A Be-type star with a black-hole companion

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 stars (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 companions. 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 classification 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.

Formation of Double Neutron Star Systems

“…Reig et al (2004) and Staubert et al (2011) explain these systems through long episodes of quiescence, presumably due to the lack of sufficient circumstellar matter in the decretion disk of the Be-star, whereby accretion mainly takes place from the stellar wind provided by the Be-star and thus causing the pulsars to spin down to the (longer) equilibrium period that is expected for wind-fed systems. Apart from MWC 656 possibly hosting a BH (Casares et al 2014), the vast majority of (if not all other) known BeHMXBs seem to host a NS. Negueruela 2010).…”

An XMM-Newton and NuSTAR Study of IGR J18214-1318: A Non-pulsating High-mass X-Ray Binary with a Neutron Star

“…However, the nature of compact objects in many IGR HMXBs that are not SFXTs remains unknown, and as several Sg HMXBs are known to harbor BHs (e.g., Cyg X-1, M33 X-7, LMC X-1, LMC X-3) compared to only one known Be-BH binary (Casares et al 2014), IGR Sg HMXBs constitute a particularly promising group to search for BHs.…”

The VLT-FLAMES Tarantula survey

“…Only one Be+black hole X-ray binary has been identified from a Galactic population of ∼80 such systems (Casares et al 2014), while the same study shows it to have a quiescent X-ray luminosity some three orders of magnitude lower than VFTS 399.…”

“…In contrast, a subset of systems with wide (P orb 100 days), low-eccentricity orbits are persistent sources with moderate luminosity (L X ∼ 10 34 −10 35 erg s −1 ) and comparatively lowlevel variability ( 10× quiescent flux), of which X Per is an exemplar (Haberl et al 1998;Reig & Roche 2007). 10 For comparison Antoniadis et al (2013) determine ∼2.01 ± 0.04 M for the pulsar PSR J0348+0432, while Clark et al (2002) report ∼2.44± 0.27 M for the non-pulsating relativistic accretor in 4U1700-37 and Casares et al (2014) find ∼3.8−6.9 M for the black hole orbiting the Be star MWC656.…”