TITLE:   GCN CIRCULAR
NUMBER:  10975
SUBJECT: GRB 100724A: Swift XRT refined analysis 
DATE:    10/07/24 18:32:29 GMT
FROM:    Boris Sbarufatti at INAF-OAB/IASFPA  <boris.sbarufatti@brera.inaf.it>

V. Mangano (INAF IASF-PA), B. Sbarufatti (INAF OAB/INAF IASF-PA) and
C. B. Markwardt (CRESST/GSFC/UMD) report on behalf of the Swift-XRT team:

We have analysed 10 ks of XRT data for GRB 100724A (Markwardt  et al.
GCN Circ. 10968), from 96 s to 25.1 ks after the BAT trigger. The data
are entirely in Photon Counting (PC) mode. The light curve can be
modelled with  a broken power-law decay with an initial decay index of
alpha1=0.3 (+/-0.1) , a break at T+525 (+/-90) s and a final decay index of
alpha2=1.07 (+/-0.03).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.98 (+0.20, -0.19).
The best-fitting absorption column is consistent with the Galactic value
of 3.4 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 4.8 x 10^-11 (5.6 x 10^-11) erg cm^-2 count^-1.

If the light curve continues to decay with a power-law decay index of
1.1, the count rate at T+24 hours will be 2.4e-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of
1.1 x 10^-13 (1.3 x 10^-13) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00429868.

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