GCN Circular 7324

P.A. Evans, K.L. Page, A.P. Beardmore, J.P. Osborne (U Leicester) and P.
Schady (MSSL-UCL) report on behalf of the Swift-XRT team:

We have analysed the first four orbits of XRT data obtained for GRB
080218B (Schady et al., GCN Circ. 7314
Not found
 
 
), totalling 4.6 ks in Photon
Counting (PC) mode. Partly due to the proximity of Swift to the SAA, the
observations did not start until 930 s after the trigger.  The
UVOT-enhanced XRT position was given in Osborne et al. (GCN Circ.
7320
Not found
 
 
).

The light curve shows a simple power-law decay, with an index of
alpha=0.92 (+0.13/-0.11). A spectrum formed from all available data can
be modelled with an absorbed power law, with gamma=2.46 (+0.53/-0.47)
and a column density nH=5.17(+0.24/-0.18)e21 cm^-2, significantly in
excess of the Galactic value of 1.35e21 cm^-2 (Kalberla et al. 2005).
Given the excess absorption, the relation given in Grupe et al (2007)
suggests that this burst lies at a redshift <3.3 (90% confidence). The
0.3-10 keV observed (unabsorbed) flux during the interval T0+1--25 ks is
5.72e-12 (1.48e-11) erg cm^-2 s^-1.

If the light curve continues to decay at alpha=0.92, the XRT count rate
at T0+24 hours will be 4.0e-3 count s^-1, which is equivalent to an
observed (unabsorbed) 0.3-10 keV flux of 2.29e-13 (5.92e-13) erg cm^-2
s^-1.

This circular is an official product of the Swift-XRT team.