Title: X-Ray and Gamma-Ray Emission from Middle-aged Supernova Remnants in Cavities. I. Spherical Symmetry,
Abstract: We present analytical and numerical studies of models of supernova-remnant
(SNR) blast waves expanding into uniform media and interacting with a denser
cavity wall, in one spatial dimension. We predict the nonthermal emission from
such blast waves: synchrotron emission at radio and X-ray energies, and
bremsstrahlung, inverse-Compton emission (from cosmic-microwave-background seed
photons, ICCMB), and emission from the decay of $\pi^0$ mesons produced in
inelastic collisions between accelerated ions and thermal gas, at GeV and TeV
energies. Accelerated particle spectra are assumed to be power-laws with
exponential cutoffs at energies limited by the remnant age or (for electrons,
if lower) by radiative losses. We compare the results with those from
homogeneous ("one-zone") models. Such models give fair representations of the
1-D results for uniform media, but cavity-wall interactions produce effects for
which one-zone models are inadequate. We study the time evolution of SNR
morphology and emission with time. Strong morphological differences exist
between ICCMB and $\pi^0$-decay emission, at some stages, the TeV emission can
be dominated by the former and the GeV by the latter, resulting in strong
energy-dependence of morphology. Integrated gamma-ray spectra show apparent
power-laws of slopes that vary with time, but do not indicate the energy
distribution of a single population of particles. As observational capabilities
at GeV and TeV energies improve, spatial inhomogeneity in SNRs will need to be
accounted for.