GCN Circular 10622

C. Pagani (U. Leicester), A. P. Beardmore (U. Leicester)�� and F. E.
Marshall (NASA/GSFC) report on behalf of the Swift-XRT team:

We have analysed 3.7 ks of XRT data for GRB 100418A (Marshall et al. GCN
Circ. 10612
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 ), from 70 s to 7.3 ks after the BAT trigger. The data
comprise 97 s in Windowed Timing (WT) mode (the first 7 s were taken
while Swift was slewing) with the remainder in Photon Counting (PC)
mode. The enhanced XRT position for this burst was given by Osborne et
al. (GCN. Circ 10614).

The light curve can be modelled with an initial power-law decay with an
index of alpha=4.23 (+0.17, -0.16), followed by a break at T+474 s to an
alpha of 0.21 (+0.22, -0.24).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index of 4.29 (+0.33, -0.30). The
best-fitting absorption column is 2.1 (+/-0.4) x 10^21 cm^-2, in excess
of the Galactic value of 4.8 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 2.0 x 10^-11 (1.3 x 10^-10) erg cm^-2
count^-1.

If the light curve continues to decay with a power-law decay index of
0.21, the count rate at T+24 hours will be 0.012 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.5 x
10^-13 (1.6 x 10^-12) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00419797.

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