Comprehensive UV and Optical spectral analysis of Cygnus X-1: Stellar and wind parameters, abundances, and evolutionary implications

Abstract

We present a comprehensive analysis of Cyg X-1, utilizing archival high-resolution UV and optical spectra in conjunction with sophisticated atmospheric models. Notably, this is the first investigation to simultaneously analyze UV and optical spectra of Cyg X-1 along with incorporating X-rays to constrain the stellar and wind parameters. Our analysis yields notably lower masses for both the donor (29$M_{\odot }$) and the black hole (12.7 to 17.8$M_{\odot }$, depending on inclination), and confirms that the donor's radius is close to reaching the inner Lagrangian point. We find super-solar Fe, Si, and Mg abundances (1.3-1.8 times solar) at the surface of the donor star, while the total CNO abundance remains solar despite evidence of CNO processing (N enrichment, O depletion) and He enrichment. This abundance pattern is distinct from the surrounding Cyg OB3 association. We observe a clear difference in wind parameters between X-ray states: $v_{\infty }\approx 1200{\mathrm{km}\mathrm{s}}^{-1}$ and $\dot{M}\approx 3\times 10^{-7}{M}_{\odot }{\mathrm{yr}}^{-1}$ in the high/soft state, increasing to $v_{\infty }\lesssim 1800{\mathrm{km}\mathrm{s}}^{-1}$ and $\dot{M}\lesssim 5\times 10^{-7}{M}_{\odot }{\mathrm{yr}}^{-1}$ in the low/hard state. The observed X-ray luminosity is consistent with wind-fed accretion. Evolutionary models show that Cyg X-1 will undergo Roche lobe overflow in the near future. Under a fully conservative mass accretion scenario, our models predict a future binary black hole merger for Cyg X-1 within $\sim 5$ Gyr. Our comprehensive analysis provides refined stellar and wind parameters of the donor star in Cyg X-1, highlighting the importance of using advanced atmospheric models and considering X-ray ionization and wind clumping. The observed abundances suggest a complex formation history involving a high initial metallicity.

arXiv commentpaper is accepted for publication in A&A

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