GCN Circular 15349

A. Melandri (INAF-OAB), S. D. Barthelmy (GSFC), D. N. Burrows (PSU),
P. D\'Avanzo (INAF-OAB), P. A. Evans (U Leicester),
N. Gehrels (NASA/GSFC), S. T. Holland (STScI), J. A. Kennea (PSU),
A. Y. Lien (NASA/GSFC/ORAU), F. E. Marshall (NASA/GSFC),
A. Maselli (INAF-IASFPA), C. Pagani (U Leicester),
K. L. Page (U Leicester), P. Romano (INAF-IASFPA), M. H. Siegel (PSU)
and R. L. C. Starling (U Leicester) report on behalf of the Swift
Team:

At 12:47:48 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 131018A (trigger=574935).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 98.462, -19.882 which is 
   RA(J2000) = 06h 33m 51s
   Dec(J2000) = -19d 52' 54"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a broad single peak
structure with a duration of about 100 sec.  The peak count rate
was ~500 counts/sec (15-350 keV), at ~56 sec after the trigger. 

The XRT began observing the field at 12:49:41.4 UT, 112.7 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source with an enhanced position: RA, Dec
98.4716, -19.8961 which is equivalent to:
   RA(J2000)  = 06h 33m 53.19s
   Dec(J2000) = -19d 53' 45.9"
with an uncertainty of 2.3 arcseconds (radius, 90% containment). This
location is 60 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 2.03
x 10^21 cm^-2 (Kalberla et al. 2005). 

The initial flux in the 2.5 s image was 4.45e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 120 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
0.26. 

Burst Advocate for this burst is A. Melandri (andrea.melandri 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.)