GCN Circular 13222

P.A. Evans (U. Leicester), O.M. Littlejohns (U. Leicester), V. Mangano
(INAF-IASFPA), A. Melandri (INAF-OAB), B. Sbarufatti (INAF-OAB/PSU),
M.C. Stroh (PSU), D.N. Burrows (PSU), J.A. Kennea (PSU), J.P. Osborne
(U. Leicester) and G. Stratta report on behalf of the Swift-XRT team:

We have analysed 7.7 ks of XRT data for GRB 120404A (Stratta  et al.
GCN Circ. 13208
Not found
 
 
), from 119 s to 23.3 ks after the  BAT trigger. The
data comprise 109 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
Osborne et al. (GCN. Circ 13218).

The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=2.15 (+0.18, -0.17). At T+544 s  the decay
flattens to an alpha of 0.1 (+0.4, -0.6) before breaking again at
T+2930 s to a final decay with index alpha=1.83 (+0.21, -0.19).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.72 (+0.14, -0.13). The
best-fitting absorption column is  2.3 (+4.7, -2.3) x 10^21 cm^-2, at a
redshift of 2.87, in addition to the Galactic value of 3.4 x 10^20
cm^-2 (Kalberla et al. 2005). The PC mode spectrum has a photon index
of 1.90 (+/-0.12) and a best-fitting absorption column of 4.5 (+4.3,
-4.0) x 10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV
flux conversion factor deduced from this spectrum  is 3.7 x 10^-11 (4.3
x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Galactic foreground: 3.4 x 10^20 cm^-2
Intrinsic column:    4.5 (+4.3, -4.0) x 10^21 cm^-2 at z=2.87
Photon index:	     1.90 (+/-0.12)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00519380.

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