TITLE:   GCN CIRCULAR
NUMBER:  20620
SUBJECT: GRB 170206B: Swift-XRT refined Analysis
DATE:    17/02/07 09:46:41 GMT
FROM:    Phil Evans at U of Leicester  <pae9@leicester.ac.uk>

S. J. LaPorte (PSU), J.A. Kennea (PSU), B. Sbarufatti (INAF-OAB/PSU),
S.L. Gibson (U. Leicester), J.P. Osborne (U. Leicester), K.L. Page (U.
Leicester), A. D'Ai (INAF-IASFPA), A. Melandri (INAF-OAB), P. D'Avanzo
(INAF-OAB) and A.Y. Lien report on behalf of the Swift-XRT team:

We have analysed 7.4 ks of XRT data for GRB 170206B (Lien et al. GCN
Circ. 20618), from 88 s to 24.5 ks after the  BAT trigger. The data
comprise 7 s in Windowed Timing (WT) mode (taken while Swift was
slewing), with the remainder in Photon Counting (PC) mode. Using 2057 s
of PC mode data and 3 UVOT images, we find an enhanced XRT position
(using the XRT-UVOT alignment and matching UVOT field sources to the
USNO-B1 catalogue): RA, Dec = 271.77062, +11.19383 which is equivalent
to:

RA (J2000): 18h 07m 04.95s
Dec(J2000): +11d 11' 37.8"

with an uncertainty of 1.6 arcsec (radius, 90% confidence).

The light curve can be modelled with a power-law decay with a decay
index of alpha=2.48 (+0.19, -0.20).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.46 (+0.18, -0.17). The
best-fitting absorption column is  4.9 (+0.8, -0.7) x 10^21 cm^-2, in
excess of the Galactic value of 1.1 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 3.1 x 10^-11 (6.9 x 10^-11) erg
cm^-2 count^-1. 

A summary of the WT-mode spectrum is thus:
Total column:	     4.9 (+0.8, -0.7) x 10^21 cm^-2
Galactic foreground: 1.1 x 10^21 cm^-2
Excess significance: 8.5 sigma
Photon index:	     2.46 (+0.18, -0.17)

If the light curve continues to decay with a power-law decay index of
2.48, the count rate at T+24 hours will be 2.5 x 10^-7 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 7.8 x
10^-18 (1.8 x 10^-17) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00737125.

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