# Comparing Neutron Star Kicks to Supernova Remnant Asymmetries

## TL;DR

This study investigates the relationship between neutron star kick velocities and supernova remnant asymmetries, finding evidence supporting ejecta asymmetries as the primary cause of neutron star kicks.

## Contribution

The paper provides observational evidence linking supernova remnant asymmetries to neutron star kicks, favoring ejecta asymmetries over neutrino emission as the main mechanism.

## Key findings

- No correlation between power-ratios and NS kick velocities.
- Neutron stars in ejecta-dominated SNRs move opposite to X-ray emission.
- Results support ejecta asymmetries as the cause of NS kicks.

## Abstract

Supernova explosions are inherently asymmetric and can accelerate new-born neutron stars (NSs) to hundreds of km/s. Two prevailing theories to explain NS kicks are ejecta asymmetries (e.g., conservation of momentum between NS and ejecta) and anisotropic neutrino emission. Observations of supernova remnants (SNRs) can give us insights into the mechanism that generates these NS kicks. In this paper, we investigate the relationship between NS kick velocities and the X-ray morphologies of 18 SNRs observed with the Chandra X-ray Observatory and the Rontgen Satellite (ROSAT). We measure SNR asymmetries using the power-ratio method (a multipole expansion technique), focusing on the dipole, quadrupole, and octupole power-ratios. Our results show no correlation between the magnitude of the power-ratios and NS kick velocities, but we find that for Cas A and G292.0+1.8, whose emission traces the ejecta distribution, their NSs are preferentially moving opposite to the bulk of the X-ray emission. In addition, we find a similar result for PKS 1209-51, CTB 109, and Puppis A; however their emission is dominated by circumstellar/interstellar material, so their asymmetries may not reflect their ejecta distributions. Our results are consistent with the theory that NS kicks are a consequence of ejecta asymmetries as opposed to anisotropic neutrino emission. In the future, additional observations to measure NS proper motions within ejecta-dominated SNRs are necessary to constrain robustly the NS kick mechanism.