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

Abstract

Using data from observations made with XMM-Newton, we present an X-ray analysis of two Wolf-Rayet (WR) binaries: V444 Cyg and CD Cru. The X-ray light curves show the phase-locked variability in both binaries, where the flux increased by a factor of ~2 for V444 Cyg and ~1.5 for CD Cru, from minimum to maximum. The maximum luminosities in the 0.3-7.5 keV energy band were found to be 5.8×10³² and 2.8×10³² erg s⁻¹ for V444 Cyg and CD Cru, respectively. The 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; these are found to be consistent with a two-temperature plasma model. At a temperature of ~0.6 keV, the cooler plasma was found to be constant at all phases for both binaries, which could be the result of 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 the orbital phase and was found to be maximum during the 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.