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arXiv · 2601.14403

Ultraviolet spectroscopy reveals a hot and luminous companion to the Be star+black hole candidate MWC 656

Johanna Müller-Horn·Varsha Ramachandran·Kareem El-Badry·Andreas A. C. Sander·Julia Bodensteiner·Douglas R. Gies·Ylva Götberg·Thomas Rivinius·Tomer Shenar·Elisa C. Schösser·Luqian Wang·Allyson Bieryla·Lars A. Buchhave·David W. Latham
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Abstract

The Galactic Be star binary MWC 656 was long considered the only known Be star+black hole (BH) system, making it a critical benchmark for models of massive binary evolution and for the expected X-ray emission of Be+BH binaries. However, recent dynamical measurements cast doubt on the presence of a BH companion. We present new multi-epoch ultraviolet spectroscopy from the Hubble Space Telescope, combined with high-resolution optical spectra, to reassess the nature of the companion. The far-ultraviolet spectra reveal high-ionisation features -- including prominent N v and He ii lines -- which are absent in the spectra of normal Be stars and are indicative of a hot, luminous companion. Spectral modelling shows that these features cannot originate from the Be star or from an accretion disc around a compact object. Instead, we find that the data are best explained by a hot ($T_\mathrm{eff} \approx 85$ kK), compact, hydrogen-deficient star with strong wind signatures, consistent with an intermediate-mass stripped star. Our revised orbital solution and composite spectroscopic modelling yield a companion mass of $M_2 = 1.48^{+0.55}_{-0.46}\,\mathrm{M}_\odot$, definitively ruling out a BH and disfavouring a white dwarf. MWC 656 thus joins the growing class of Be+stripped star binaries. The system's unusual properties -- including a high companion temperature and wind strength -- extend the known parameter space of such binaries. The continued absence of confirmed OBe+BH binaries in the Galaxy highlights a growing tension with population synthesis models.

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Johanna Müller-Horn, Varsha Ramachandran, Kareem El-Badry, Andreas A. C. Sander, Julia Bodensteiner, Douglas R. Gies, Ylva Götberg, Thomas Rivinius, Tomer Shenar, Elisa C. Schösser, Luqian Wang, Allyson Bieryla, Lars A. Buchhave, David W. Latham. 2026-01-20. Ultraviolet spectroscopy reveals a hot and luminous companion to the Be star+black hole candidate MWC 656. https://doi.org/10.1051/0004-6361/202557960

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