X-ray spectral imaging and Doppler mapping of Cassiopeia A

¹ Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK
² SRON Space Research Institute, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands e-mail: J.A.M.Bleeker@sron.nl; K.J.van.der.Heyden@sron.nl; j.s.kaastra@sron.nl
³ Columbia Astrophysics Laboratory, Columbia University, 550 West 120th Street, New York, NY 10027, USA e-mail: jvink@astro.columbia.edu

Corresponding author: R. Willingale, rw@star.le.ac.uk

Received: 17 July 2001
Accepted: 7 November 2001

Abstract

We present a detailed X-ray spectral analysis of Cas A using a deep exposure from the EPIC-MOS cameras on-board XMM-Newton. Spectral fitting was performed on a 1515 grid of pixels using a two component non-equilibrium ionisation model (NEI) giving maps of ionisation age, temperature, interstellar column density, abundances for Ne, Mg, Si, S, Ca, Fe and Ni and Doppler velocities for the bright Si-K, S-K and Fe-K line complexes. The abundance maps of Si, S, Ar and Ca are strongly correlated. The correlation is particularly tight between Si and S. The measured abundance ratios are consistent with the nucleosynthesis yield from the collapse of a progenitor star of 12 at the time of explosion. The distributions of the abundance ratios Ne/Si, Mg/Si, Fe/Si and Ni/Si are very variable and distinctly different from S/Si, Ar/Si and Ca/Si. This is also expected from the current models of explosive nucleosynthesis. The ionisation age and temperature of both the hot and cool NEI components varies considerably over the remnant. Accurate determination of these parameters has enabled us to extract reliable Doppler velocities for the hot and cold components. The combination of radial positions in the plane of the sky and velocities along the line of sight have been used to measure the dynamics of the X-ray emitting plasma. The data are consistent with a linear radial velocity field for the plasma within the remnant with  km s^-1 at  arcsec implying a primary shock velocity of  km s^-1 at this shock radius. The Si-K and S-K line emission from the cool plasma component is confined to a relatively narrow shell with radius 100–150 arcsec. This component is almost certainly ejecta material which has been heated by a combination of the reverse shock and heating of ejecta clumps as they plough through the medium which has been pre-heated by the primary shock. The Fe-K line emission is expanding somewhat faster and spans a radius range 110–170 arcsec. The bulk of the Fe emission is confined to two large clumps and it is likely that these too are the result of ablation from ejecta bullets rather than swept up circumstellar medium.