GCN Circular 15706

P. D'Avanzo (INAF-OAB), J. L. Racusin (NASA/GSFC), P. A. Evans (U.
Leicester) report on behalf of the Swift-XRT team:

We have analysed 10 ks of XRT data for GRB 140108A (Racusin  et al. GCN
Circ. 15699
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 ),  from 58 s to 41.8 ks after the  BAT trigger. The data
comprise 2.4 ks 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 Osborne et
al. (GCN. Circ 15705).

The light curve shows some initial flaring activity, with the flares
peaking at around 90 and 220 s after the trigger. The underlying
emission can be modelled with a series of power-law decays. The initial
decay index is alpha=2.8 (+/-0.7). At T+184 s  the decay flattens to an
alpha of 0.52 (+/-0.03) before breaking again at T+7014 s to a final
decay with index alpha=1.20 (+0.08, -0.07).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.69 (+0.04, -0.03). The
best-fitting absorption column is  consistent with the Galactic value
of 7.0 x 10^21 cm^-2 (Kalberla et al. 2005). The PC mode spectrum has a
photon index of 1.91 (+0.16, -0.15) and a best-fitting absorption
column of 9.3 (+1.3, -1.2) x 10^21 cm^-2. The counts to observed
(unabsorbed) 0.3-10 keV flux conversion factor deduced from this
spectrum  is 5.7 x 10^-11 (1.1 x 10^-10) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     9.3 (+1.3, -1.2) x 10^21 cm^-2
Galactic foreground: 7.0 x 10^21 cm^-2
Excess significance: 3.1 sigma
Photon index:	     1.91 (+0.16, -0.15)

If the light curve continues to decay with a power-law decay index of
1.20, the count rate at T+24 hours will be 0.035 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.0 x
10^-12 (3.8 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/00583338.

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