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MWC 656 is unlikely to contain a black hole. (arXiv:2308.08642v1 [astro-ph.SR])

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Context. MWC 656 was reported as the first known Be star with a black-hole
(BH) companion in a 60 d period. The mass of the proposed BH companion is
estimated to be between 4 - 7 MSun. This estimate is based on radial velocity
(RV) measurements derived from the Fe ii 4583 emission line of the Be star disc
and from the He ii 4686 emission line, assumed to be formed in a disc around
the putative BH. Aims. Using new high-resolution spectroscopic data, we
investigate whether MWC 656 truly contains a BH. Methods. We used the
cross-correlation method to calculate the RVs of both the Be star and the He ii
4686 emission line and we derive a new orbital solution. We also performed
disentangling to look for the spectral signature of a companion. Results. We
derive an orbital period of 59.028 +- 0.011 d and a mass ratio q = M_Heii/M_Be
= 0.12 +- 0.03, much lower than the previously reported q = 0.41 +- 0.07.
Adopting a mass of the Be star of M_Be = 7.8 +- 2.0MSun, the companion has a
mass of 0.94 +- 0.34MSun. For the upper limit of M_Be = 16MSun and q = 0.15,
the companion has a mass 2.4MSun. Performing disentangling on mock spectra
shows that the spectral signature of a non-degenerate stellar companion with
such a low mass cannot be retrieved using our data. Conclusions. Our
measurements do not support the presence of a BH companion in MWC 656. The
derived upper limit on the mass of the companion rather indicates that it is a
neutron star, a white dwarf, or a hot helium star. Far-UV data will help to
reject or confirm a hot helium-star companion.

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