GCN Circular 15372

A. Maselli  (INAF-IASFPA), A. Melandri (INAF-OAB), V. Mangano (PSU),
M.C. Stroh (PSU), D.N. Burrows (PSU), P.A. Evans (U. Leicester), B.P.
Gompertz (U. Leicester), J.P. Osborne (U. Leicester), B. Sbarufatti
(INAF-OAB/PSU) and J.L. Racusin report on behalf of the Swift-XRT team:

We have analysed 6.5 ks of XRT data for GRB 131024A (Racusin  et al.
GCN Circ. 15365
Not found
 
 
),  from 151 s to 24.0 ks after the  BAT trigger. The
data comprise 20 s in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. The enhanced XRT position for this burst was
given by Osborne et al. (GCN. Circ 15367).

The light curve can be modelled with  a power-law decay with a decay
index of alpha=0.46 (+/-0.05).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.08 (+0.18, -0.17). The
best-fitting absorption column is  7.7 (+3.7, -3.2) x 10^20 cm^-2,
consistent with the Galactic value of 4.4 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 3.4 x 10^-11 (4.3 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     7.7 (+3.7, -3.2) x 10^20 cm^-2
Galactic foreground: 4.4 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     2.08 (+0.18, -0.17)

If the light curve continues to decay with a power-law decay index of
0.46, the count rate at T+24 hours will be 0.036 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.2 x
10^-12 (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/00575707.

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