New Evidence for a Black Hole in the Compact Binary Cygnus X-3

TL;DR

This study analyzes Cygnus X-3's high-energy spectra to estimate the mass of its compact object, providing strong evidence that it is a black hole with a mass exceeding 4.2 solar masses, thus resolving a longstanding debate.

Contribution

The paper applies a scaling relationship to spectral data to estimate the compact object’s mass, conclusively identifying it as a black hole in Cygnus X-3.

Findings

Compact object mass > 4.2 solar masses

Favored mass estimate ~10 solar masses

Supports Cygnus X-3 as a black hole

Abstract

The bright and highly variable X-ray and radio source known as Cygnus X-3 was among the first X-ray sources discovered, yet it remains in many ways an enigma. Its known to consist of a massive, Wolf-Rayet primary in an extremely tight orbit with a compact object. Yet one of the most basic of parameters - the mass of the compact object - is not known. Nor is it even clear whether its is a neutron star or a black hole. In this Paper we present our analysis of the broad-band high-energy continua covering a substantial range in luminosity and spectral morphology. We apply these results to a recently identified scaling relationship which has been demonstrated to provide reliable estimates of the compact object mass in a number of accretion powered binaries. This analysis leads us to conclude that the compact object in Cygnus X-3 has a mass greater than $4.2M_\odot$ thus clearly indicative of a black hole and as such resolving a long-standing issue. The full range of uncertainty in our analysis and from using a range of recently published distance estimates constrains the compact object mass to lie between $4.2M_\odot$ and $14.4M_\odot$. Our favored estimate, based on a 9.0 kpc distance estimate is $\sim 10 M_\odot$ with the error margin of 3.2 solar masses. This result may thus pose challenges to shared-envelope evolutionary models of compact binaries, as well as establishing Cygnus X-3 as the first confirmed accretion-powered galactic gamma-ray source.