Cassiopeia A

About: Cassiopeia A is a research topic. Over the lifetime, 773 publications have been published within this topic receiving 26680 citations.

The radio and X-ray emission from type II supernovae.

Abstract: The interaction of the outer parts of a supernova envelope with circumstellar matter gives rise to a high-energy density shell. The equation of motion of the shell is deduced based on the approximations that the shell is thin and that the supernova density profile is a power law in radius. The density structure in the shell is Rayleigh-Taylor unstable, and the energy density created by the instability can be a substantial fraction of the original thermal energy density. The instability can drive turbulent motions, and these may amplify the magnetic field and accelerate relativistic electrons. If the efficiency of these processes is comparable to that inferred for the Cassiopeia A supernova remnant, the observed radio luminosity from SN 1980k and SN 1979c can be reproduced. Several mechanisms are considered for the early low-frequency absorption of the radio emission. Free-free absorption by circumstellar matter is the most likely mechanism because of the steep time dependence of the radio emission and the magnitude of the absorption effect. If the circumstellar matter is smoothly distributed, it is inferred that the presupernova star of SN 1980k had a mass loss rate of about 10/sup -5/ M/sub sun/ yr/sup -1/ and that of SN 1979cmore » about 5 x 10/sup -5/ M/sub sun/ yr/sup -1/. Clumping of the matter would reduce the estimated mass loss rates. It is also inferred that SN 1979c had more high velocity matter than did SN 1980k. Thermal X-ray emission is expected from both the shocked circumstellar medium and the shocked supernova matter. The shocked supernova matter dominates the emission in the band observed with the Einstein Observatory, and it can produce the X-ray flux observed from SN 1980k. Inverse Compton emission is another possibility for the observed X-ray emission, but it is less likely because it would decrease the number of radio-emitting electrons. Subject headings: nebulae: supernova remnants: radiation mechanisms: radio sources: general: X-rays: sources« less

Evolution of Nonradiative Supernova Remnants

Abstract: We conduct an analytic and numerical study of the dynamics of supernova remnant (SNR) evolution from the ejecta-dominated stage through the Sedov-Taylor (ST) stage, the stages that precede the onset of dynamically significant radiative losses and/or pressure confinement by the ambient medium. We assume spherical symmetry and focus on the evolution of ejecta described by a power-law density distribution expanding into a uniform ambient medium. We emphasize that all nonradiative remnants of a given power-law structure evolve according to a single unified solution, and we discuss this general property in detail. Use of dimensionless quantities constructed from the characteristic dimensional parameters of the problem—the ejecta energy, ejecta mass, and ambient density—makes the unified nature of the solution manifest. It is also possible to obtain a unified solution for the ST and radiative stages of evolution, and we place our work in the context of scaling laws for solutions for SNR evolution in those stages. We present numerical simulations of the flow and approximate analytic solutions for the motions of both the reverse shock and blast-wave shock. These solutions follow the shocks through the nonradiative stages of remnant evolution across periods of self-similar flow linked by non-self-similar behavior. We elucidate the dependence of the ejecta-dominated evolution on the ejecta power-law index n by developing a general trajectory for all n and explaining its relation to the solutions of Chevalier and Nadyozhin for n>5 and Hamilton & Sarazin for n=0. We demonstrate excellent agreement between our analytic solutions and numerical simulations. These solutions should be valuable in describing remnants such as SN 1006, Tycho, Kepler, Cassiopeia A, and other relatively young SNRs that are between the early ejecta-dominated stage and the late Sedov-Taylor stage. In appendices, we extend our results to power-law ambient media, and we describe an early period of the evolution in which the SNR is radiative and evolves according to a nonunified solution.

High-energy particle acceleration in the shell of a supernova remnant.

Abstract: A significant fraction of the energy density of the interstellar medium is in the form of high-energy charged particles (cosmic rays)1. The origin of these particles remains uncertain. Although it is generally accepted that the only sources capable of supplying the energy required to accelerate the bulk of Galactic cosmic rays are supernova explosions, and even though the mechanism of particle acceleration in expanding supernova remnant (SNR) shocks is thought to be well understood theoretically2,3, unequivocal evidence for the production of high-energy particles in supernova shells has proven remarkably hard to find. Here we report on observations of the SNR RX J1713.7 - 3946 (G347.3 - 0.5), which was discovered by ROSAT4 in the X-ray spectrum and later claimed as a source of high-energy γ-rays5,6 of TeV energies (1 TeV = 1012 eV). We present a TeV γ-ray image of the SNR: the spatially resolved remnant has a shell morphology similar to that seen in X-rays, which demonstrates that very-high-energy particles are accelerated there. The energy spectrum indicates efficient acceleration of charged particles to energies beyond 100 TeV, consistent with current ideas of particle acceleration in young SNR shocks.

A New Σ-D Relation and Its Application to the Galactic Supernova Remnant Distribution

Abstract: Technological advances in radio telescopes and X-ray instruments over the last 20 years have greatly increased the number of known supernova remnants (SNRs) and have led to a better determination of their properties. In particular, more SNRs now have reasonably determined distances. However, many of these distances were determined kinematically using old rotation curves (based on R☉ = 10 kpc and V☉ = 250 km s-1). A more modern rotation curve (based on R☉ = 8.5 kpc and V☉ = 220 km s-1) is used to verify or recalculate the distances to these remnants. We use a sample of 36 shell SNRs (37 including Cassiopeia A) with known distances to derive a new radio surface brightness-to-diameter (Σ-D) relation. The slopes derived here (β = -2.64 including Cas A, β = -2.38 without Cas A) are significantly flatter than those derived in previous studies. An independent test of the accuracy of the Σ-D relation was performed by using the extragalactic SNRs in the Large and Small Magellanic Clouds. The limitations of the Σ-D relation and the assumptions necessary for its use are discussed. A revised Galactic distribution of SNRs is presented based on the revised distances as well as those calculated from this Σ-D relation. A scaling method is employed to compensate for observational selection effects by computing scale factors based on individual telescope survey sensitivities, angular resolutions, and sky coverage. The radial distribution of the surface density of shell SNRs, corrected for selection effects, is presented and compared with previous works.