TITLE:   GCN CIRCULAR
NUMBER:  34568
SUBJECT: GRB 230826A: Swift-XRT refined Analysis
DATE:    23/08/27 07:26:29 GMT
FROM:    Phil Evans at U of Leicester <pae9@star.le.ac.uk>

J.A. Kennea (PSU), A. Tohuvavohu (U. Toronto), J.P. Osborne (U.
Leicester), K.L. Page (U. Leicester), M. Capalbi (INAF-IASFPA), M.
Perri (SSDC & INAF-OAR), V. D'Elia (SSDC & INAF-OAR), D.N. Burrows
(PSU) and P.A. Evans report on behalf of the Swift-XRT team:

We have analysed 8.3 ks of XRT data for GRB 230826A, from 65 s to 29.5
ks after the  BAT trigger. The data comprise 56 s in Windowed Timing
(WT) mode (the first 8 s were taken while Swift was slewing) with the
remainder in Photon Counting (PC) mode. The refined XRT position is RA,
Dec = 83.0642, +66.1244 which is equivalent to:

RA (J2000): 05 32 15.41
Dec(J2000): +66 07 27.9

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

The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=3.44 (+0.25, -0.24). At T+213 s  the decay
flattens to an alpha of -0.06 (+0.24, -0.26) before breaking again at
T+1264 s to a final decay with index alpha=1.39 (+0.22, -0.14).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.93 (+0.32, -0.29). The
best-fitting absorption column is  4.7 (+1.0, -0.9) x 10^21 cm^-2, in
excess of the Galactic value of 1.5 x 10^21 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 2.15 (+0.22, -0.21)
and a best-fitting absorption column of 4.4 (+1.1, -1.0) x 10^21 cm^-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 3.6 x 10^-11 (6.3 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     4.4 (+1.1, -1.0) x 10^21 cm^-2
Galactic foreground: 1.5 x 10^21 cm^-2
Excess significance: 4.8 sigma
Photon index:	     2.15 (+0.22, -0.21)

If the light curve continues to decay with a power-law decay index of
1.39, the count rate at T+24 hours will be 2.8 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.0 x
10^-13 (1.8 x 10^-13) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01187463.

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