GCN Circular 43275

T. Sbarrato (INAF-OAB), P. D'Avanzo (INAF-OAB), M.G. Bernardini
(INAF-OAB), D.N. Burrows (PSU), S. Lanava (PSU), S. Dichiara (PSU),
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 5.6 ks of XRT data for GRB 251230A, from 92 s to 50.9
ks after the   trigger. The data comprise 329 s in Windowed Timing (WT)
mode (the first 6 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=7.1 (+/-0.5). At T+129 s  the decay
flattens to an alpha of 0.2 (+0.3, -1.1) before breaking again at T+194
s to a final decay with index alpha=1.51 (+/-0.05).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.80 (+/-0.06). The
best-fitting absorption column is  4.4 (+1.3, -1.2) x 10^20 cm^-2, in
excess of the Galactic value of 1.9 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.82 (+/-0.14) and a
best-fitting absorption column of 5.7 (+3.6, -3.2) x 10^20 cm^-2. The
counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 3.6 x 10^-11 (4.1 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     5.7 (+3.6, -3.2) x 10^20 cm^-2
Galactic foreground: 1.9 x 10^20 cm^-2
Excess significance: 2.0 sigma
Photon index:	     1.82 (+/-0.14)

If the light curve continues to decay with a power-law decay index of
1.51, the count rate at T+24 hours will be 2.3 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 8.4 x
10^-14 (9.4 x 10^-14) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
https://www.swift.ac.uk/xrt_products/01429020.

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