MWC 656 ( = HD 215227 ) was recently discovered to be the first binary system composed of a Be star and a black hole ( BH ) .
We observed it with XMM-Newton , and detected a faint X-ray source compatible with the position of the optical star , thus proving it to be the first Be/BH X-ray binary .
The spectrum analysis requires a model fit with two components , a black body plus a power law , with k _ { B } T = 0.07 ^ { +0.04 } _ { -0.03 } keV and a photon index \Gamma = 1.0 \pm 0.8 , respectively .
The non-thermal component dominates above \simeq 0.8 keV .
The obtained total flux is F ( 0.3 – 5.5 ~ { } { keV } ) = ( 4.6 ^ { +1.3 } _ { -1.1 } ) \times 10 ^ { -14 } erg cm ^ { -2 } s ^ { -1 } .
At a distance of 2.6 \pm 0.6 kpc the total flux translates into a luminosity L _ { X } = ( 3.7 \pm 1.7 ) \times 10 ^ { 31 } erg s ^ { -1 } .
Considering the estimated range of BH masses to be 3.8–6.9 M _ { \odot } , this luminosity represents ( 6.7 \pm 4.4 ) \times 10 ^ { -8 } ~ { } L _ { Edd } , which is typical of stellar-mass BHs in quiescence .
We discuss the origin of the two spectral components : the thermal component is associated with the hot wind of the Be star , whereas the power law component is associated with emission from the vicinity of the BH .
We also find that the position of MWC 656 in the radio versus X-ray luminosity diagram may be consistent with the radio/X-ray correlation observed in BH low-mass X-ray binaries .
This suggests that this correlation might also be valid for BH high-mass X-ray binaries ( HMXBs ) with X-ray luminosities down to \sim 10 ^ { -8 } L _ { Edd } .
MWC 656 will allow the accretion processes and the accretion/ejection coupling at very low luminosities for BH HMXBs to be studied .