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A Be-type star with a black-hole companion

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Abstract

Stellar-mass black holes have all been discovered through X-ray emission, which arises from the accretion of gas from their binary companions (this gas is either stripped from low-mass stars or supplied as winds from massive ones). Binary evolution models also predict the existence of black holes accreting from the equatorial envelope of rapidly spinning Be-type stars1,2,3 (stars of the Be type are hot blue irregular variables showing characteristic spectral emission lines of hydrogen). Of the approximately 80 Be X-ray binaries known in the Galaxy, however, only pulsating neutron stars have been found as companions2,3,4. A black hole was formally allowed as a solution for the companion to the Be star MWC 656 (ref. 5; also known as HD 215227), although that conclusion was based on a single radial velocity curve of the Be star, a mistaken spectral classification6 and rough estimates of the inclination angle. Here we report observations of an accretion disk line mirroring the orbit of MWC 656. This, together with an improved radial velocity curve of the Be star through fitting sharp Fe ii profiles from the equatorial disk, and a refined Be classification (to that of a B1.5–B2 III star), indicates that a black hole of 3.8 to 6.9 solar masses orbits MWC 656, the candidate counterpart of the γ-ray source AGL J2241+4454 (refs 5, 6). The black hole is X-ray quiescent and fed by a radiatively inefficient accretion flow giving a luminosity less than 1.6 × 10−7 times the Eddington luminosity. This implies that Be binaries with black-hole companions are difficult to detect in conventional X-ray surveys.

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Figure 1: Optical spectrum of MWC 656, obtained with the Mercator telescope.
Figure 2: Orbital evolution of the Fe ii 4,583 Å and He ii 4,686 Å emission lines.
Figure 3: The radial velocity curves of the Be star and its companion.

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Acknowledgements

We thank T. Maccarone and P. Charles for comments on the paper. This work made use of the molly software package developed by T. R. Marsh. The Liverpool telescope and the Mercator telescope are operated on the island of La Palma by the Liverpool John Moores University and the University of Leuven/Observatory of Geneva, respectively, in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias. The Liverpool telescope is funded by the UK Science and Technology Facilities Council. This research was supported by the Spanish MINECO and FEDER under grants AYA2010-18080, AYA2010-21782-C03-01, AYA2010-21967-C05-04/05, AYA2012-39364-C02-01/02, AYA2012-39612-C03-01, FPA2010-22056-C06-02 and SEV2011-0187-01; it was also funded by grant PID 2010119 from the Gobierno de Canarias. J.M.P. acknowledges financial support from ICREA Academia.

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Authors and Affiliations

Authors

Contributions

J.C. performed the radial velocity analysis of the spectra and wrote the paper. I.N. obtained the Mercator spectrum and contributed to the interpretation of the data. I.R. computed the eccentric orbital fits to the radial velocity curves. M.R. calculated the distance and X-ray luminosity, and contributed to the interpretation of the data. J.M.P. also contributed to the interpretation of the data. A.H. computed the rotational broadening of the star and, together with I.N., performed the spectral calibration of the star. S.S-D. observed the standard stars and reduced the Mercator spectra. J.M.P. and M.R. assisted in writing the section on γ-ray binaries in Methods.

Corresponding author

Correspondence to J. Casares.

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Extended data figures and tables

Extended Data Figure 1 Time evolution of the He ii 4,686 Å emission line in MWC 656.

a, Radial velocities obtained from single Gaussian fits to the line profile. The best fitting sine wave, with a period of 59.5 days, is overplotted. Maximum velocity occurs at HJD 2455722.2 or photometric phase 0.06. b, Equivalent width (EW) as a function of time. We used the convention of positive equivalent widths for emission lines. Error bars, 1 s.d.

Extended Data Figure 2 Diagnostic diagram for the He ii 4,686 Å line in MWC 656.

It has been computed using the double-Gaussian technique with a Gaussian width equal to the instrumental resolution full-width at half-maximum, 55 km s−1. The vertical dotted line indicates the Gaussian separation for which the continuum noise starts to dominate. Error bars, 1 s.d. Panels display the evolution of the sine wave fitting parameters with Gaussian separation a. From top to bottom: the systemic velocity γ, the sine wave ϕ0 phase, the velocity semiamplitude K and the control parameter σ(K)/K.

Extended Data Figure 3 Classification spectrum of the Be star MWC 656.

From top to bottom, spectra of MWC 656 and the MK standards HD 214993 (B1.5 III) and HD 35468 (B2 III) (ref. 54). The standards have been artificially broadened by 330 km s−1 to mimic the rotational broadening of MWC 656.

Extended Data Table 1 Observing log of MWC 656
Extended Data Table 2 Orbital elements derived from radial velocities of the He ii 4,686 Å and Fe ii 4,583 Å lines

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Casares, J., Negueruela, I., Ribó, M. et al. A Be-type star with a black-hole companion. Nature 505, 378–381 (2014). https://doi.org/10.1038/nature12916

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