A high resolution X-ray image of Tycho's supernova remnant obtained from the Einstein Observatory reveals three components of X-ray emission that we identify with shocked interstellar material, diffuse ejecta, and clumpy ejecta. This picture is applied to derive the mass of X-ray emitting material. Assuming a distance of 3 kpc, an absorbing column density of 3 × 1021 atoms/cm2, and using an ion-electron non-equilibrium calculation for the emissivity, we find the average density of the ISM is 0.4 atoms/cm3, and the energy contained in the remnant is 1.4 × 1051 ergs. The total mass of X-ray emitting material in the remnant is ≈4 M⊙, ≈2 M⊙ ejecta and ≈2 M⊙ swept up, putting the remnant at an intermediate state between a free expansion and the Sedov phase. There is no evidence for neutron star. The upper limit on the surface temperature is in the range 1.1 to 1.8 × 106K.

The early identification of the strong X-ray source in Taurus with the Crab nebula (Bowyer et al. 1964) was the first milestone in the association of X-ray emission with supernova remnants. Unfortunately, it proved to be “red herring” which clouded the interpretation of X-ray emission from supernova remnants for a decade. Because the Crab was one of the brightest X-ray sources in the sky at a few keV, the interrogation energy of the early surveys, and because it was the first (and for several years the only) X-ray source conclusively identified, the potential association of a supernova origin with the large body of unidentified X-ray sources was not an unreasonable hypothesis.

We present the results of fitting an ionization nonequilibrium (NIE) model to the high energy (> 5 keV) X-ray spectra of the young supernova remnants Cas A and Tycho. As an additional constraint, we demand that the models simultaneously fit lower energy, higher resolution data. For Cas A, a single NIE component can not adequately reproduce the features for the entire X-ray spectrum because 1) the ionization structure of iron ions responsible for the K emission is inconsistent with that of the ions responsible for the lower energy lines, and 2) the flux of the highest energy X-rays is underestimated. The iron K line and the high energy continuum could arise from the same NIE component but the identification of this component with either the blast wave or the ejecta in the “standard” model is difficult. In Tycho, the high energy data rule out a class of models for the lower energy data which have too large a continuum contribution.

Structural changes have been found and investigated in a handful of young supernova remnants (SNR). The variations in the Crab Nebula (Trimble, 1968) have been known for some time. Here I am concerned with young shell remnants of which the best studied is Cas A. It is, like the Crab nebula, undergoing an overall radial expansion, although other forms of motion have also been observed. Simple expansion has been found in Tycho (3C10) and the remnant of SN 1006. Optical measurements of the remnant of Kepler's supernova (SN 1604), however, have failed to show motion of the magnitude expected from this relatively young SNR.

This paper describes a detailed comparison of radio maps of Cas A obtained with the Cambridge 5 km Radio Telescope in 1974 and 1978. Accurate proper motions and brightness changes are presented for 342 distinct radio peaks, ranging in angular size from 1 arc minute for fragments of the intense ring to less than the resolution limit of 2.0 × 2.3 arc seconds for the compact radio knots.

During the last few years radio and x-ray astronomers have produced high-resolution imagery of the remnant of Cas A. Since the most recent published optical photographs of Cas A date back to 1975 (Kamper and van den Bergh 1976 a, b) it seemed worthwhile to present new optical results based on plates obtained with the 5-m Hale telescope in 1976, 1977 and 1980. A photograph taken in 1980 is shown in Fig. 1.

We report on evidence for weak large scale (r ≥ 6 arcmin) X-ray emission from Cassiopeia A. We investigate several mechanisms for producing such an X-ray halo. Further observations will be required to determine which mechanism is operative.

The radio source Sgr A has been mapped with the Very Large Array (VLA) at 6 and 20 cm with an angular resolution of 5″ x 8″ arc. In agreement with the earlier “WORST” map, the non-thermal source Sgr A East shows a shell structure, while the thermal source Sgr A West shows a spiral-like morphology. We suggest that Sgr A East is a supernova remnant (SNR) near the galactic centre. Its surface brightness is the third largest in our galaxy after Cas A and the Crab Nebula. The diameter is 9 pc and the source fits the surface-brightness diameter relationship of Clark and Caswell (1976) if a distance of 10 kpc is assumed.

Plausible environments for supernovae are the interstellar medium with constant density or a circumstellar medium built up by mass loss with ρ ∝ r−n Self-similar solutions for the interaction region between the expanding supernova gas and the ambient gas exist provided that the expanding gas has ρ ∝ r−n with n > 5. The circumstellar medium case is likely to be important for the early evolution of Type II supernovae because their progenitor stars are probably red supergiants. The radio and X-ray emission observed from extragalactic supernovae may be from this interaction region. The early self-similar solutions can also be applied to the young galactic remnants.

