GCN Circular 10674

M. C. Stroh, D. Grupe and E. A. Hoversten (PSU) report on behalf of the Swift-XRT team:

We have analysed 641 s of XRT data for GRB 100424A (Hoversten  et al.
GCN Circ. 10667
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 ), from 126 s to 769 s after the  BAT trigger. The data
comprise 509 s in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. The light curve can be modelled with a
series of power-law decays. The initial decay index is alpha=0.63
(+/-0.14). At T+288 s the decay steepens to an alpha of 1.41 (+0.27,
-0.19) before breaking again at T+526 s to a final decay with index
alpha=2.8 (+0.9, -0.6).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.26 (+/-0.06). The
best-fitting absorption column is 1.62 (+0.22, -0.21) x 10^21 cm^-2,
in excess of the Galactic value of 2.3 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 5.8 x 10^-11 (6.5 x 10^-11) erg
cm^-2 count^-1.

If the light curve continues to decay with a power-law decay index of
2.8, the count rate at T+24 hours will be 5.1 x 10^-6 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.0 x
10^-16 (3.3 x 10^-16) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00420367.

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