Neutron - mirror neutron mixing and neutron stars

TL;DR

This paper explores how neutron-mirror neutron oscillations could cause neutron stars to transform into mixed stars with mirror dark matter, affecting their observable properties and evolution, with implications for astrophysical signals.

Contribution

It introduces a model of neutron-mirror neutron mixing affecting neutron star composition, mass, and evolution, predicting observable signatures and collapse conditions.

Findings

Mixed stars can have the same mass but different radii compared to ordinary neutron stars.

Maximum mass of maximally mixed stars is reduced by a factor of √2.

Stars exceeding a critical mass may collapse into black holes after a certain time.

Abstract

The oscillation of neutrons $n$ into mirror neutrons $n'$, their mass degenerate partners from dark mirror sector, can have interesting implications for neutron stars: an ordinary neutron star could gradually transform into a mixed star consisting in part of mirror dark matter. Mixed stars can be detectable as twin partners of ordinary neutron stars: namely, there can exist compact stars with the same masses but having different radii. For a given equation of state (identical between the ordinary and mirror components), the mass and radius of a mixed star depend on the proportion between the ordinary and mirror components in its interior which in turn depends on its age. If $50 \% - 50\%$ proportion between two fractions can be reached asymptotically in time, then the maximum mass of such "maximally mixed stars" should be $\sqrt2$ times smaller than that of ordinary neutron star while the stars exceeding a critical mass value $M^{\rm max}_{NS}/\sqrt2$ should collapse in black holes after certain time. We evaluate the evolution time and discuss the implications of $n-n'$ transition for the pulsar observations as well as for the gravitational waves from the neutron star mergers and associated electromagnetic signals.