X-ray emission characteristics of two WR binaries : V444 Cyg and CD Cru

TL;DR

This study analyzes X-ray emissions from two Wolf-Rayet binaries, revealing phase-locked variability, hot plasma characteristics, and evidence of colliding wind shocks, advancing understanding of their high-energy phenomena.

Contribution

First detailed X-ray analysis of V444 Cyg and CD Cru, demonstrating phase-dependent hot plasma behavior and shock interactions in WR binaries.

Findings

X-ray flux increases by factors of 2 and 1.5 in V444 Cyg and CD Cru.

Hot plasma temperatures vary with orbital phase, indicating colliding wind shocks.

Maximum absorption occurs during primary eclipse of V444 Cyg.

Abstract

We present X-ray analysis of two Wolf-Rayet (WR) binaries: V444 Cyg and CD Cru using the data from observations with XMM-Newton. The X-ray light curves show the phase locked variability in both binaries, where the flux increased by a factor of $\sim 2$ in the case of V444 Cyg and $\sim 1.5$ in the case of CD Cru from minimum to maximum. The maximum luminosities in the 0.3--7.5 keV energy band were found to be $5.8\times10^{32}$ and $2.8\times10^{32}$ erg s$^{-1}$ for V444 Cyg and CD Cru, respectively. X-ray spectra of these stars confirmed large extinction and revealed hot plasma with prominent emission line features of highly ionized Ne, Mg, Si, S, Ar, Ca and Fe, and are found to be consistent with a two-temperature plasma model. The cooler plasma at a temperature of $\sim$ 0.6 keV was found to be constant at all phases of both binaries, and could be due to a distribution of small-scale shocks in radiation-driven outflows. The hot components in these binaries were found to be phase dependent. They varied from 1.85 to 9.61 keV for V444 Cyg and from 1.63 to 4.27 keV for CD Cru. The absorption of the hard component varied with orbital phase and found to be maximum during primary eclipse of V444 Cyg. The high plasma temperature and variability with orbital phase suggest that the hard-component emission is caused by a colliding wind shock between the binary components.