -15.438 which is 
   RA(J2000) = 19h 25m 29s
   Dec(J2000) = -15d 26' 16"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a weak complex
peak structure with a duration of about 20 sec.  The peak count rate
was ~1000 counts/sec (15-350 keV), at ~3 sec after the trigger. 

The XRT began observing the field at 04:08:30.6 UT, 141.6 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 291.33917, -15.42535 which
is equivalent to:
   RA(J2000)  = 19h 25m 21.40s
   Dec(J2000) = -15d 25' 31.3"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 119 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
http://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 consistent with the Galactic value of 1.54
x 10^21 cm^-2 (Willingale et al. 2013). 

The initial flux in the 2.5 s image was 4.59e-10 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 147 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
0.17. 

Burst Advocate for this burst is P. A. Evans (pae9 AT star.le.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/too.html.)