TITLE:   GCN CIRCULAR
NUMBER:  11277
SUBJECT: GRB 100915A: Swift detection of a likely burst
DATE:    10/09/15 01:51:42 GMT
FROM:    Jamie A. Kennea at PSU/Swift-XRT  <kennea@astro.psu.edu>

O. M. Littlejohns (U Leicester), S. D. Barthelmy (GSFC),
D. N. Burrows (PSU), J. R. Cummings (NASA/UMBC),
N. Gehrels (NASA/GSFC), J. M. Gelbord (PSU),
S. T. Holland (CRESST/USRA/GSFC), J. A. Kennea (PSU),
C. B. Markwardt (NASA/GSFC), F. E. Marshall (NASA/GSFC),
K. L. Page (U Leicester), A. Rowlinson (U Leicester),
M. H. Siegel (PSU) and M. Stamatikos (OSU/NASA/GSFC) report on behalf
of the Swift Team:

At 01:31:05 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 100915A (trigger=434178).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is RA, Dec 315.648, +65.667 which is 
   RA(J2000) = 21h 02m 36s
   Dec(J2000) = +65d 39' 60"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  This is a BAT image-only trigger with significance
reported on-board of 8.67 sigma.  Ground processing of the BAT scaled map
image indicates a slightly weaker, 6.4 sigma excess.  As is typical for 
such triggers, there is nothing obvious in the raw light curve.  We will 
need to wait for event-by-event data on the ground to clarify the nature 
of the light curve. 

The XRT began observing the field at 01:33:17.7 UT, 131.9 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source with an enhanced position: RA, Dec
315.6937, 65.6731 which is equivalent to:
   RA(J2000)  = 21h 02m 46.50s
   Dec(J2000) = +65d 40' 23.0"
with an uncertainty of 2.2 arcseconds (radius, 90% containment). This
location is 71 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 gives a column density consistent with the Galactic value of 2.27
x 10^21 cm^-2 (Kalberla et al. 2005). 

The initial flux in the 2.5 s image was 5.89e-10 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 140 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
0.51. 

Burst Advocate for this burst is O. M. Littlejohns (oml2 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.)