GCN Circular 21978

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

We have analysed 6.1 ks of XRT data for GRB 171007A (Cannizzo et al.
GCN Circ. 21976
Not found
 
 
), from 62 s to 12.7 ks after the  BAT trigger. The data
comprise 107 s in Windowed Timing (WT) mode (the first 5 s were taken
while Swift was slewing) with the remainder in Photon Counting (PC)
mode. The best available XRT position  (using the promptly downlinked
event data, the XRT-UVOT alignment and matching UVOT field sources to
the USNO-B1 catalogue) is RA, Dec = 135.6008, 42.8193 which is
equivalent to:

RA (J2000): 09 02 24.19
Dec(J2000): +42 49 09.5

with an uncertainty of 2.3 arcsec (radius, 90% confidence).

The light curve can be modelled with a power-law decay with a decay
index of alpha=3.56 (+0.21, -0.18).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.38 (+0.19, -0.18). The
best-fitting absorption column is  1.2 (+0.9, -0.7) x 10^21 cm^-2, in
excess of the Galactic value of 2.1 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 4.7 x 10^-11 (5.2 x 10^-11) erg
cm^-2 count^-1. 

A summary of the WT-mode spectrum is thus:
Total column:	     1.2 (+0.9, -0.7) x 10^21 cm^-2
Galactic foreground: 2.1 x 10^20 cm^-2
Excess significance: 2.4 sigma
Photon index:	     1.38 (+0.19, -0.18)

If the light curve continues to decay with a power-law decay index of
3.56, the count rate at T+24 hours will be 1.1 x 10^-10 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.2 x
10^-21 (5.7 x 10^-21) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00777215.

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