Non-Maxwellian Proton Velocity Distributions in Nonradiative Shocks

TL;DR

This paper investigates non-Maxwellian proton velocity distributions in nonradiative shocks of supernova remnants, showing that assuming Maxwellian distributions can lead to underestimating shock speeds and affecting magnetic field estimates.

Contribution

It predicts nonthermal proton velocity distributions from neutral atom ionization and assesses their impact on shock parameter measurements from H alpha line profiles.

Findings

Maxwellian assumption can underestimate shock speed by up to 15%.

Pickup ion populations can significantly heat thermal particles.

Profiles can reveal magnetic field strength and direction.

Abstract

The Balmer line profiles of nonradiative supernova remnant shocks provide the means to measure the post-shock proton velocity distribution. While most analyses assume a Maxwellian velocity distribution, this is unlikely to be correct. In particular, neutral atoms that pass through the shock and become ionized downstream form a nonthermal distribution similar to that of pickup ions in the solar wind. We predict the H alpha line profiles from the combination of pickup protons and the ordinary shocked protons, and we consider the extent to which this distribution could affect the shock parameters derived from H alpha profiles. The Maxwellian assumption could lead to an underestimate of shock speed by up to about 15%. The isotropization of the pickup ion population generates wave energy, and we find that for the most favorable parameters this energy could significantly heat the thermal particles. Sufficiently accurate profiles could constrain the strength and direction of the magnetic field in the shocked plasma, and we discuss the distortions from a Gaussian profile to be expected in Tycho's supernova remnant.