[vsnet-grb-info 6803] GRB 081024: Swift detection of a burst

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  8398
SUBJECT: GRB 081024: Swift detection of a burst
DATE:    08/10/24 06:18:55 GMT
FROM:    Neil Gehrels at GSFC  <gehrels at milkyway.gsfc.nasa.gov>

G. Stratta (ASDC), W. H. Baumgartner (GSFC/UMBC),
N. Gehrels (NASA/GSFC), E. A. Hoversten (PSU),
C. B. Markwardt (CRESST/GSFC/UMD), M. Perri (ASDC) and
L. Vetere (PSU) report on behalf of the Swift Team:

At 05:53:08 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 081024 (trigger=332516).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 27.898, +61.346 which is 
   RA(J2000) = 01h 51m 36s
   Dec(J2000) = +61d 20' 47"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a short hard peak with 
a duration of about 0.3 sec at T+0 with a short precursor peak at T-2.5 seconds. 
The peak count rate was ~6,000 counts/sec (15-350 keV), at ~0 sec after
the trigger. 

The XRT began observing the field at 05:54:21.1 UT, 72.3 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source with an enhanced position: RA, Dec 27.8745,
61.3317 which is equivalent to:
   RA(J2000)  = 01h 51m 29.88s
   Dec(J2000) = +61d 19' 54.1"
with an uncertainty of 2.3 arcseconds (radius, 90% containment). This
location is 65 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
7.71e+21 cm^-2 (Kalberla et al. 2005). 

UVOT took a finding chart exposure of nominal 150 seconds with the White
(160-650 nm) filter starting 76 seconds after the BAT trigger. No afterglow
candidate has been found in the initial data products. Image catalog data are
not available at this time. The 8'x8' region for the list of sources generated
on-board covers 100% of the BAT error circle. The list of sources is typically
complete to about 18 mag. No correction has been made for the large, but
uncertain extinction expected. 

This appears to be a short hard GRB with a precursor and a fading X-ray 
afterglow.  We note that the position in galactic coordinates is l=130deg, 
b=-0.7 deg.  It is not impossible that this is a galactic transient or
even an SGR. 

Burst Advocate for this burst is G. Stratta (giulia.stratta 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.)