Jets with entrained clouds - II. The synchrotron spectrum and emission structure

Abstract

Hydrodynamic simulations of adiabatic bow shocks in a supersonic flow are used to investigate the properties of synchrotron sources such as the knots in radio jets and protrusions in supernova remnants. Standard treatments of shock acceleration are used to obtain the relativistic electron distribution function. Source characteristics are derived by using simulation results for density and field strength to obtain the spectral emissivity throughout the flow. Numerical integration over volume then yields the source spectrum, and integration along the line-of-sight gives the surface brightness distribution. Simulated spectra closely resemble the spectra of knots in the M87 jet, which are power laws at low frequency, and break sharply (Deltaalpha ≈ 1) in the optical. A comparison of the surface brightness distributions with observations of protrusions in SN remnant Cas A supports the idea that these features are bow shocks in the supernova ejecta.