GCN Circular 43320

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

We have analysed 6.3 ks of XRT data for GRB 260104A, from 101 s to 28.3
ks after the   trigger. The data comprise 10 s in Windowed Timing (WT)
mode (taken while Swift was slewing), with the remainder in Photon
Counting (PC) mode. 

The late-time light curve (from T0+5.1 ks) can be modelled with a
power-law decay with a decay index of alpha=1.22 (+0.26, -0.23).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.78 (+0.16, -0.10). The
best-fitting absorption column is  3.55 (+3.69, -0.15) x 10^20 cm^-2,
consistent with the Galactic value of 3.4 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 3.7 x 10^-11 (4.0 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     3.55 (+3.69, -0.15) x 10^20 cm^-2
Galactic foreground: 3.4 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.78 (+0.16, -0.10)

If the light curve continues to decay with a power-law decay index of
1.22, the count rate at T+24 hours will be 4.6 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.7 x
10^-13 (1.8 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/01430212.

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