Polarized Balmer Line Emission from Supernova Remnant Shock Waves Efficiently Accelerating Cosmic Rays

TL;DR

This paper investigates how polarized Balmer line emissions from supernova remnant shocks reveal the energy loss due to cosmic ray acceleration, linking polarization degree to shock temperature and velocity differences.

Contribution

It introduces a method to estimate shock energy loss and cosmic ray acceleration efficiency from polarized Balmer line observations, considering nonthermal particle effects.

Findings

Higher energy loss rates increase polarization degree.

Polarized intensity ratio of Hβ to Hα depends on energy loss rate.

Downstream temperature lower than adiabatic indicates energy transfer to particles.

Abstract

Linearly polarized Balmer line emissions from supernova remnant shocks are studied taking into account the energy loss of the shock owing to the production of nonthermal particles. The polarization degree depends on the downstream temperature and the velocity difference between upstream and downstream regions. The former is derived once the line width of the broad component of the H$\alpha$ emission is observed. Then, the observation of the polarization degree tells us the latter. At the same time, the estimated value of the velocity difference independently predicts adiabatic downstream temperature that is derived from Rankine-Hugoniot relations for adiabatic shocks. If the actually observed downstream temperature is lower than the adiabatic temperature, there is a missing thermal energy which is consumed for particle acceleration. It is shown that a larger energy loss rate leads to more highly polarized H$\alpha$ emission. Furthermore, we find that polarized intensity ratio of H$\beta$ to H$\alpha$ also depends on the energy loss rate and that it is independent of uncertain quantities such as electron temperature, the effect of Lyman line trapping and our line of sight.