Cyg X-3: a low-mass black hole or a neutron star

TL;DR

This study estimates the masses of the components in Cyg X-3, suggesting the compact object could be a low-mass black hole or a neutron star, with observational data favoring a black hole interpretation.

Contribution

The paper provides new mass estimates for Cyg X-3's components using a novel relationship between mass-loss rate and mass for WR stars, clarifying the nature of the compact object.

Findings

Mass of WR star: approximately 10.3 solar masses

Mass of compact object: approximately 2.4 solar masses

Properties favor a low-mass black hole over a neutron star

Abstract

Cyg X-3 is a highly interesting accreting X-ray binary, emitting from the radio to high-energy gamma-rays. It consists of a compact object wind-fed by a Wolf-Rayet (WR) star, but the masses of the components and the mass-loss rate have been a subject of controversies. Here, we determine its masses, inclination, and the mass-loss rate using our derived relationship between the mass-loss rate and the mass for WR stars of the WN type, published infrared and X-ray data, and a relation between the mass-loss rate and the binary period derivative (observed to be >0 in Cyg X-3). Our obtained mass-loss rate is almost identical to that from two independent estimates and consistent with other ones, which strongly supports the validity of this solution. The found WR and compact object masses are 10.3_{-2.8}^{+3.9}, 2.4_{-1.1}^{+2.1} solar masses, respectively. Thus, our solution still allows for the presence of either a neutron star or a black hole, but the latter only with a low mass. However, the radio, infrared and X-ray properties of the system suggest that the compact object is a black hole. Such a low-mass black-hole could be formed via accretion-induced collapse or directly from a supernova.