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GCN Circular 15018

Subject
GRB 130722A: Swift-XRT refined Analysis
Date
2013-07-22T21:30:03Z (11 years ago)
From
Phil Evans at U of Leicester <pae9@leicester.ac.uk>
D.N. Burrows (PSU), C. Pagani (U. Leicester), A.P. Beardmore (U.
Leicester), P.A. Evans (U. Leicester), A. Melandri (INAF-OAB), B.
Sbarufatti (INAF-OAB/PSU), P. D'Avanzo (INAF-OAB), J.A. Kennea (PSU),
V. Mangano (PSU) and E. Troja report on behalf of the Swift-XRT team:

We have analysed 7.3 ks of XRT data for GRB 130722A (Troja  et al. GCN
Circ. 15013),  from 117 s to 17.7 ks after the	BAT trigger. The data
comprise 739 s in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. The refined XRT position is RA, Dec =
260.65405, -2.97376 which is equivalent to:

RA (J2000): 17 22 36.97
Dec(J2000): -02 58 25.5

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

The late-time light curve (from T0+3.6 ks) can be modelled with  a
power-law decay with a decay index of alpha=1.10 (+/-0.11).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.21 (+/-0.05). The
best-fitting absorption column is  4.18 (+0.18, -0.17) x 10^21 cm^-2,
in excess of the Galactic value of 1.2 x 10^21 cm^-2 (Kalberla et al.
2005). The PC mode spectrum has a photon index of 1.89 (+0.16, -0.15)
and a best-fitting absorption column of 4.9 (+0.9, -0.8) x 10^21 cm^-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 4.8 x 10^-11 (7.8 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     4.9 (+0.9, -0.8) x 10^21 cm^-2
Galactic foreground: 1.2 x 10^21 cm^-2
Excess significance: 7.3 sigma
Photon index:	     1.89 (+0.16, -0.15)

If the light curve continues to decay with a power-law decay index of
1.10, the count rate at T+24 hours will be 0.023 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.1 x
10^-12 (1.8 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/00563213.

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