ABSTRACT We present a statistical analysis of the He ii 4686 emission line in the spectra of the black hole and Wolf–Rayet (WR) star of the high-mass X-ray binary IC10 X-1. This line is visibly skewed, and the third moment (skewness) varies with the binary’s orbital phase. We describe a new method of extracting such weak/faint features lying barely above a noisy continuum. Using the moments of these features, we have been able to decompose these skewed lines into two symmetric Gaussian profiles as a function of the orbital phase. The astrophysical implications of this decomposition are significant due to the complex nature of wind–accretion stream interactions in such binary systems. Previous studies have already shown a 0.25 phase lag in the radial velocity curve of the star and the X-ray eclipse, which indicates that the He ii emitters might be in the stellar wind, hence not tracing the star’s orbital motion. Results from this work further suggest the existence of two separate emitting regions, one in the stellar wind in the shadow of the WR star and another in the accretion stream that impacts the black hole’s outer accretion disc; and the observed skewed He ii lines can be reproduced by superposition of the two corresponding time-dependent Gaussian emission profiles.

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