This is the first study of the expansion rate of Cas A as seen in X-rays. A similar study was done for Tycho's SNR (Hughes 1997) using a somewhat different method. Measuring the expansion timescale of a SNR in X-rays has one major advantage over similar studies in the radio and the optical bands: the X-ray emission is directly connected to the bulk mass of SNRs.
We have found an overall expansion timescale of yr, corresponding to an expansion rate of %yr-1. The optical expansion timescale is shorter, namely of the order of 340 yr. This can be easily understood, since the optical knots consist of dense gas which is expected to be less decelerated than the bulk of the ejecta. More intricate is the discrepancy between our expansion timescale and the expansion timescale based on the compact radio features ( yr), since also these radio features are thought to be the marks of denser gas moving through a more diffuse medium. This explanation is clearly not in agreement with our expansion timescale as we discussed in the previous paragraph. However, we lack a coherent picture of the dynamics of the compact radio features. Furthermore, as we already pointed out, the radio image is brightest in the West of Cas A, whereas at the photon energies where the ROSAT and Einstein HRI are most sensitive Cas A is brightest in the East. This may have caused different biases in the radio and X-ray measurements.
An expansion timescale of 501 yr means that the standard reverse-shock Chevalier (1982) model for a SNR moving through the wind of its progenitor is not applicable to Cas A. It either means that Cas A is in a transition phase between the reverse-shock dominated and the blast-wave dominated phase or it means that the situation is too complex to be well described by simple analytical SNR models.
In the near future high spatial resolution images of Cas A obtained with AXAF will be able to improve the existing measurements. Meanwhile there are other outstanding issues concerning the dynamics of Cas A such as the front back asymmetry of optical knots (Reed et al. 1995) and the North/South asymmetry in the Doppler shifts observed in the X-ray spectra (Markert et al. 1983). Together with the expansion rate these facts should ultimately be incorporated into a complete three dimensional dynamical model of Cas A.
Note. During the refereeing process a preprint appeared with a similar measurement as presented in this paper (Koralesky et al. 1998). Their results confirm our analysis.
© European Southern Observatory (ESO) 1998
Online publication: September 30, 1998