GCN Circular 15648

V. Mangano (PSU), K. Page (U. Leicester), D. Malesani (DARK/NBI) 
report on behalf of the Swift-XRT team:

We have analysed 4.0 ks of XRT data for the Fermi/LAT-detected burst:
GRB 131231A,  from 52.1 ks to 62.5 ks after the  Fermi/LAT trigger. The
data are entirely in Photon Counting (PC) mode. 
An X-ray source is detected at a position consistent with the optical afterglow 
(Xu et al., GCN 15641
Not found
 
 
) at a mean count rate of 0.4 ct/s.
The refined XRT position is RA, Dec = 10.5904, -1.6519  which is equivalent to:

RA (J2000): 00 42 21.70
Dec(J2000): -01 39 06.7

with an uncertainty of 3.5 arcsec (radius, 90% confidence). This
position is 695 arcsec from the Fermi/LAT position and 4 arcsec
from the optical afterglow.

The light curve can be modelled with a power-law decay with an
index of alpha=1.6 (+/-0.7). 

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.86 (+0.13, -0.13). The
best-fitting absorption column at the redshift of 0.642 (Xu et al., GCN 15645
Not found
 
 
)
is  1.4 (+0.7, -0.6) x 10^21 cm^-2, in excess of the Galactic value of 
2.6 x 10^20 cm^-2 (Kalberla et al. 2005). 
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced 
from this spectrum  is 3.0 x 10^-11 (4.0 x 10^-11) erg cm^-2 count^-1. 

Further ToO observations have already been scheduled for today.

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