A classification of the X-ray and radio states of Cyg X-3 and their long-term correlations

TL;DR

This paper classifies the X-ray and radio states of Cyg X-3, revealing correlations and hysteresis patterns, and compares these states to black-hole and neutron-star binaries, supporting the presence of a black hole.

Contribution

It introduces a detailed spectral and long-term correlated classification of Cyg X-3's X-ray and radio states, highlighting their similarities to black-hole binaries.

Findings

Positive radio-soft X-ray correlation in hard state

Negative correlation during transition to soft state

Hysteresis loop in flux-flux diagram

Abstract

We present a detailed classification of the X-ray states of Cyg X-3 based on the spectral shape and a new classification of the radio states based on the long-term correlated behaviour of the radio and soft X-ray light curves. We find a sequence of correlations, starting with a positive correlation between the radio and soft X-ray fluxes in the hard spectral state, changing to a negative one at the transition to soft spectral states. The temporal evolution can be in either direction on that sequence, unless the source goes into a very weak radio state, from which it can return only following a major radio flare. The flare decline is via relatively bright radio states, which results in a hysteresis loop on the flux-flux diagram. We also study the hard X-ray light curve, and find its overall anticorrelation with the soft X-rays. During major radio flares, the radio flux responds exponentially to the level of a hard X-ray high-energy tail. We also specify the detailed correspondence between the radio states and the X-ray spectral states. We compare our results to those of black-hole and neutron-star binaries. Except for the effect of strong absorption and the energy of the high-energy break in the hard state, the X-ray spectral states of Cyg X-3 closely correspond to the canonical X-ray states of black-hole binaries. Also, the radio/X-ray correlation closely corresponds to that found in black-hole binaries, but it significantly differs from that in neutron-star binaries. Overall, our results strongly support the presence of a black hole in Cyg X-3.