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We propose multi-epoch STIS FUV spectroscopy of the only Be star + black hole binary candidate comma MWC 656. These observations will reveal the nature of the Be star.s elusive companiondoublePoint a black hole comma or an intermediate-mass (virgul3 Msun) stripped helium star. Either scenario would be extraordinarydoublePoint intermediate-mass stripped stars are the main progenitors of stripped-envelope supernovae comma X-ray binaries comma and merging neutron stars comma yet they have remained elusive for decades the black hole would be in the mass gap and would be the only known X-ray faint high-mass X-ray binary containing a black hole. Our proposed observations will unambiguously distinguish between these two scenarios comma as the UV spectrum of a massive stripped star will be very different from that of a black hole accretion disk. FUV spectra will also constrain the spectral type comma mass comma and RV variability of the Be star more reliably than has been possible with optical data comma which are strongly contaminated by the Be star.s circumstellar disk.

我们提出对唯一的Be星+黑洞双星候选体MWC 656开展多历元STIS远紫外（FUV）光谱观测。这些观测将揭示Be星神秘伴星的本质：黑洞，或是中等质量（3 Msun）剥离氦星（stripped helium star）。两种情形均具有非凡意义：中等质量剥离星是剥离包层超新星（stripped-envelope supernovae）、X射线双星及并合中子星的主要前身星，然而数十年来它们始终难以被探测到；若为伴星为黑洞，则其处于质量间隙内，且将是已知唯一包含黑洞的X射线暗弱的大质量X射线双星。我们提出的观测将明确区分这两种情形，因为大质量剥离星的紫外光谱与黑洞吸积盘的紫外光谱存在显著差异。远紫外光谱还将比光学数据更可靠地限制Be星的光谱型、质量及视向速度（RV）变化——光学数据因受到Be星星周盘的强烈污染而可靠性受限。