GCN Circular 35291

S. Dichiara (PSU), A.P. Beardmore (U. Leicester), J.P. Osborne (U.
Leicester), E. Ambrosi	(INAF-IASFPA) , M. Capalbi (INAF-IASFPA), M.
Perri (SSDC & INAF-OAR), J.A. Kennea (PSU), A. Tohuvavohu (U. Toronto)
and P.A. Evans report on behalf of the Swift-XRT team:

We have analysed 6.5 ks of XRT data for GRB 231205B, from 224 s to 68.4
ks after the  BAT trigger. The data comprise 3 s in Windowed Timing
(WT) mode (taken while Swift was slewing), with the remainder in Photon
Counting (PC) mode. 

The light curve can be modelled with an initial power-law decay with an
index of alpha=0.27 (+0.04, -0.05), followed by a break at T+6137 s to
an alpha of 1.32 (+/-0.08).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.06 (+/-0.10). The
best-fitting absorption column is  4.5 (+/-0.5) x 10^21 cm^-2, in
excess of the Galactic value of 1.5 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 3.8 x 10^-11 (6.2 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     4.5 (+/-0.5) x 10^21 cm^-2
Galactic foreground: 1.5 x 10^21 cm^-2
Excess significance: 9.6 sigma
Photon index:	     2.06 (+/-0.10)

If the light curve continues to decay with a power-law decay index of
1.32, the count rate at T+24 hours will be 0.063 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.3 x
10^-12 (3.9 x 10^-12) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01200812.

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