V. D'Elia (ASDC), A. Maselli (INAF-IASFPA), V. Mangano (INAF-IASFPA),
J.A. Kennea (PSU), M.C. Stroh (PSU), D.N. Burrows (PSU), A.P. Beardmore
(U. Leicester), P.A. Evans (U. Leicester), B.P. Gompertz (U. Leicester)
and A.Y. Lien report on behalf of the Swift-XRT team:
We have analysed 6.2 ks of XRT data for GRB 130408A (Lien et al. GCN
Circ. 14361), from 134 s to 28.6 ks after the BAT trigger. The data
comprise 28 s in Windowed Timing (WT) mode (the first 9 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 14367).
The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=0.39 (+0.06, -0.07). At T+3999 s the
decay steepens to an alpha of 8.0 (+0.0, -3.0) before breaking again at
T+4518 s to a final decay with index alpha=0.97 (+0.14, -0.15).
A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.00 (+0.08, -0.07). The
best-fitting absorption column is consistent with the Galactic value
of 2.0 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 3.9 x 10^-11 (5.6 x 10^-11) erg cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
Galactic foreground: 2.0 x 10^21 cm^-2
Intrinsic column: 0 (+4.9, -0) x 10^21 cm^-2 at z=3.758
Photon index: 2.00 (+0.08, -0.07)
If the light curve continues to decay with a power-law decay index of
0.97, the count rate at T+24 hours will be 0.020 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 7.8 x
10^-13 (1.1 x 10^-12) erg cm^-2 s^-1.
The results of the XRT-team automatic analysis are available at
This circular is an official product of the Swift-XRT team.