TITLE:   GCN CIRCULAR
NUMBER:  10518
SUBJECT: GRB 100316B: Swift-XRT refined analysis
DATE:    10/03/17 09:30:12 GMT
FROM:    Owen Littlejohns at U of Leicester  <oml2@star.le.ac.uk>

O. M. Littlejohns, P. A. Evans, K. L. Page (U. Leicester) and W. H.
Baumgartner (GSFC/UMBC) report on behalf of the Swift-XRT team:

We have analysed 2.9 ks of XRT data for GRB 100316B (Baumgartner et al.
GCN Circ. 10489), from 75 s to 40.1 ks after the BAT trigger. The data
are entirely in Photon Counting (PC) mode. The enhanced XRT position for
this burst was given by Beardmore et al. (GCN. Circ 10502).

The light curve can be modelled with a broken power-law decay with an
initial decay index of alpha=0.1 (+/-0.2), followed by a break at T~1100
s to a final decay of alpha=1.2 (+0.5, -0.1).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.35 (+0.24, -0.21). The
best-fitting absorption column is 4.1 (+2.7, -1.3) x 10^21 cm^-2, at a
redshift of 1.18 (Vergani et al. GCN Circ 10495), in addition to the
Galactic value of 9.9 x 10^20 cm^-2 (Kalberla et al. 2005). The counts
to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from
this spectrum is 3.4 x 10^-11 (5.8 x 10^-11) erg cm^-2 count^-1.

If the light curve continues to decay with a power-law decay index of
1.2, the count rate at T+48 hours will be 6.6 x 10^-4 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.2 x
10^-14 (3.8 x 10^-14) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00416103.

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