GCN Circular 21408

B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows (PSU), J.P. Osborne (U.
Leicester), K.L. Page (U. Leicester), B. Mingo (U. Leicester), A.
Melandri (INAF-OAB), P. D'Avanzo (INAF-OAB), V. D'Elia (ASDC), A.
Cholden-Brown (PSU) and S.J. LaPorte report on behalf of the Swift-XRT
team:

We have analysed 6.1 ks of XRT data for GRB 170803A (LaPorte et al. GCN
Circ. 21405
Not found
 
 
), from 68 s to 30.1 ks after the  BAT trigger. The data
comprise 312 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. 21407
Not found
 
 
).

The late-time light curve (from T0+5.6 ks) can be modelled with a
power-law decay with a decay index of alpha=1.00 (+/-0.28).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.86 (+0.13, -0.12). The
best-fitting absorption column is  1.3 (+/-0.4) x 10^21 cm^-2, in
excess of the Galactic value of 3.5 x 10^20 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 3.7 x 10^-11 (4.5 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.3 (+/-0.4) x 10^21 cm^-2
Galactic foreground: 3.5 x 10^20 cm^-2
Excess significance: 4.5 sigma
Photon index:	     1.86 (+0.13, -0.12)

If the light curve continues to decay with a power-law decay index of
1.00, the count rate at T+24 hours will be 6.3 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.3 x
10^-13 (2.8 x 10^-13) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00766081.

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