GCN Circular 20997

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

We have analysed 6.5 ks of XRT data for GRB 170405A, from 111 s to 57.3
ks after the  BAT trigger. The data comprise 489 s in Windowed Timing
(WT) mode (the first 8 s were taken while Swift was slewing) with the
remainder in Photon Counting (PC) mode. 

The late-time light curve (from T0+4.5 ks) can be modelled with a
power-law decay with a decay index of alpha=1.66 (+0.14, -0.12).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.592 (+0.027, -0.019). The
best-fitting absorption column is  consistent with the Galactic value
of 8.7 x 10^20 cm^-2 (Willingale et al. 2013). The PC mode spectrum has
a photon index of 1.84 (+0.09, -0.07) 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 3.6 x 10^-11 (4.2 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Galactic foreground: 8.7 x 10^20 cm^-2
Intrinsic column:    8.7 (+68.1, -0.0) x 10^20 cm^-2 at z=3.51
Photon index:	     1.84 (+0.09, -0.07)

If the light curve continues to decay with a power-law decay index of
1.66, the count rate at T+24 hours will be 5.5 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.0 x
10^-13 (2.3 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/00745797.

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