On the Determination of Ejecta Structure and Explosion Asymmetry from the X‐Ray Knots of Cassiopeia A

We present a detailed analysis of Chandra X-ray spectra from individualejecta knots in the supernova remnant Cassiopeia A. The spectra are fitted togive the electron temperature and (single) ionization age. These quantities arecompared with the predictions of self similar hydrodynamic models incorporatingtime dependent ionization and radiation losses, and Coulomb electron-ionequilibration behind the reverse shock, for a variety of different ejectadensity 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 actuallyin the jet itself, have a systematically shallower outer envelope than ejectaelsewhere in the remnant, and we interpret this as being due to more energy ofthe initial explosion being directed in this polar direction as opposed toequatorially. The degree of asymmetry we infer is at the low end of thatgenerally modelled in asymmetric core-collapse simulations, and may possibly beused to rule out highly asymmetric explosion models.Comment: 32 pages, submitted to Ap