Orbital modulation of X-ray emission lines in Cygnus X-3

Abstract

<i>Aims. <i/>We address the problem where the X-ray emission lines are formed and investigate orbital dynamics using <i>Chandra HETG observations, photoionizing calculations and numerical wind-particle simulations. The aims were to set constraints on the masses of the components of this close binary system consisting of a Wolf-Rayet (WR) star and a compact component and to investigate the nature of the latter (neutron star or black hole). The goal was also to investigate P Cygni signatures in line profiles. <i/><i>Methods. <i/>The observed Si xiv (6.185 Å) and S xvi (4.733 Å) line profiles at four orbital phases were fitted with P Cygni-type profiles consisting of an emission and a blue-shifted absorption component. Numerical models were constructed using photoionizing calculations and particle simulations. In the models, the emission originates in the photoionized wind of the WR companion illuminated by a hybrid source: the X-ray radiation of the compact star and the photospheric EUV-radiation from the WR star. <i>Results. <i/>Spectral lines with moderate excitation (such as Si xiv and S xvi) arise in the photoionized wind. The emission component exhibits maximum blue-shift at phase 0.5 (when the compact star is in front), while the velocity of the absorption component is constant (around -900 km s<sup>-1<sup/>). Both components, like the continuum flux, have intensity maxima around phase 0.5. The simulated Fe xxvi Ly<i>α<i/> line (1.78 Å, H-like) from the wind is weak compared to the observed one. We suggest that it originates in the vicinity of the compact star, with a maximum blue shift at phase 0.25 (compact star approaching). By combining the mass function derived with that from the infrared He i absorption (arising from the WR companion), we constrain the masses and the inclination of the system. <i>Conclusions. <i/>The Si xiv and S xvi lines and their radial velocity curves can be understood in the framework of a photoionized wind involving a hybrid ionizer. Constraints on the compact star mass and orbital inclination (<i>i<i/>) are given using the mass functions derived from the Fe xxvi line and He i 2.06 <i>μ<i/>m absorption. Both a neutron star at large inclination (<i>i<i/> <i>≥<i/> 60 degrees) and a black hole at small inclination are possible solutions. The radial velocity amplitude of the He ii 2.09 <i>μ<i/>m emission (formed in the X-ray shadow behind the WR star) suggests <i>i<i/> = 30 degrees, implying a possible compact star mass between 2.8–8.0 . For <i>i<i/> = 60 degrees the same range is 1.0–3.2 .