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

摘要

Aims. We address the problem where the X-ray emission lines are formed and investigate orbital dynamics using 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. Methods. The observed Si XIV (6.185?) and S XVI (4.733?)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 WRcompanion illuminated by a hybrid source: the X-ray radiation of the compact star and the photospheric EUV-radiation from the WRstar. Results. 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 phase0.5 (when the compact star is in front), while the velocity of the absorption component is constant (around -900kms-1). Both components, like the continuum flux, have intensity maxima around phase0.5. The simulated Fe XXVI Lyline (1.78?, 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 phase0.25 (compact star approaching). By combining the mass function derived with that from the infrared He I absorption (arising from the WRcompanion), we constrain the masses and the inclination of the system. Conclusions. The Si XIV and S XVIlines 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) are given using the mass functions derived from the Fe xxvi line and He I2.06m absorption. Both a neutron star at large inclination (degrees) and a black hole at small inclination are possible solutions. The radial velocity amplitude of the He II 2.09m emission (formed in the X-ray shadow behind the WRstar) suggests i=30degrees, implying a possible compact star mass between 2.8-8.0.For i=60 degrees the same range is 1.0-3.2.Key words: black hole physics - accretion, accretion disks - stars: binaries: spectroscopic - stars: winds, outflows - stars: individual: Cygnus X-3 - stars: neutron