GCN Circular 18104

B. Sbarufatti (INAF-OAB/PSU), S. D. Barthelmy (GSFC),
J. R. Cummings (NASA/UMBC), L. M. Z. Hagen (PSU),
N. P. M. Kuin (UCL-MSSL), F. E. Marshall (NASA/GSFC),
D. M. Palmer (LANL) and M. H. Siegel (PSU) report on behalf of the
Swift Team:

At 22:41:48 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 150801B (trigger=650977).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 82.959, -5.387 which is 
   RA(J2000) = 05h 31m 50s
   Dec(J2000) = -05d 23' 14"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows several overlapping peaks
with a total duration of about 20 sec.  The peak count rate
was ~2600 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 22:43:07.3 UT, 78.8 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source with an enhanced position: RA, Dec 82.9666, -5.3883 which
is equivalent to:
   RA(J2000)  = 05h 31m 51.99s
   Dec(J2000) = -05d 23' 18.0"
with an uncertainty of 2.3 arcseconds (radius, 90% containment). This
location is 27 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. 

A power-law fit to a spectrum formed from promptly downlinked event
data does not constrain the column density. 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 83 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
0.24. 

We note that this event is in the vicinity of the Orion Nebula. 
However, it appears in all other respects to be a typical GRB. 

Burst Advocate for this burst is B. Sbarufatti (boris.sbarufatti AT brera.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/too.html.)