GCN Circular 18830

A. Melandri, P. D'Avanzo (INAF-OAB) and P.A. Evans (U. Leicester)
report on behalf of the Swift-XRT team:

We have analysed 7.5 ks of XRT data for GRB 160104A (Melandri et al.
GCN Circ. 18815
Loading...
 
 ), from 63 s to 36.3 ks after the  BAT trigger. The data
comprise 914 s in Windowed Timing (WT) mode (the first 9 s were taken
while Swift was slewing) with the remainder in Photon Counting (PC)
mode. The enhanced XRT position for this burst was given by Beardmore
et al. (GCN Circ. 18821
Loading...
 
 ).

The light curve can be modelled with an initial power-law decay with an
index of alpha=3.1 +/- 0.4, followed by a break at T+269 s to an alpha
of 0.53 (+0.14, -0.17). Flaring activity is observed between T0+200 s
and T0+2.0 ks.

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.95 (+0.18, -0.08). The
best-fitting absorption column is  consistent with the Galactic value
of 2.7 x 10^21 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 (5.2 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.7 (+/-0.6) x 10^21 cm^-2
Galactic foreground: 2.7 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.95 (+0.18, -0.08)

If the light curve continues to decay with a power-law decay index of
0.53, the count rate at T+24 hours will be 0.01 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.7 x
10^-13 (5.2 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/00669319.

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