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GCN Circular 34345

Subject
GRB 230805B: Swift-XRT refined Analysis
Date
2023-08-05T21:19:54Z (9 months ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
A. D'Ai (INAF-IASFPA), A. Melandri (INAF-OAR), T. Sbarrato (INAF-OAB),
S. Dichiara (PSU), J.A. Kennea (PSU), A. Tohuvavohu (U. Toronto), J.P.
Osborne (U. Leicester), K.L. Page (U. Leicester) and P.A. Evans report
on behalf of the Swift-XRT team:

We have analysed 6.0 ks of XRT data for GRB 230805B, from 210 s to 25.0
ks after the  BAT trigger. The data are entirely in Photon Counting
(PC) mode. The best available XRT position  (using the promptly
downlinked event data, the XRT-UVOT alignment and matching UVOT field
sources to the USNO-B1 catalogue) is RA, Dec = 207.74271, 31.19102
which is equivalent to:

RA (J2000): 13 50 58.25
Dec(J2000): +31 11 27.7

with an uncertainty of 1.9 arcsec (radius, 90% confidence).

The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=3.7 (+0.9, -0.4). At T+650 s  the decay
flattens to an alpha of -0.22 (+0.72, -0.08) before breaking again at
T+4027 s to a final decay with index alpha=1.7 (+0.6, -0.5).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.76 (+0.26, -0.24). The
best-fitting absorption column is  8.9 (+7.5, -6.2) x 10^20 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 3.8 x 10^-11 (4.4 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     8.9 (+7.5, -6.2) x 10^20 cm^-2
Galactic foreground: 1.4 x 10^20 cm^-2
Excess significance: 2.0 sigma
Photon index:	     1.76 (+0.26, -0.24)

If the light curve continues to decay with a power-law decay index of
1.7, the count rate at T+24 hours will be 5.6 x 10^-4 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.1 x
10^-14 (2.5 x 10^-14) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01183217.

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