GCN Circular 21182

P. D'Avanzo (INAF-OAB), J.A. Kennea (PSU), B. Sbarufatti
(INAF-OAB/PSU), D.N. Burrows (PSU), K.L. Page (U. Leicester), B. Mingo
(U. Leicester), A.P. Beardmore (U. Leicester), V. D'Elia (ASDC), A.
D'Ai (INAF-IASFPA) and P.A. Evans report on behalf of the Swift-XRT
team:

We have analysed 6.8 ks of XRT data for GRB 170531B (Evans et al. GCN
Circ. 21171
Not found
 
 
), from 129 s to 52.1 ks after the  BAT trigger. The data
comprise 303 s in Windowed Timing (WT) mode (the first 10 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. 21175
Not found
 
 
).

The late-time light curve (from T0+5.5 ks) can be modelled with a
power-law decay with a decay index of alpha=1.18 (+0.25, -0.20).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.45 (+/-0.04). The
best-fitting absorption column is  6.5 (+2.7, -2.5) x 10^21 cm^-2, at a
redshift of 2.366, in addition to the Galactic value of 1.4 x 10^21
cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index
of 2.07 (+0.13, -0.12) and a best-fitting absorption column consistent
with the Galactic value. The counts to observed (unabsorbed) 0.3-10 keV
flux conversion factor deduced from this spectrum  is 3.4 x 10^-11 (4.7
x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Galactic foreground: 1.4 x 10^21 cm^-2
Intrinsic column:    1.4 (+2.2, -0.0) x 10^21 cm^-2 at z=2.366
Photon index:	     2.07 (+0.13, -0.12)

If the light curve continues to decay with a power-law decay index of
1.18, the count rate at T+24 hours will be 3.3 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.1 x
10^-13 (1.6 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/00755354.

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