[vsnet-grb-info 2033] GRB 050824: XRT refined analysis and possible afterglow detection

[GCN Circulars]

TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  3872
SUBJECT: GRB 050824: XRT refined analysis and possible afterglow detection
DATE:    05/08/25 05:28:52 GMT
FROM:    Sergio Campana at INAF-OAB  <campana at merate.mi.astro.it>

S. Campana (INAF-OAB), V. Mangano (INAF-IASF Palermo), G. Chincarini 
(INAF-OAB &
Uni. Bicocca), G. Tagliaferri (INAF-OAB), D.N. Burrows (PSU), L. 
Cominsky (Sonoma
State U.) and J. Norris (GSFC) report on behalf of the Swift XRT team:

We have analyzed the Swift XRT data from the first orbit observation of 
GRB 050824
(Campana et al., GCN3866). The new refined coordinates are:

RA(J2000)  = 0h48m56.05s
Dec(J2000) = +22:36:28.5

This position is 59 arcseconds from the BAT position given in GCN3866 
(and 83
arcsec from the refined BAT position, Krimm et al. GCN3871) and 3.4 
arcsec from
the suggested optical counterpart (Gorosabel et al., GCN3865).
We estimated an uncertainty of 6.8 arcseconds radius (90% confidence 
level, throughout
the circular). A second fainter X-ray source is located 2.5 arcmin from 
the suggested
afterglow position (and further away from the BAT refined position).

The 0.5-10 keV light curve XRT started 2005-08-25 UT00:53:45, 6089 
seconds after
the BAT trigger, following an uploaded TOO since Swift did not 
automatically
slewed on source (due to the Moon constraint). At the moment we have 
data from
one Swift orbit. During the first orbit we accumulated 1346 seconds of 
good data.
The afterglow candidate is detected with a count rate of (4.1+/-1.1)x10^-2
counts/second (55 total counts). We found no indication of count rate decay.

The number of counts is too low to perform a full spectral analysis.
Fixing the column density to the Galactic value (3.6x10^20 cm-2) a 
preliminary
spectral fit to the PC data gives a spectral power law photon index of 
2.6+/-0.6
in the 0.5-10 keV band. The average unabsorbed 0.5-10 keV flux is about
(1.2+/-0.7)E-12 erg cm-2 s-1.