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GCN Circular 21198

Subject
GRB 170604A: Swift-XRT refined Analysis
Date
2017-06-05T08:21:54Z (7 years ago)
From
Phil Evans at U of Leicester <pae9@leicester.ac.uk>
B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows (PSU), A. Cholden-Brown
(PSU), B. Mingo (U. Leicester), A.P. Beardmore (U. Leicester), P.A.
Evans (U. Leicester), V. D'Elia (ASDC), A. D'Ai (INAF-IASFPA), A.
Melandri (INAF-OAB) and K.L. Page report on behalf of the Swift-XRT
team:

We have analysed 7.6 ks of XRT data for GRB 170604A (Page et al. GCN
Circ. 21191), from 110 s to 39.1 ks after the  BAT trigger. The data
comprise 308 s in Windowed Timing (WT) mode (the first 7 s were taken
while Swift was slewing) with the remainder in Photon Counting (PC)
mode. Using 4539 s of PC mode data and 10 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 = 342.65623, -15.41233
which is equivalent to:

RA (J2000): 22h 50m 37.49s
Dec(J2000): -15d 24' 44.4"

with an uncertainty of 1.4 arcsec (radius, 90% confidence).

The late-time light curve (from T0+3.8 ks) can be modelled with a
power-law decay with a decay index of alpha=0.98 (+/-0.05).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.27 (+/-0.04). The
best-fitting absorption column is  1.95 (+/-0.11) x 10^21 cm^-2, in
excess of the Galactic value of 3.8 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.85 (+/-0.09) and a
best-fitting absorption column of 6.9 (+2.3, -2.2) x 10^20 cm^-2. The
counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 3.6 x 10^-11 (4.0 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     6.9 (+2.3, -2.2) x 10^20 cm^-2
Galactic foreground: 3.8 x 10^20 cm^-2
Excess significance: 2.3 sigma
Photon index:	     1.85 (+/-0.09)

If the light curve continues to decay with a power-law decay index of
0.98, the count rate at T+24 hours will be 0.046 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.6 x
10^-12 (1.9 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/00755867.

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