New possible class of neutron stars: hot and fast non-accreting rotators

TL;DR

This paper proposes a new class of neutron stars called HOFNARs, which are hot, rapidly rotating, non-accreting, and maintained by r-mode dissipation, with potential implications for gravitational wave emission and dense matter physics.

Contribution

The paper introduces the concept of HOFNARs, describes their expected properties, observational criteria, and discusses their potential existence and significance in astrophysics.

Findings

HOFNARs can be distinguished from qLMXBs based on specific observational criteria.

Some stable X-ray sources may belong to the HOFNAR class.

HOFNARs could emit gravitational waves due to mass-current multipole moments.

Abstract

A new class of neutron stars (NSs) -- hot rapidly rotating non-accreting NSs, which we propose to call HOFNARs (HOt and Fast Non-Accreting Rotators) or "hot widows" (in analogy with "black widow" pulsars) -- is suggested. We argue that such stars should originate from the low-mass X-ray binaries (LMXBs) provided that they were unstable with respect to excitation of $r$-modes at the end of accretion epoch (when their low-mass companions ceased to fill the Roche lobe). High temperature of "hot widows"/HOFNARs is maintained by $r$-mode dissipation rather than by accretion. We analyse observational properties of "hot widows"/HOFNARs and demonstrate that these objects form a specific separate class of neutron stars. In particular, some of the most stable X-ray sources among the candidates to quiescent LMXB systems (qLMXBs), can, in fact, belong to that new class. We formulate observational criteria which allow to distinguish "hot widows"/HOFNARs from qLMXB systems, and argue that available observations of X-ray sources 47 Tuc X5 and X7 satisfy (or, at least, do not contradict) these criteria. In addition, we discuss indirect evidences in favor of "hot widows"/HOFNARs existence, following from the analysis of observations and predictions of population synthesis theories. If that new class of NSs does exist, it would prove the possibility to emit gravitational waves by mass-current multipole. Various applications of our results, such as prospects for constraining superdense matter properties with hot widows"/HOFNARs, are analyzed.