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

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 M . For i = 60 degrees the same range is 1.0-3.2 M .

Revisiting the dynamical case for a massive black hole in IC10 X-1

“…v wind in the line-forming region is substantially lower than the terminal wind velocity. Vilhu et al (2009) used Chandra HETG spectra to map the ionization and velocity fields of the wind in Cyg X-3, largely confirming the picture developed by van Kerkwijk et al (1996) and Hanson et al (2000) and showing that the wind is highly ionized out to many times the binary separation. The X-ray FeXXVI line RV curve appears to trace the compact object's motion, with the required phase relationship to the IR emission and absorption features, resulting in a mass function consistent with that obtained by Hanson et al (2000).…”

“…At the more moderate ionization fraction expected deep in the core of the wind (and especially in the shadow of the WR star), single-electron ions of e.g. O, C, Si, and Fe will be present (as seen by Vilhu et al (2009) in Cyg X-3) and contribute significant soft X-ray absorption.…”

A Precise Measurement of the Orbital Period Parameters of Cygnus X-3

“…Zdziarski et al (2013) have presented arguments based on the radial velocity variation of the compact object from X-ray spectral lines and the mass loss rate to conclude that the mass of the compact object is 2.4 +2.1 −1.1 M , which could either be a neutron star or a black hole. The broadband spectral properties, however, suggest a low mass black hole (Vilhu et al 2009;. The orbital modulation of the X-ray flux has been explained by a low temperature cloud of plasma surrounding the compact object asymmetrically, which Compton scatters the radiation (Zdziarski et al 2010).…”

Modeling the X-ray light curves of Cygnus X-3

“…To estimate cooling times we use the photoionisation model of Vilhu et al (2009), where the compact star luminosity is L x = 2.46 × 10 38 erg/s. The clump densities probably vary, as do their sizes, but as an example we use a density of 10 13 cm −3 .…”

“…The data were limited to moderately high states (daily means ≥15 counts/s), to be more consistent with the Cygnus X-3 spectral state during the Chandra/HETG observations used. The emission line light curves (Si XIV Lyα 6.185 Å, FeXXVI Lyα 1.780 Å (H-type) and FeXXV 1.859 Å (He-type)) were taken from Vilhu et al (2009) and observed during a high state by Chandra/HETG (PI McCollough). The number of phase bins used was 30 and 10 for the continuum and lines, respectively.…”