A propelling neutron star in the enigmatic Be-star $\gamma$~Cassiopeia

TL;DR

This paper proposes that the peculiar X-ray emission from $ ext{γ}$ Cassiopeia can be explained by a fast-spinning neutron star companion undergoing the propeller mechanism, forming a new subclass of Be-star X-ray binaries.

Contribution

It introduces a model where a fast-spinning neutron star explains $ ext{γ}$ Cas's X-ray emission, suggesting a new subclass of Be-star X-ray binaries with specific evolutionary paths.

Findings

The hot shell around the neutron star reproduces observed X-ray properties.

The model links $ ext{γ}$ Cas to a new subclass of Be-star X-ray binaries.

Evolutionary connection to low-luminosity X Per-type binaries.

Abstract

The enigmatic X-ray emission from the bright optical star, $\gamma$ Cassiopeia, is a long-standing problem. $\gamma$ Cas is known to be a binary system consisting of a Be-type star and a low-mass ($M\sim 1\,M_\odot$) companion of unknown nature orbiting in the Be-disk plane. Here we apply the quasi-spherical accretion theory onto a compact magnetized star and show that if the low-mass companion of $\gamma$ Cas is a fast spinning neutron star, the key observational signatures of $\gamma$ Cas are remarkably well reproduced. Direct accretion onto this fast rotating neutron star is impeded by the propeller mechanism. In this case, around the neutron star magnetosphere a hot shell is formed that emits thermal X-rays in qualitative and quantitative agreement with observed properties of the X-ray emission from $\gamma$ Cas. We suggest that $\gamma$ Cas and its analogs constitute a new subclass of Be-type X-ray binaries hosting rapidly rotating neutron stars formed in supernova explosions with small kicks. The subsequent evolutionary stage of $\gamma$ Cas and its analogs should be the X Per-type binaries comprising low-luminosity slowly rotating X-ray pulsars. The model explains the enigmatic X-ray emission from $\gamma$ Cas, and also establishes evolutionary connections between various types of rotating magnetized neutron stars in Be-binaries.