GCN Circular 42860

K.L. Page (U. Leicester), M. Capalbi (INAF-OAR), M. Perri (SSDC &
INAF-OAR), V. D'Elia (SSDC & INAF-OAR), S. Dichiara (PSU), J.A. Kennea
(PSU), D.N. Burrows (PSU), A.P. Beardmore (U. Leicester) and P.A. Evans
report on behalf of the Swift-XRT team:

We have analysed 6.2 ks of XRT data for GRB 251126A, from 66 s to 56.7
ks after the   trigger. The data comprise 9 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 a series of power-law decays. The
initial decay index is alpha=3.5 (+0.6, -0.5). At T+210 s  the decay
flattens to an alpha of -0.50 (+0.00, -0.17) before breaking again at
T+1954 s to a final decay with index alpha=1.13 (+0.21, -0.16).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.11 (+0.25, -0.24). The
best-fitting absorption column is  1.7 (+0.8, -0.7) x 10^21 cm^-2, in
excess of the Galactic value of 9.5 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 3.3 x 10^-11 (4.6 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.7 (+0.8, -0.7) x 10^21 cm^-2
Galactic foreground: 9.5 x 10^20 cm^-2
Excess significance: 1.8 sigma
Photon index:	     2.11 (+0.25, -0.24)

If the light curve continues to decay with a power-law decay index of
1.13, the count rate at T+24 hours will be 8.5 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.8 x
10^-13 (3.9 x 10^-13) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
https://www.swift.ac.uk/xrt_products/01417793.

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