Is there a propeller neutron star in $\gamma$ Cas?

Abstract

$\gamma$ Cas is the prototype of a small population of B0-B1.5 III-V classical Be (cBe) stars that emit anomalous and hard X-rays with a unique array of properties. $\gamma$ Cas is known to host, like other cBe stars, a decretion disk and also a low mass companion. Recently Postnov et al. have posited that this companion is a magnetized rapidly spinning neutron star that deflects direct gravitational accretion from a stellar/disk wind via the "propeller mechanism." These authors state that the key X-ray observations are "remarkably well produced" in this scenario. We reexamine this mechanism in detail and conclude that there are a number of fatal objections in its application to the $\gamma$ Cas case. Among other considerations these issues include the prediction under the propeller scenario of a much smaller population of $\gamma$ Cas stars than is observed and the lack of allowance for observed correlations of X-ray and UV and/or optical properties over a variety of timescales.

arXiv comment9 pages

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• Gamma Cas Stars as Be + White Dwarf Binary Systems
• Intriguing X-ray and optical variations of the gamma Cas analog HD45314
• The X-ray emission of gamma Cassiopeiae during the 2020-2021 disc eruption
• Optical spectroscopy of Galactic field Classical Be stars
• Fluorescent Fe K line emission of gamma Cas stars I. Do gamma Cas stars host propelling neutron stars?
• X-ray emission of massive stars and their winds
• Concerning the occurrence of bow shocks around high-mass X-ray binaries
• Fluorescent Fe K line emission of gamma Cas stars II. Predictions for magnetic interactions and white-dwarf accretion scenarios