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

¹ Observatory,Tähtitorninmäki (PO Box 14), 00014 University of Helsinki, Finland e-mail: osmi.vilhu@astro.helsinki.fi
² Tuorla Observatory, University of Turku, Väisäläntie 20, 21500 Piikkiö, Finland e-mail: pahakala@astro.utu.fi
³ Metsähovi Radio Observatory/TKK, Metsähovintie 114, 02540 Kylmälä, Finland e-mail: diana@kurp.hut.fi
⁴ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA e-mail: mccolml@head-cfa.harvard.edu

Received: 5 November 2008
Accepted: 19 April 2009

Abstract

Aims. We address the problem where the X-ray emission lines are formed and investigate orbital dynamics using 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.

Methods. 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.

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 phase 0.5 (when the compact star is in front), while the velocity of the absorption component is constant (around -900 km s^-1). Both components, like the continuum flux, have intensity maxima around phase 0.5. The simulated Fe xxvi Lyα 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.

Conclusions. 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) are given using the mass functions derived from the Fe xxvi line and He i 2.06 μm absorption. Both a neutron star at large inclination (i ≥ 60 degrees) and a black hole at small inclination
are possible solutions. The radial velocity amplitude of the He ii 2.09 μm emission (formed in the X-ray shadow behind the WR star) suggests i = 30 degrees, implying a possible compact star mass between 2.8–8.0 . For i = 60 degrees the same range is 1.0–3.2 .