X-Ray Spectra of Cassiopeia A
We calculated X-ray spectra for our 1D hydrodynamical models as described in our
earlier work on Kepler's SNR (Borkowski, Sarazin, & Blondin 1994). Briefly,
in order to find how electron temperatures and electron
densities varied for each fluid element, we performed simulations with
a Lagrangian version of the VH-1 hydrocode, and stored pressures and densities
as a function of time for each fluid element. Next, we calculated electron
temperatures taking into account possible collisionless heating of electrons at the
blast wave, and calculating their subsequent heating in Coulomb collisions with ions.
(We found the best agreement for the case without any collisionless heating at
the reverse shock. We therefore present results for the case with Coulomb
collisions being the sole heating source for electrons.)
Ionic states
in each fluid element were then found by solving the time-dependent
ionization equations.
We used an updated version of the X-ray code written by
Hamilton & Sarazin (1984) to accomplish this task.
The chemical abundances of elements producing the X-ray lines seen
in the
Figure can be determined by fitting model spectra to the
ASCA spectrum, and allowing elemental abundances to vary. This procedure is
straightforward for the CSM which is chemically homogeneous, but for the
SN ejecta we must allow for the element stratification. Because we use
very simple SN ejecta models, a two-zone, chemically stratified model seems
appropriate. The outer zone is He-rich, without any H, with the abundances
of heavier elements equal to those of the CSM shell, while in the innermost
ejecta we allowed elements heavier than Ne to vary.
We also allowed for small
changes in the normalization of the model spectrum, and left the interstellar
absorption as a free parameter. All spectral fitting was performed using
XSPEC v9.