Post-adiabatic supernova remnants in an interstellar magnetic field: oblique shocks and non-uniform environment

TL;DR

This study uses high-resolution MHD simulations to explore how magnetic fields influence shock dynamics, cosmic-ray acceleration, and emission in late-stage supernova remnants, highlighting the importance of magnetic orientation and environmental gradients.

Contribution

It provides new insights into the role of magnetic fields in post-adiabatic SNRs, especially regarding shock compression, cosmic-ray spectra, and polarization, through detailed simulations.

Findings

Magnetic pressure supports the radiative shell, limiting shock compression.

Shock obliquity and density gradients significantly affect SNR evolution.

Tangential magnetic fields dominate downstream regions, explaining observed polarization patterns.

Abstract

We present very-high-resolution 1D MHD simulations of the late-stage supernova remnants (SNR). In the post-adiabatic stage the magnetic field has an important and significant dynamical effect on the shock dynamics, the flow structure, and hence the acceleration and emission of cosmic rays. We find that the tangential component of the magnetic field provides pressure support that to a fair degree prevents the collapse of the radiative shell and thus limits the total compression ratio of the partially or fully radiative forward shock. A consequence is that the spectra of cosmic rays would not be as hard as in hydrodynamic simulations. We also investigated the effect on the flow profiles of the magnetic-field inclination and a large-scale gradient in the gas density and/or the magnetic field. A positive density gradient shortens the evolutionary stages whereas a shock obliquity lowers the shock compression. The compression of the tangential component of the magnetic field leads to its dominance in the downstream region of post-adiabatic shocks for a wide range of orientation of the upstream field, which may explain why one preferentially observes tangential radio polarization in old SNRs. As most cosmic rays are produced at late stages of SNR evolution, the post-adiabatic phase and the influence of the magnetic field during it are most important for modeling the cosmic-ray acceleration at old SNRs and the gamma-ray emission from late-stage SNRs interacting with clouds.