After the supernova shock wave has swepted up about 8–10 stellar masses of interstellar material, the SNR structure is well described by blast wave theory (eg. Sedov 1959, Chevalier 1977). In fact, both numerical calculations of the early phases (Jones, Smith, and Straka 1981) and small scale, laboratory simulations (Wilke 1982) show transition to blast wave at 8–10 masses. While the late stages have been well understood for some time, the early stages have only been crudely modeled until very recently.

Young SNRs expand to much larger radii in a cloudy ISM than in a homogeneous medium, and they can have large variations in the pressure. The collision between supernova ejecta and an ambient cloud can result in an expanding high pressure region (a “secondary blast wave”). Observations of MSH 15–52 can be accounted for in this manner. X-ray emission from both young and older SNRs can provide an important probe for inferring the structure of the ISM.

X-ray spectra of young supernova remnants (SNR's) are perhaps the most spectacular examples of hot, line-emitting astrophysical plasmas. Heated to temperatures of 1 to 10 keV and enriched with the heavy element products of stellar nucleosynthesis, the plasma inside these SNR's emits prodigiously in lines of 0, Ne, Mg, Si, S, Ar, Ca, and Fe. Theoretical models of this emission provide measures of the plasma temperature and density, elemental abundances, and the degree of approach to ionization equilibrium. Thus, astrophysicists are offered the opportunity to test their understanding of the supernova explosion, its interaction with the interstellar medium, and the nucleo-synthetic processes which enrich our galaxy with heavy elements.

An extensive grid of nonequilibrium ionization models for the X-ray spectra of adiabatic supernova remnants (SNRs) is described. The models are compared to the SSS spectra of remnants in the LMC, the Tycho SNR, and SNR 1006. In Tycho, we show that the observed spectrum requires significantly enhanced abundances of Si and S, and that this conclusion is independent of the detailed ionization and thermal structure in the remnant. We find that the SSS spectrum of SNR 1006 can be fit reasonably by a thermal emission model with abundances of about one half solar. In this model, the weak line emission results from the very low ionization state in the remnant, and not because the X-ray emission is non-thermal. We argue that the failure to detect strong Fe line emission in young Type I SNRs poses a severe problem for models of Type I SN, which predict that most of the ejecta be iron. Finally, the results of UV observations of a star behind SNR 1006 are mentioned; these observations show that the remnant contains a large amount of rapidly moving, cold iron.

A technique is presented for characterizing the ionization structure and consequent thermal x-ray emission of a SNR when non-equilibrium ionization effects are important. The technique allows different theoretical SNR models to be compared and contrasted rapidly in advance of detailed numerical computations. In particular it is shown that the spectrum of a Sedov remnant can probably be applied satisfactorily in a variety of SNR structures, including the reverse shock model advocated by Chevalier (1982) for Type I SN, the isothermal similarity solution of Solinger, Rappaport and Buff (1975), and various inhomogenous or ‘messy’ structures.

Einstein X-ray observations of the young supernova remnants Cassiopeia A (Murray et al. 1980) and Tycho (Seward, Gorenstein and Tucker 1982) indicate that the swept-up mass does not much exceed that of the observed ejecta. The initial density distribution of the ejecta and surrounding material is then important in determining the X-ray structure and evolution. Some aspects of this behaviour have been dealt with in previous numerical (e.g. Gull 1973; Itoh 1977; Jones, Smith and Straka 1981) and analytical (e.g. Chevalier 1982a,b) studies. We present here results obtained from numerical models covering a wider range of initial conditions. In particular, we consider the effect of a constant stellar wind from the progenitor star on the expansion of the remnant. We have previously suggested that variable mass loss from SN1006 may explain its warm filled interior (Fabian, Stewart and Brinkmann 1982).

The “snow-plough” approximation is applied to the problem of the propagation of the shock wave from a nonsymmetrical supernova explosion in a uniform medium. The spherization of the shape of the remnant is obtained. The results are in good agreement with the observed properties of Cas A.

The Crab Nebula has been intensely studied by X-ray astronomers ever since its discovery as the first, optically identified X-ray object (Bowyer et al. 1964); and a large majority of X-ray experiments during the past two decades have observed the Crab, seeking not only the answers to scientific questions but also assurance that the instruments' calibrations were understood. It is therefore no surprise that, following its launch in 1978 November, the Einstein X-ray Observatory too had the Crab Nebula on its list of mandatory targets.