GCN Circular 7951
Subject
GRB 080707: Swift XRT refined analysis
Date
2008-07-07T20:35:47Z (17 years ago)
From
Rhaana Starling at U of Leicester <rlcs1@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.