Letter to the Editor

MWC 656 is unlikely to contain a black hole

¹ Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
e-mail: soetkin.janssens@kuleuven.be
² Anton Pannekoek Institute for Astronomy, Science Park 904, 1098 XH Amsterdam, The Netherlands
³ European Organisation for Astronomical Research in the Southern Hemisphere (ESO), Karl-Schwarzschild-Str 2, 85748 Garching, Germany
⁴ European Southern Observatory, Alonso de Cordova 3107, Vitacura, Casilla, 19001 Santiago de Chile, Chile

Received: 29 June 2023
Accepted: 4 August 2023

Abstract

Contact. MWC 656 was reported to be the first known Be star with a black-hole (BH) companion on a 60 d period. The mass of the proposed BH companion is estimated to be between 4 and 7 M_⊙. This estimate is based on radial velocity (RV) measurements derived from the Fe IIλ4583 emission line of the Be star disc and from the He IIλ4686 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 IIλ4686 emission line to derive a new orbital solution. We also used a spectral disentangling method to look for the spectral signature of a companion.

Results. We derived an orbital period of 59.028 ± 0.011 d and a mass ratio of q = M_He II/M_Be = 0.12 ± 0.03, much lower than the previously reported value of q = 0.41 ± 0.07. By adopting a mass for the Be star of M_Be = 7.8 ± 2.0 M_⊙, the mass of the companion is 0.94 ± 0.34 M_⊙. For the upper limit of M_Be = 16 M_⊙ and q = 0.15, the mass of the companion is 2.4 M_⊙. Performing the 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. Rather, the derived upper limit on the mass of the companion indicates that it is either a neutron star, a white dwarf, or a hot helium star. Far-UV data will aid in rejecting or confirming a hot helium-star companion.