K.L. Page, J.P. Osborne (U. Leicester) & S.T. Holland (CRESST/GSFC/USRA)
report on behalf of the Swift-XRT team:
We have analysed the first four orbits of data obtained for GRB 080307
(Holland et al., GCN Circ. 7362), which includes 509 seconds in Windowed
Timing (WT) mode and 7.9 ks in Photon Counting (PC mode).
The UVOT-enhanced XRT position has been given by Goad et al. in GCN Circ
In the first orbit of the XRT data, the emission rises slowly, peaking
around 200-300 seconds after the BAT trigger, after which the decay can be
modelled with a single power-law with alpha = 1.83 +/- 0.08. The shape of
the early light-curve is reminiscent of GRB 060218 (Campana, S. et al,
2006, Nature, 442, 1008).
As the X-ray emission rises, it also softens; the hardness ratio then
becomes close to constant as the light-curve decay sets in. The PC data at
the end of the first orbit can be fitted with an absorbed power-law, with
Gamma = 1.74 +0.23/-0.21 and NH = (1.5 +0.8/-0.6)e21 cm^-2, which is in
excess of the Galactic value of 2.37e20 cm^-2. From the relation in Grupe
et al. (2007, AJ, 133, 2216), this indicates a redshift of less than 4.0.
The mean observed (unabsorbed) flux for the PC data at the end of the
first orbit (615-1030 seconds) is 7.86e-11 (9.83e-11) erg cm^-2 s^-1. The
corresponding counts to (observed) flux conversion is 1 count s^-1 =
4.9e-11 erg cm-2 s^-1.
If the light-curve continues to decay following a power-law of 1.83, the
count rate at 24 hours is predicted to be 3.6e-4 count s^-1 (an observed
flux of 1.8e-14 erg cm^-2 s^-1).
This circular is an official product of the Swift-XRT team.