Near-IR and X-ray Variability of Cyg X-3: Evidence for a Compact IR Source and Complex Wind Structures

TL;DR

This study analyzes near-IR and X-ray variability of Cyg X-3, revealing a compact IR source and complex wind structures, and estimates system parameters suggesting a black hole or neutron star companion.

Contribution

It provides the first detailed modeling of IR and X-ray light curves of Cyg X-3, identifying wind structures and estimating system parameters including the nature of the compact object.

Findings

IR flux originates from both free-free wind emission and a compact IR source.

Detected wind structures include a bow shock and clumpy trail.

Estimated system parameters suggest a black hole or neutron star companion.

Abstract

We study near-infrared (JHK) and X-ray light curves of Cyg X-3 obtained with the 2.5-m telescope of the Caucasian Mountain Observatory of MSU SAI and collected from RXTE ASM and MAXI archives. The light curves in the X-ray and IR domains are strongly affected by irregular variations. However, the mean curves are remarkably stable and qualitatively similar in both domains. This means that the IR flux of the system originates not only from the free-free radiation of the WR wind but also from a compact IR source located near the relativistic companion. The shape of the mean X-ray and IR light curves suggest the existence of two additional structures in the WR wind - a bow shock near the relativistic companion and a so-called "clumpy trail" (Vilhu et al. 2013). Modeling of the mean X-ray and IR light curves allowed us to obtain important system parameters: the orbital phase of the superior conjunction of the relativistic companion $\phi_0=-0.066\pm 0.006$, the orbital inclination angle $i=29.5^\circ\pm 1.2^\circ$, and the WR mass-loss rate $\dot{M} = (0.96\pm 0.14)\times 10^{-5}\rm M_\odot yr^{-1}$. By using relations between $\dot{M}$ and the rate of the period change and between $\dot{M}$ and the WR mass, we estimated the probable mass of the relativistic companion $M_{\rm C}\simeq 7.2\rm M_\odot$ which points towards the black hole hypothesis. However, this estimate is based on the assumption of a smooth WR wind. Considering the uncertainty associated with clumping, the mass-loss rate can be lower which leaves room for the neutron star hypothesis.