GCN Circular 9073

A. Rowlinson (U. Leicester) and P. Schady (MSSL-UCL) report on behalf of
the Swift-XRT team:

We have analysed 3.7 ks of XRT data for GRB 090401B (Schady et al. GCN
Circ. 9066
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 ), from 79 s to 7.2 ks after the BAT trigger. The data
comprise 1.3 ks in Windowed Timing (WT) mode and 2.4ks in Photon
Counting (PC) mode. The enhanced XRT position for this burst was given
by Osborne et al. (GCN. Circ 9070). The light curve can be modelled with
an initial power-law decay with an index of alpha=1.145 (+/-0.020),
followed by a break at T+474 s to an alpha of 1.409 (+0.028, -0.026).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.736 (+/-0.026). The
best-fitting absorption column is 3.68 (+/-0.12) x 10^21 cm^-2, in
excess of the Galactic value of 2.3 x 10^21 cm^-2 (Kalberla et al.
2005). The PC mode spectrum has a photon index of 2.13 (+/-0.11) and a
best-fitting absorption column of 4.4 (+0.5, -0.4) x 10^21 cm^-2. The
counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum is 4.5 x 10^-11 (8.4 x 10^-11) erg cm^-2
count^-1.

If the light curve continues to decay with a power-law decay index of
1.409, the count rate at T+24 hours will be 0.017 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 7.7 x
10^-13 (1.4 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/00348152.

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

[GCN OPS NOTE(02apr09):  Per author's request, the typo in the Subject line
of "080401B" was changed to "090401B".]