GCN Circular 21876

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

We have analysed 6.5 ks of XRT data for GRB 170912A (Tohuvavohu et al.
GCN Circ. 21861
Not found
 
 
), from 104 s to 62.2 ks after the  BAT trigger. The
data comprise 17 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 enhanced XRT position for this burst was given by Goad
et al. (GCN Circ. 21862
Not found
 
 
).

The light curve can be modelled with a power-law decay with a decay
index of alpha=4.8 (+nan, -nan).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.85 (+0.54, -0.22). The
best-fitting absorption column is  consistent with the Galactic value
of 3.1 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 3.9 x 10^-11 (5.4 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     3.1 (+/-2.3) x 10^21 cm^-2
Galactic foreground: 3.1 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.85 (+0.54, -0.22)

If the light curve continues to decay with a power-law decay index of
4.8, the count rate at T+24 hours will be 1.4 x 10^-13 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.4 x
10^-24 (7.4 x 10^-24) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00772006.

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