TITLE:   GCN CIRCULAR
NUMBER:  21376
SUBJECT: GRB 170728B: Swift-XRT refined Analysis
DATE:    17/07/29 05:05:26 GMT
FROM:    Phil Evans at U of Leicester  <pae9@leicester.ac.uk>

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

We have analysed 7.3 ks of XRT data for GRB 170728B (Cenko et al. GCN
Circ. 21371), from 454 s to 14.4 ks after the  BAT trigger. The data
comprise 231 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 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 = 237.9806, 70.1227 which is
equivalent to:

RA (J2000): 15 51 55.35
Dec(J2000): +70 07 21.7

with an uncertainty of 2.0 arcsec (radius, 90% confidence).

The light curve can be modelled with an initial power-law decay with an
index of alpha=0.66 (+0.08, -0.12), followed by a break at T+1874 s to
an alpha of 1.20 (+/-0.07).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.94 (+/-0.12). The
best-fitting absorption column is  1.7 (+0.5, -0.4) x 10^21 cm^-2, in
excess of the Galactic value of 3.2 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.81 (+/-0.08) and a
best-fitting absorption column of 2.5 (+/-0.3) x 10^21 cm^-2. The
counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 4.1 x 10^-11 (5.3 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.5 (+/-0.3) x 10^21 cm^-2
Galactic foreground: 3.2 x 10^20 cm^-2
Excess significance: 11.6 sigma
Photon index:	     1.81 (+/-0.08)

If the light curve continues to decay with a power-law decay index of
1.20, the count rate at T+24 hours will be 0.043 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.8 x
10^-12 (2.3 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/00765130.

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