# On the Origin of Asymmetries in Bilateral Supernova Remnants

## TL;DR

This study uses 3-D MHD simulations to determine whether the asymmetries in bilateral supernova remnants are primarily caused by variations in the interstellar medium or magnetic field, providing diagnostic parameters for observations.

## Contribution

It introduces a simulation-based approach to distinguish the origins of asymmetries in bilateral supernova remnants and offers new parameters for observational diagnostics.

## Key findings

- Asymmetries occur when the line-of-sight is misaligned with density or magnetic field gradients.
- Different asymmetry patterns can be explained by the orientation of ambient gradients.
- Diagnostic parameters can help infer the microphysics of shock waves from observations.

## Abstract

AIMS: We investigate whether the morphology of bilateral supernova remnants (BSNRs) observed in the radio band is determined mainly either by a non-uniform interstellar medium (ISM) or by a non-uniform ambient magnetic field. METHODS: We perform 3-D MHD simulations of a spherical SNR shock propagating through a magnetized ISM. Two cases of shock propagation are considered: 1) through a gradient of ambient density with a uniform ambient magnetic field; 2) through a homogeneous medium with a gradient of ambient magnetic field strength. From the simulations, we synthesize the synchrotron radio emission, making different assumptions about the details of acceleration and injection of relativistic electrons. RESULTS: We find that asymmetric BSNRs are produced if the line-of-sight is not aligned with the gradient of ambient plasma density or with the gradient of ambient magnetic field strength. We derive useful parameters to quantify the degree of asymmetry of the remnants that may provide a powerful diagnostic of the microphysics of strong shock waves through the comparison between models and observations. CONCLUSIONS: BSNRs with two radio limbs of different brightness can be explained if a gradient of ambient density or, most likely, of ambient magnetic field strength is perpendicular to the radio limbs. BSNRs with converging similar radio arcs can be explained if the gradient runs between the two arcs.