GCN Circular 35443

A. Tohuvavohu (U. Toronto), D.N. Burrows (PSU), J. D. Gropp (PSU), K.L.
Page (U. Leicester), A.P. Beardmore (U. Leicester), R. Brivio
(INAF-OAB), A. D'Ai (INAF-IASFPA), A. Melandri (INAF-OAR) and P.A.
Evans report on behalf of the Swift-XRT team:

We have analysed 9.1 ks of XRT data for GRB 231230A, from 127 s to 41.2
ks after the  BAT trigger. The data comprise 68 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 light curve can be modelled with an initial power-law decay with an
index of alpha=0.835 (+0.027, -0.028), followed by a break at T+17.4 ks
to an alpha of 2.0 (+/-0.4).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.93 (+/-0.13). The
best-fitting absorption column is  4.0 (+0.7, -0.6) x 10^21 cm^-2, in
excess of the Galactic value of 1.3 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 4.1 x 10^-11 (6.1 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     4.0 (+0.7, -0.6) x 10^21 cm^-2
Galactic foreground: 1.3 x 10^20 cm^-2
Excess significance: 9.9 sigma
Photon index:	     1.93 (+/-0.13)

If the light curve continues to decay with a power-law decay index of
2.0, the count rate at T+24 hours will be 5.1 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.1 x
10^-13 (3.1 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/01205319.

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