TITLE:   GCN CIRCULAR
NUMBER:  21267
SUBJECT: GRB 170626A: Swift-XRT refined Analysis
DATE:    17/06/26 20:01:54 GMT
FROM:    Phil Evans at U of Leicester  <pae9@leicester.ac.uk>

A. Cholden-Brown (PSU), P.A. Evans (U. Leicester), S.L. Gibson (U.
Leicester), J.P. Osborne (U. Leicester), A. Melandri (INAF-OAB), P.
D'Avanzo (INAF-OAB), V. D'Elia (ASDC), S. J. LaPorte (PSU), J.A. Kennea
(PSU) and F.E. Marshall report on behalf of the Swift-XRT team:

We have analysed 9.4 ks of XRT data for GRB 170626A (Marshall et al.
GCN Circ. 21264), from 51 s to 28.8 ks after the  BAT trigger. The data
comprise 1.2 ks in Windowed Timing (WT) mode (the first 6 s were taken
while Swift was slewing) with the remainder in Photon Counting (PC)
mode. The refined XRT position is RA, Dec = 165.3936, +56.4768 which is
equivalent to:

RA (J2000): 11 01 34.46
Dec(J2000): +56 28 36.5

with an uncertainty of 3.5 arcsec (radius, 90% confidence).

The late-time light curve (from T0+5.4 ks) can be modelled with a
power-law decay with a decay index of alpha=1.22 (+/-0.09).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.019 (+/-0.025). The
best-fitting absorption column is  3.2 (+/-0.5) x 10^20 cm^-2, in
excess of the Galactic value of 6.7 x 10^19 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.73 (+/-0.08) and a
best-fitting absorption column of 3.2 (+1.8, -1.7) x 10^20 cm^-2. The
counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 3.7 x 10^-11 (3.9 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     3.2 (+1.8, -1.7) x 10^20 cm^-2
Galactic foreground: 6.7 x 10^19 cm^-2
Excess significance: 2.4 sigma
Photon index:	     1.73 (+/-0.08)

If the light curve continues to decay with a power-law decay index of
1.22, the count rate at T+24 hours will be 0.039 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.4 x
10^-12 (1.5 x 10^-12) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00758766.

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