GCN Circular 10639

A. Rowlinson (U Leicester) and W. H. Baumgartner (GSFC/UMBC) report on
behalf of the Swift-XRT team:

We have analysed 6.9 ks of XRT data for GRB 100420A (Baumgartner et al.
GCN Circ. 10628
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 ), from 115 s to 13.6 ks after the BAT trigger. The data
comprise 120 s in Windowed Timing (WT) mode (the first 8 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 Beardmore et
al. (GCN. Circ 10638).

The light curve can be modelled with a power-law decay with a decay
index of alpha=2.69 (+0.47, -0.39), followed by a break at T+721 s to an
alpha of 1.22 (+0.34, -4.02).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.83 (+0.22, -0.20). The
best-fitting absorption column is consistent with the Galactic value of
1.2 x 10^21 cm^-2 (Kalberla et al. 2005). The counts to observed
(unabsorbed) 0.3-10 keV flux conversion factor deduced from this
spectrum is 4.2 x 10^-11 (5.3 x 10^-11) erg cm^-2 count^-1.

If the light curve continues to decay with a power-law decay index of
1.22, the count rate at T+24 hours will be 2.3 x 10^-4 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 9.7 x
10^-15 (1.2 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/00419932.

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