The radio/X-ray correlation in Cyg X-3 and the nature of its hard spectral state

TL;DR

This study investigates the complex radio/X-ray correlation in Cyg X-3, revealing that observed fluxes are affected by local absorption, and compares its spectral and radio properties with other X-ray binaries, highlighting its unique characteristics.

Contribution

It demonstrates that the apparent uncorrelation between total X-ray flux and radio flux in Cyg X-3 is due to absorption effects, and clarifies the intrinsic spectral state and radio properties of the source.

Findings

Observed 1-100 keV flux is uncorrelated with radio flux due to absorption.

Intrinsic X-ray spectrum in the hard state resembles an intermediate state.

Cyg X-3 is more radio loud and interacts strongly with stellar wind.

Abstract

We study the radio/X-ray correlation in Cyg X-3. It has been known that the soft and hard X-ray fluxes in the hard spectral state are correlated positively and negatively, respectively, with the radio flux. We show that this implies that the observed $\sim$1--100 keV flux (which is a fair approximation to the bolometric flux) is completely uncorrelated with the radio flux. We can recover a positive correlation (seen in other sources and expected theoretically) if the soft X-rays are strongly absorbed by a local medium. Then, however, the intrinsic X-ray spectrum of Cyg X-3 in its hard state becomes relatively soft, similar to that of an intermediate spectral state of black-hole binaries, but not to their true hard state. We also find the radio spectra in the hard state of Cyg X-3 are hard on average, and the flux distributions of the radio emission and soft X-rays can be described by sums of two log-normal functions. We compare Cyg X-3 with other X-ray binaries using colour-colour, colour-Eddington ratio and Eddington ratio-radio flux diagrams. We find Cyg X-3 to be spectrally most similar to GRS 1915+105, except that Cyg X-3 is substantially more radio loud, which appears to be due to its jet emission enhanced by interaction with the powerful stellar wind from the Wolf-Rayet donor.