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GCN Circular 34318

GRB 230802A: Swift-XRT refined Analysis
2023-08-02T20:28:29Z (10 months ago)
Phil Evans at U of Leicester <>
A.P. Beardmore (U. Leicester), B. Sbarufatti (INAF-OAB), A. D'Ai
(INAF-IASFPA), A. Melandri (INAF-OAR), J. D. Gropp (PSU), S. Dichiara
(PSU), J.A. Kennea (PSU), J.P. Osborne (U. Leicester) and P.A. Evans
report on behalf of the Swift-XRT team:

We have analysed 7.9 ks of XRT data for GRB 230802A, from 52 s to 40.1
ks after the  BAT trigger. The data comprise 386 s in Windowed Timing
(WT) mode (the first 10 s were taken while Swift was slewing) with the
remainder in Photon Counting (PC) mode. 

The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=0.4 (+/-0.4). At T+345 s  the decay
steepens to an alpha of 2.5 (+0.4, -0.3) before breaking again at
T+1364 s to a final decay with index alpha=1.26 (+0.16, -0.30).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.02 (+/-0.04). The
best-fitting absorption column is  6.5 (+/-0.5) x 10^21 cm^-2, in
excess of the Galactic value of 3.4 x 10^21 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.66 (+0.22, -0.21)
and a best-fitting absorption column of 4.5 (+1.5, -1.1) x 10^21 cm^-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 4.9 x 10^-11 (6.5 x 10^-11) erg cm^-2

A summary of the PC-mode spectrum is thus:
Total column:	     4.5 (+1.5, -1.1) x 10^21 cm^-2
Galactic foreground: 3.4 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.66 (+0.22, -0.21)

If the light curve continues to decay with a power-law decay index of
1.26, the count rate at T+24 hours will be 1.5 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 7.2 x
10^-14 (9.7 x 10^-14) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at

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

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