GCN Circular 21325

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

We have analysed 6.1 ks of XRT data for GRB 170711A (D'Elia et al. GCN
Circ. 21321
Not found
 
 
), from 51 s to 13.0 ks after the  BAT trigger. The data
comprise 6 s in Windowed Timing (WT) mode (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.
21322
Not found
 
 
).

The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=3.2 (+0.8, -0.7). At T+160 s  the decay
flattens to an alpha of -0.14 (+0.25, -0.65) before breaking again at
T+1004 s to a final decay with index alpha=0.59 (+0.36, -0.11).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.08 (+0.22, -0.21). The
best-fitting absorption column is  6.7 (+1.5, -1.3) x 10^21 cm^-2, in
excess of the Galactic value of 3.2 x 10^21 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 4.2 x 10^-11 (7.6 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     6.7 (+1.5, -1.3) x 10^21 cm^-2
Galactic foreground: 3.2 x 10^21 cm^-2
Excess significance: 4.2 sigma
Photon index:	     2.08 (+0.22, -0.21)

If the light curve continues to decay with a power-law decay index of
0.59, the count rate at T+24 hours will be 0.018 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/00761622.

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