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

Open Access
Publisher Website
Google Scholar
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 for two limiting cases, in which pressure is due to either thermal plasma (γ = 5/3) or relativistic particles (γ = 4/3). Downstream evolution of the electron distribution function is mediated by both adiabatic and radiative (synchrotron) losses, producing a high-momentum cut-off which migrates to lower momenta with increasing distance from the shock. Source characteristics are derived by using simulation results for density and field strength (Paper I) 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 (Δα ≈ 1) in the optical. The spectral break is sensitive to the pre-shock toroidal field strength, but a good fit to the low-frequency spectral index is obtained for Mach numbers M ≈ 5 (γ = 5/3) and M ≈ 7 (γ = 4/3). A comparison of the surface brightness distributions with observations of protrusions in SN remnant CasA supports the idea that these features are bow shocks in the supernova ejecta.

This publication has 0 references indexed in Scilit: