GCN Circular 10588

S. Campana (INAF-OAB), P.A. Evans (U. Leicester) and S. T. Holland
(CRESST/USRA/GSFC) report on behalf of the Swift-XRT team:

Swift XRT observed GRB 100413A (Holland et al. 2010, GCN 10581
Not found
 
 
)
in Windowed Timing (WT) mode in the 146-550 s time interval.
During this interval the 1.5-10 keV to 0.3-1.5 keV hardness ratio slightly
decayed, testifying for small spectral variations.
We assume a Galactic column density of 7.3x10^20 cm^-2 (Kalberla et
al. 2005, A&A 440 775) and fit the WT spectrum with an absorbed
(tbabs*ztbabs) cutoff power law model within XSPEC (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).
Data were grouped to have 30 counts per spectral bin. The resulting fit 
is good
(reduced chi2=0.9, 194 dof).
Filtering the data for single pixel events does not change the results.
In the intrinsic column density vs. redshift plane there is a low 
significance (3 sigma)
solution at low redshifts but the minimum lies at higher redshifts with 
a 2 sigma confidence
level solution of z=3.9+1.7-0.7 and N_H(z)=(5.5+9.8-2.1)x1022 cm-2.
Thus, even if we cannot exclude that GRB100413A is at low redshift, a 
high redshift (z~4),
highly absorbed (N_H(z)~6x1022 cm-2) solution is preferred.
This compares well with the non-detection with UVOT (Holland et al. 
2010) and
other robotic telescopes.

The contour plot is available at
http://www.brera.inaf.it/utenti/campana/100413.gif