GCN Circular 18315

V. D'Elia (ASDC), N. Gehrels (NASA/GSFC), L. Izzo (URoma/ICRA),
F. E. Marshall (NASA/GSFC), D. M. Palmer (LANL) and M. H. Siegel (PSU)
report on behalf of the Swift Team:

At 21:18:24 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 150915A (trigger=655721).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 319.731, -34.854 which is 
   RA(J2000) = 21h 18m 55s
   Dec(J2000) = -34d 51' 12"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As is usual for an image trigger, there
is no obvious variation in the immediately available BAT lightcurve. 

The XRT began observing the field at 21:20:33.5 UT, 128.7 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source located at RA, Dec 319.65874,
-34.91306 which is equivalent to:
   RA(J2000)  = 21h 18m 38.10s
   Dec(J2000) = -34d 54' 47.0"
with an uncertainty of 3.6 arcseconds (radius, 90% containment). This
location is 301 arcseconds from the BAT onboard position. This position
may be improved as more data are received; the latest position is
available at http://www.swift.ac.uk/sper. 

Although this position is outside the nominal 90% BAT error circle, 
it is probably the GRB. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density in excess of the Galactic value (8.65 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 2.7
(+2.23/-1.97) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 2.5 s image was 5.57e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 133 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 19% 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
0.10. 

Burst Advocate for this burst is V. D'Elia (delia AT asdc.asi.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.)