X-RAY novae are binary systems in which a compact object accretes gas from a companion star. In several cases there is good evidence that the compact object is a black hole1–5. The best evidence for the presence of a black hole comes from measuring the orbital velocity of the companion star and thereby determining the minimum mass of the compact object; when this mass exceeds the maximum mass for a neutron star (≲3 solar masses6), a black hole seems the only remaining possibility. The unusual X-ray nova GRO J1655 - 40 (refs 7-10) is unique because it emits superlumi-nal radio jets, suggesting that it is a low-luminosity counterpart to active galactic nuclei7,9, which are thought to be powered by accretion onto a massive black hole. Here we report observations of the optical counterpart10 of GRO J1655 - 40, which show that the system undergoes periodic eclipses; the edge-on geometry thus implied allows a determination of the companion star's true velocity. The mass of the compact object derived from the velocity curve is at least 3.16 ±0.15 solar masses, which strongly supports its identification as a black hole.
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Bailyn, C., Orosz, J., McClintock, J. et al. Dynamical evidence for a black hole in the eclipsing X-ray nova GRO J1655 – 40. Nature 378, 157–159 (1995). https://doi.org/10.1038/378157a0
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