  MWC 656 is the first known Be/black hole (BH) binary system. Be/BH binaries
are important in the context of binary system evolution and sources of
detectable gravitational waves because they are possible precursors of
coalescing neutron star/BH binaries. X-ray observations conducted in 2013
revealed that MWC 656 is a quiescent high-mass X-ray binary (HMXB), opening the
possibility to explore X-ray/radio correlations and the accretion/ejection
coupling down to low luminosities for BH HMXBs. Here we report on a deep joint
Chandra/VLA observation of MWC 656 (and contemporaneous optical data) conducted
in 2015 July that has allowed us to unambiguously identify the X-ray
counterpart of the source. The X-ray spectrum can be fitted with a power law
with $\Gamma\sim2$, providing a flux of $\simeq4\times10^{-15}$ erg cm$^{-2}$
s$^{-1}$ in the 0.5-8 keV energy range and a luminosity of $L_{\rm
X}\simeq3\times10^{30}$ erg s$^{-1}$ at a 2.6 kpc distance. For a 5 M$_\odot$
BH this translates into $\simeq5\times10^{-9}$ $L_{\rm Edd}$. These results
imply that MWC 656 is about 7 times fainter in X-rays than it was two years
before and reaches the faintest X-ray luminosities ever detected in
stellar-mass BHs. The radio data provide a detection with a peak flux density
of $3.5\pm1.1$ $\mu$Jy beam$^{-1}$. The obtained X-ray/radio luminosities for
this quiescent BH HMXB are fully compatible with those of the X-ray/radio
correlations derived from quiescent BH low-mass X-ray binaries. These results
show that the accretion/ejection coupling in stellar-mass BHs is independent of
the nature of the donor star.
