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GCN Circular 32350

Subject
GRB 220708B: Swift-XRT afterglow detection
Date
2022-07-08T14:27:31Z (2 years ago)
From
Phil Evans at U of Leicester <pae9@leicester.ac.uk>
J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), A.P. Beardmore
(U. Leicester), M.G. Bernardini (INAF-OAB), E. Ambrosi	(INAF-IASFPA) ,
M. Capalbi (INAF-IASFPA), B. Sbarufatti (PSU), D.N. Burrows (PSU), J.
D. Gropp (PSU) and P.A. Evans (U. Leicester) report on behalf of the
Swift-XRT team:

Swift-XRT has performed follow-up observations of the
Swift/BAT-GUANO-detected burst GRB 220708B, collecting 1.7 ks of Photon
Counting (PC) mode data between T0+35.1 ks and T0+36.8 ks. 

An uncatalogued X-ray source is detected and is above the RASS 3-sigma
upper limit at this position, and is therefore likely the GRB
afterglow.  The position of this source is RA, Dec=248.5790, +36.3478
which is equivalent to:

RA (J2000): 16:34:18.97
Dec(J2000): +36:20:52.0

with an uncertainty of 4.0 arcsec (radius, 90% confidence).  This
position is 3.6 arcmin from the Swift/BAT-GUANO position.  The light
curve is consistent with a constant source of mean count rate 6.5e-02
ct/sec. A power-law fit gives an index of 0.0 (+0.0, -3.3).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.3 (+0.6, -0.5). The
best-fitting absorption column is  3.2 (+2.2, -1.7) x 10^21 cm^-2, in
excess of the Galactic value of 1.4 x 10^20 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 3.2 x 10^-11 (5.7 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     3.2 (+2.2, -1.7) x 10^21 cm^-2
Galactic foreground: 1.4 x 10^20 cm^-2
Excess significance: 2.9 sigma
Photon index:	     2.3 (+0.6, -0.5)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00021508.
The results of the full analysis of the XRT observations are available
at https://www.swift.ac.uk/ToO_GRBs/00021508.

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