Discovery of X-ray emission from the first Be/black hole system

Abstract

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 \textit{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 $k_{\mathrm{B}}T=0.07_{-0.03}^{+0.04}$~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 $F(0.3$--$5.5\mathrm{keV})=(4.6_{-1.1}^{+1.3})\times 10^{-14}$ erg cm${}^{-2}$ s${}^{-1}$. At a distance of $2.6\pm 0.6$~kpc the total flux translates into a luminosity $L_{\mathrm{X}}=(3.7\pm 1.7)\times 10^{31}$ erg s${}^{-1}$. Considering the estimated range of BH masses to be 3.8--6.9 $M_{\odot }$, this luminosity represents $(6.7\pm 4.4)\times 10^{-8}{L}_{\mathrm{Edd}}$, 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 $\sim 10^{-8}{L}_{\mathrm{Edd}}$. MWC~656 will allow the accretion processes and the accretion/ejection coupling at very low luminosities for BH~HMXBs to be studied.

in press in ApJ Letters, 5 pages, 3 figures

References in corpus (2)

• X-ray Properties of Black-Hole Binaries
• Jets from the faintest black holes

Cited by in corpus (12)

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• On the formation of Be stars through binary interaction
• Giant Outbursts in Be/X-ray Binaries
• The First Simultaneous X-Ray/Radio Detection of the First Be/BH System MWC 656
• Swift J011511.0-725611: Discovery of a rare Be Star / White Dwarf binary system in the SMC
• MWC 656 is unlikely to contain a black hole
• MAGIC observations of MWC 656, the only known Be/BH system
• The gamma-ray source AGL J2241+4454 as the possible counterpart of MWC 656
• Whispering in the dark: Faint X-ray emission from black holes with OB star companions
• The newly discovered Be/X-ray binary Swift J004516.6-734703 in the SMC: witnessing the emergence of a circumstellar disc
• Identifying four $INTEGRAL$ sources in the Galactic Plane via VLT/optical and $XMM$-$Newton$/X-ray spectroscopy
• Optical spectroscopy of the Be/black hole binary MWC656 -- interaction of a black hole with a circumstellar disc