  We present a detailed analysis of Chandra X-ray spectra from individual
ejecta knots in the supernova remnant Cassiopeia A. The spectra are fitted to
give the electron temperature and (single) ionization age. These quantities are
compared with the predictions of self similar hydrodynamic models incorporating
time dependent ionization and radiation losses, and Coulomb electron-ion
equilibration behind the reverse shock, for a variety of different ejecta
density profiles described by a uniform density core and a power law envelope.
We find that the ejecta close to the "jet" region in the NE, but not actually
in the jet itself, have a systematically shallower outer envelope than ejecta
elsewhere in the remnant, and we interpret this as being due to more energy of
the initial explosion being directed in this polar direction as opposed to
equatorially. The degree of asymmetry we infer is at the low end of that
generally modelled in asymmetric core-collapse simulations, and may possibly be
used to rule out highly asymmetric explosion models.
