GCN Circular 34444

P. D'Avanzo (INAF-OAB), M.G. Bernardini (INAF-OAB), D.N. Burrows (PSU),
J. D. Gropp (PSU), S. Dichiara (PSU), J.P. Osborne (U. Leicester), K.L.
Page (U. Leicester), A.P. Beardmore (U. Leicester), E. Ambrosi 
(INAF-IASFPA)  and P.A. Evans report on behalf of the Swift-XRT team:

We have analysed 6.0 ks of XRT data for GRB 230815A, from 117 s to 45.2
ks after the  BAT trigger. The data comprise 574 s in Windowed Timing
(WT) mode (the first 9 s were taken while Swift was slewing) with the
remainder in Photon Counting (PC) mode. 

The late-time light curve (from T0+6.1 ks) can be modelled with a
power-law decay with a decay index of alpha=1.94 (+0.25, -0.29).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.92 (+0.06, -0.05). The
best-fitting absorption column is  7.4 (+0.5, -0.4) x 10^21 cm^-2, in
excess of the Galactic value of 4.9 x 10^21 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.94 (+0.18, -0.17)
and a best-fitting absorption column of 9.1 (+1.7, -1.5) 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 (8.5 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     9.1 (+1.7, -1.5) x 10^21 cm^-2
Galactic foreground: 4.9 x 10^21 cm^-2
Excess significance: 4.5 sigma
Photon index:	     1.94 (+0.18, -0.17)

If the light curve continues to decay with a power-law decay index of
1.94, the count rate at T+24 hours will be 2.5 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.2 x
10^-13 (2.1 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/01185505.

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