GCN Circular 45979

T. Sbarrato (INAF-OAB), P. D'Avanzo (INAF-OAB), J.J. DeLaunay (PSU), S.
Lanava (PSU), S. Dichiara (PSU), J.P. Osborne (U. Leicester), K.L. Page
(U. Leicester), M.G. Bernardini (INAF-OAB) and P.A. Evans report on
behalf of the Swift-XRT team:

We have analysed 6.3 ks of XRT data for GRB 261017A, from 70 s to 43.9
ks after the   trigger. The data comprise 98 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 light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=1.7 (+0.5, -0.4). At T+100 s  the decay
steepens to an alpha of 5.0 (+0.7, -0.3) before breaking again at T+260
s to a final decay with index alpha=1.12 (+0.30, -0.26).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.16 (+0.08, -0.07). The
best-fitting absorption column is  4.7 (+3.5, -1.5) x 10^20 cm^-2,
consistent with the Galactic value of 3.2 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 5.4 x 10^-11 (5.5 x
10^-11) erg cm^-2 count^-1. 

A summary of the WT-mode spectrum is thus:
Total column:	     4.7 (+3.5, -1.5) x 10^20 cm^-2
Galactic foreground: 3.2 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.16 (+0.08, -0.07)

If the light curve continues to decay with a power-law decay index of
1.12, the count rate at T+24 hours will be 8.1 x 10^-4 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 4.4 x
10^-14 (4.5 x 10^-14) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
https://www.swift.ac.uk/xrt_products/01519536.

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