[vsnet-grb-info 9019] GRB 100316C: Swift-XRT team refined analysis

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  10507
SUBJECT: GRB 100316C: Swift-XRT team refined analysis 
DATE:    10/03/16 19:15:43 GMT
FROM:    Kim Page at U.of Leicester  <kpa at star.le.ac.uk>

K.L. Page (U. Leicester) and M. Stamatikos (OSU/NASA/GSFC) report on 
behalf of the Swift-XRT team:

We have analysed 5.7 ks of XRT data for GRB 100316C (Stamatikos et al. GCN 
Circ. 10491), from 87 s to 13.5 ks after the BAT trigger. The data are 
entirely in Photon Counting (PC) mode. The enhanced XRT position for this 
burst was given by Evans et al. (GCN Circ. 10506).

The light curve can be modelled with a doubly-broken power-law, with 
alpha1 = 0.28 (+0.66,-1.18) until T = 258 (+87/-66) s after the trigger, 
at which point the decay steepens to alpha2 = 2.53 (+1.18, -0.44). At T = 
883 (+1857, -271) s, the decay flattens to alpha3 = 0.76 (+0.27, -0.26).

A spectrum formed from the PC mode data can be fitted with an absorbed 
power-law with a photon spectral index of 1.6 (+0.5, -0.4). The best-fitting 
absorption column is 1.8 (+1.6, -1.3) x 10^21 cm^-2, in excess of the Galactic 
value of 3.9 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 
5.1 x 10^-11 (6.2 x 10^-11) erg cm^-2 count^-1.

If the light curve continues to decay with a power-law decay index of 0.76, the 
count rate at T+24 hours will be 0.0013 count s^-1, corresponding to an 
observed (unabsorbed) 0.3-10 keV flux of 6.6 x 10^-14 (8.1 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/00416115.

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