Wave Propagation at Oblique Shocks: How Did Tycho Get Its Stripes?

TL;DR

This paper presents a new model explaining the origin of the stripe patterns in Tycho's supernova remnant through cosmic ray-induced Alfven waves and their interaction with oblique shocks, linking wave physics to observed synchrotron features.

Contribution

It introduces a novel wave-based model for Tycho's stripes, emphasizing the role of circularly polarized Alfven waves generated by cosmic rays and their transformation at oblique shocks.

Findings

The model explains stripe formation via surviving Alfven modes after wave dissipation.

Stripe visibility depends on shock obliquity and wave damping mechanisms.

Maximum cosmic ray energy in Tycho's SNR is estimated between 6×10^{14} and 10^{15} eV.

Abstract

We describe a new model for the "stripes" of synchrotron radiation seen in the remnant of Tycho's supernova. In our picture, cosmic rays streaming ahead of the forward shock generate parallel (with respect to the local magnetic field direction) circularly polarized Alfven waves that are almost free of dissipation, and due to being circularly polarized exhibit no spatial variation of magnetic field strength. Following interaction with the SNR shock with nonzero obliquity, these parallel waves become obliquely propagating, due the the wave refraction (different in principle for the different plane wave components), and dissipation sets in. The magnetosonic polarization decays faster, due to transit time damping, leaving only the Alfven mode. This surviving mode now exhibits a spatial variation of the magnetic field, leading to local maxima and minima in the synchrotron emission, i.e. the stripes. We attribute the initial wave generation to the Bell instability, which in contrast to the resonant generation of upstream Alfven waves, gives rise to a preferred wavelength, and hence the single wave period at which the stripes are seen. Based on estimates for damping rates due to turbulent cascade and transit time damping, we estimate the dependence of the visibility of the stripes on the shock obliquity, and determine a maximum cosmic ray energy in Tycho's SNR in the range $6\times 10^{14} - 1\times 10^{15}$ eV.