[vsnet-grb-info 6332] GRB 080707: Swift XRT refined analysis

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  7951
SUBJECT: GRB 080707: Swift XRT refined analysis
DATE:    08/07/07 20:35:47 GMT
FROM:    Rhaana Starling at U of Leicester  <rlcs1 at star.le.ac.uk>

R.L.C. Starling (U. Leicester) and P. Schady (MSSL-UCL) report, on behalf 
of the Swift XRT team:

We have analysed the first 4 orbits of Swift XRT data (from T0+64s to 
T0+17.8 ks) for GRB 080707 (trigger=316204, Schady et al., GCN Circ. 
7947).

Using 1482 seconds of overlapping XRT Photon Counting (PC) mode and UVOT 
data, we find an updated astrometrically corrected X-ray position (using 
the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 
catalogue): RA, Dec = 32.61860, 33.10983 deg which is equivalent to:
 
RA(J2000) = 02 10 28.46 
Dec(J2000)= +33 06 35.4 

with an uncertainty of 1.5 arcseconds (radius, 90% confidence). This 
position lies 1.35 arcseconds from the optical counterpart reported in 
Clemens et al. (GCN Circ. 7948).

The light curve can be fit by an initial steep decay followed by a shallow 
plateau phase. The final orbit of data so far collected may indicate a 
further break to a steeper slope, but this is not required by the fit. 
A  singly broken power law gives alpha1=5.6+1/-0.8, Tbk=145+17/-13 and 
alpha2=0.35+0.06/-0.07. 
A doubly broken power law model gives the following parameters: 
alpha1=5.5+0.9/-0.8, Tbk1=151+6/-15s, alpha2=0.2+0.1/-0.5, 
Tbk2=5300+5400/-4200s and alpha3=0.7+0.6/-0.3.

The PC mode spectrum comprising 1.64 ks of data from the latter part of 
orbit 1 and from orbit 2 (T0+96s - T0+6384s) is well fit with a power law 
of photon index Gamma=2.1+0.4/-0.5 (90%) absorbed by the Galactic column 
of 6.99E+20 cm^-2 (Kalberla et al. 2005) and by a column at the inferred 
redshift of the host galaxy (z=1.23, Fynbo et al. GCN Circ. 7949) which 
can only be constrained to be nH<1.23e22 cm^-2. The 0.3-10 keV observed 
(unabsorbed) flux is 5.4e-12 (6.73-12) erg/cm^2/s, corresponding to a 
count rate of 0.12 count/s, hence the observed count rate to flux 
conversion is 1 count/s = 4.5e-11 erg/cm^2/s.

The predicted count rate at T0+24h is 0.011 counts/s, assuming a decay 
rate of alpha=0.7 from the doubly broken power law model.

This is an official product of the Swift XRT team.