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GCN Circular 18030

Subject
GRB 150716A: Swift-XRT refined Analysis
Date
2015-07-17T07:19:04Z (9 years ago)
From
Phil Evans at U of Leicester <pae9@leicester.ac.uk>
B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows (PSU), T.G.R. Roegiers
(PSU), A.P. Beardmore (U. Leicester), P.A. Evans (U. Leicester), B.P.
Gompertz (U. Leicester), A. Maselli  (INAF-IASFPA), A. Melandri
(INAF-OAB), P. D'Avanzo (INAF-OAB) and D. Kocevski report on behalf of
the Swift-XRT team:

We have analysed 3.6 ks of XRT data for GRB 150716A (Kocevski et al.
GCN Circ. 18027), from 66 s to 22.7 ks after the  BAT trigger. The data
comprise 126 s in Windowed Timing (WT) mode (the first 11 s were taken
while Swift was slewing) with the remainder in Photon Counting (PC)
mode. The refined XRT position is RA, Dec = 278.4879, -12.9802 which is
equivalent to:

RA (J2000): 18 33 57.10
Dec(J2000): -12 58 48.9

with an uncertainty of 4.1 arcsec (radius, 90% confidence).

The light curve can be modelled with a power-law decay with a decay
index of alpha=1.67 (+0.08, -0.06).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.67 (+0.25, -0.23). The
best-fitting absorption column is  1.35 (+0.24, -0.22) x 10^22 cm^-2,
in excess of the Galactic value of 7.0 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 3.6 x 10^-11 (1.4 x 10^-10) erg
cm^-2 count^-1. 

A summary of the WT-mode spectrum is thus:
Total column:	     1.35 (+0.24, -0.22) x 10^22 cm^-2
Galactic foreground: 7.0 x 10^21 cm^-2
Excess significance: 4.8 sigma
Photon index:	     2.67 (+0.25, -0.23)

If the light curve continues to decay with a power-law decay index of
1.67, the count rate at T+24 hours will be 2.4 x 10^-4 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 8.8 x
10^-15 (3.5 x 10^-14) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00649157.

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