The First Simultaneous X-Ray/Radio Detection of the First Be/BH System MWC 656

Abstract

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 $\Gamma \sim 2$, providing a flux of $\simeq 4\times 10^{-15}$ erg cm${}^{-2}$ s${}^{-1}$ in the 0.5-8 keV energy range and a luminosity of $L_{\mathrm{X}}\simeq 3\times 10^{30}$ erg s${}^{-1}$ at a 2.6 kpc distance. For a 5 M${}_{\odot }$ BH this translates into $\simeq 5\times 10^{-9}$ $L_{\mathrm{Edd}}$. 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 $3.5\pm 1.1$ $\mu$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.

6 pages, 4 figures. Accepted for publication in ApJ Letters

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