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GCN Circular 32899

Subject
GRB 221028A: Swift-XRT refined Analysis
Date
2022-10-31T07:29:01Z (2 years ago)
From
Phil Evans at U of Leicester <pae9@leicester.ac.uk>
V. D'Elia (SSDC & INAF-OAR), D.N. Burrows (PSU), J. D. Gropp (PSU), S.
Dichiara (PSU), P.A. Evans (U. Leicester), J.P. Osborne (U. Leicester),
K.L. Page (U. Leicester), B. Sbarufatti (INAF-OAB), A. D'Ai
(INAF-IASFPA) and A.Y. Lien report on behalf of the Swift-XRT team:

We have analysed 5.6 ks of XRT data for GRB 221028A (Lien et al. GCN
Circ. 32874), from 92 s to 28.1 ks after the  BAT trigger. The data
comprise 23 s in Windowed Timing (WT) mode (the first 2 s were taken
while Swift was slewing) with the remainder in Photon Counting (PC)
mode. The refined XRT position is RA, Dec = 314.2027, +41.0898 which is
equivalent to:

RA (J2000): 20 56 48.64
Dec(J2000): +41 05 23.3

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

The light curve can be modelled with a power-law decay with a decay
index of alpha=2.04 (+0.13, -0.12).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.5 (+0.5, -0.4). The
best-fitting absorption column is  2.4 (+1.0, -0.8) x 10^22 cm^-2, in
excess of the Galactic value of 7.8 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 8.2 x 10^-11 (1.3 x 10^-10) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.4 (+1.0, -0.8) x 10^22 cm^-2
Galactic foreground: 7.8 x 10^21 cm^-2
Excess significance: 3.3 sigma
Photon index:	     1.5 (+0.5, -0.4)

If the light curve continues to decay with a power-law decay index of
2.04, the count rate at T+24 hours will be 8.3 x 10^-6 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 6.8 x
10^-16 (1.1 x 10^-15) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01131910.

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