Ultraviolet spectroscopy reveals a hot and luminous companion to the Be star+black hole candidate MWC 656
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| Format: | Preprint |
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2026
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| _version_ | 1866913011130695680 |
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| author | Müller-Horn, Johanna Ramachandran, Varsha El-Badry, Kareem Sander, Andreas A. C. Bodensteiner, Julia Gies, Douglas R. Götberg, Ylva Rivinius, Thomas Shenar, Tomer Schösser, Elisa C. Wang, Luqian Bieryla, Allyson Buchhave, Lars A. Latham, David W. |
| author_facet | Müller-Horn, Johanna Ramachandran, Varsha El-Badry, Kareem Sander, Andreas A. C. Bodensteiner, Julia Gies, Douglas R. Götberg, Ylva Rivinius, Thomas Shenar, Tomer Schösser, Elisa C. Wang, Luqian Bieryla, Allyson Buchhave, Lars A. Latham, David W. |
| contents | 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. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2601_14403 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | Ultraviolet spectroscopy reveals a hot and luminous companion to the Be star+black hole candidate MWC 656 Müller-Horn, Johanna Ramachandran, Varsha El-Badry, Kareem Sander, Andreas A. C. Bodensteiner, Julia Gies, Douglas R. Götberg, Ylva Rivinius, Thomas Shenar, Tomer Schösser, Elisa C. Wang, Luqian Bieryla, Allyson Buchhave, Lars A. Latham, David W. Solar and Stellar Astrophysics High Energy Astrophysical Phenomena 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. |
| title | Ultraviolet spectroscopy reveals a hot and luminous companion to the Be star+black hole candidate MWC 656 |
| topic | Solar and Stellar Astrophysics High Energy Astrophysical Phenomena |
| url | https://arxiv.org/abs/2601.14403 |