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GCN Circular 31410

Subject
GRB 220107A: Swift-XRT afterglow detection
Date
2022-01-08T06:39:25Z (2 years ago)
From
Phil Evans at U of Leicester <pae9@leicester.ac.uk>
D.N. Burrows (PSU), J. D. Gropp (PSU), J.A. Kennea (PSU), K.L. Page (U.
Leicester), A.P. Beardmore (U. Leicester), A. Melandri (INAF-OAB), T.
Sbarrato (INAF-OAB), P. D'Avanzo (INAF-OAB) 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 220107A (DeLaunay et al. GCN Circ.
31402), collecting 699 s of Photon Counting (PC) mode data between
T0+26.7 ks and T0+27.4 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. Using 698 s of PC mode data and 1 UVOT image, we find an
enhanced XRT position (using the XRT-UVOT alignment and matching UVOT
field sources to the USNO-B1 catalogue): RA, Dec = 169.80648, +34.17014
which is equivalent to:

RA (J2000): 11h 19m 13.55s
Dec(J2000): +34d 10' 12.5"

with an uncertainty of 2.3 arcsec (radius, 90% confidence). This
position is 3.0 arcmin from the Swift/BAT-GUANO position. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=2.5 (+0.5, -2.4).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.84 (+0.27, -0.18). The
best-fitting absorption column is  3.1 (+6.1, -0.9) x 10^20 cm^-2,
consistent with the Galactic value of 2.2 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 3.5 x 10^-11 (3.7 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     3.1 (+6.1, -0.9) x 10^20 cm^-2
Galactic foreground: 2.2 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.84 (+0.27, -0.18)

If the light curve continues to decay with a power-law decay index of
2.5, the count rate at T+24 hours will be 0.054 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.9 x
10^-12 (2.0 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/00021476.
The results of the full analysis of the XRT observations are available
at https://www.swift.ac.uk/ToO_GRBs/00021476.

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