GCN Circular 10517

S. Campana (INAF-OAB) reports on behalf of the Swift-XRT team:

Swift XRT observed GRB 100316D (Stamatikos et al. 2010, GCN10496
Loading...
 
 )
in Windowed Timing (WT) mode in the 144-737 s time interval.
During this interval the light curve decays very slowly (decay
index alpha=-0.13, Starling et al. 2010, GCN 10505
Loading...
 
 ).
In addition, the 1.5-10 keV to 0.3-1.5 keV hardness ratio remains
constant (reduced chi2=0.91 with 97 degrees of freedom, dof).
Motivated by the high count rate (around 30 c s^-1, but far from
the pile up limit) and by the constant spectral shape we tried to
estimate the GRB redshift from the X-ray data.
We assume a Galactic column density of 7x10^20 cm^-2 (Kalberla et
al. 2005, A&A 440 775) and fit the WT spectrum taken from the
Leicester pages (http://www.swift.ac.uk/xrt_spectra/00416135/)
with an absorbed (phabs*zphabs) cutoff power law model (the cutoff
power law model provides much better results in terms of column
density evaluation with respect to a simple power law model when
small spectral variations are present). Given the high count rate
we selected single pixel events only.
The resulting fit is good (reduced chi2=1.2, 333 dof). The redshift
is constrained to lie within the 90% confidence level (delta
chi2=4.61) of 0.014<z<0.28 (with a best fit redshift of z=0.1 and
intrinsic column density of N_H(z)=7x10^21 cm^-2).
Filtering WT counts with grades 0-2 (standard WT grades) slightly
shifts the best fit range to larger redshifts (in the approximate
range 0.1-0.6).

The contour plot is available at
http://www.brera.inaf.it/utenti/campana/100316d.gif

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

[GCN OPS NOTE(18mar10):  Per author's request, the chi2=2.71 
was changed to chi2=4.61.]