F. Bocchino, A. Maggio, S. Sciortino
Istituto e Osservatorio Astronomico di Palermo
Piazza del Parlamento 1, 90134 Palermo, ITALY
The Astrophysical Journal, June 1, 1997 issue, Vol. 481.
t
{
We have developed a grid of Non Equilibrium of Ionization (NEI) models
which depends on the electron temperature and ionization time, and
which is suitable to be used for fitting X-ray spectra. Using this
grid, we have carried out spatially resolved spectral analysis of a
ROSAT Position Sensitive Proportional Counter (PSPC) observation of the
North-East shock region of the Vela supernova remnant (SNR) on an
angular scale of 
(
pc at the distance of the
remnant). The adopted model passes the 
test at the 95%
confidence level on the 45% of the considered spatial bins, giving
unexpected low values of the ionization time (
yr
cm
). We discuss the possibility that this result may be due to
a reduced contribution from
line emission, which may be related to locking of metals in grains. We
also find significant temperature gradients on the scale explored, and
we argue that they are correlated to ISM inhomogeneities encountered by
the shock front. The emission from the remaining 55% of the bins is
incompatible with the single-tau single-temperature NEI model, and a feasible
description rests on the two temperature (2T) Collisional Equilibrium of
Ionization (CIE) model we have adopted in our previous analysis of the
same data set. We temptatively interpret our results
in terms of superposition of shocks propagating in an
inhomogeneous medium, which produces a multi-temperature and
multi-ionization stage plasma.