2014ApJ...786L..11M

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 blackbody plus a power law, with k_BT = 0.07_–0.03^+0.04 keV and a photon index Γ = 1.0±0.8, respectively. The non-thermal component dominates above ≃0.8 keV. The obtained total flux is F(0.3-5.5 KeV) = (4.6_–1.1^+1.3) x 10^–14 erg/cm2/s. At a distance of 2.6±0.6 kpc the total flux translates into a luminosity L_X= (3.7±1.7)x10³¹ erg/s. Considering the estimated range of BH masses to be 3.8-6.9 M_☉, this luminosity represents (6.7±4.4)x10^–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 ∼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.