[vsnet-grb-info 14057] GRB 131024B: Swift detection of a burst

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  15371
SUBJECT: GRB 131024B: Swift detection of a burst
DATE:    13/10/24 21:56:41 GMT
FROM:    Hans Krimm at NASA-GSFC  <krimm at milkyway.gsfc.nasa.gov>

H. A. Krimm (CRESST/GSFC/USRA), M. M. Chester (PSU),
S. T. Holland (STScI), J. A. Kennea (PSU), V. Mangano (PSU),
C. B. Markwardt (NASA/GSFC), F. E. Marshall (NASA/GSFC),
A. Maselli (INAF-IASFPA), J. L. Racusin (NASA/GSFC),
B. Sbarufatti (INAF-OAB/PSU), C. A. Swenson (PSU),
E. Troja (NASA/GSFC/UMCP) and B.-B. Zhang (UAH) report on behalf of
the Swift Team:

At 21:35:31 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 131024A (trigger=575754).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 144.403, +44.240 which is 
   RA(J2000) = 09h 37m 37s
   Dec(J2000) = +44d 14' 23"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a multiple-peak
structure with a duration of about 30 sec.  The peak count rate
was ~1000 counts/sec (15-350 keV), at ~2 sec after the trigger. 

The XRT began observing the field at 21:37:21.8 UT, 110.5 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source with an enhanced position: RA, Dec
144.5032, 44.2715 which is equivalent to:
   RA(J2000)  = 09h 38m 00.76s
   Dec(J2000) = +44d 16' 17.6"
with an uncertainty of 2.1 arcseconds (radius, 90% containment). This
location is 282 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. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density in excess of the Galactic value (1.27 x
10^20 cm^-2, Kalberla et al. 2005), with an excess column of 2.6
(+1.51/-1.36) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 2.5 s image was 5.91e-10 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 250 seconds with the U filter starting
278 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 19.2 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 18.0 mag. No
correction has been made for the expected extinction corresponding to E(B-V) of
0.02. 

Burst Advocate for this burst is H. A. Krimm (krimm AT milkyway.gsfc.nasa.gov). 
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.)