Title: Complex spectral variability and hints of a luminous companion in the Be star + black hole binary candidate ALS 8814 Show Chinese title

Title: Be星和黑洞双星候选体ALS 8814的复杂光谱变化和发光伴星的迹象

Abstract: The emission-line binary ALS 8814 was recently proposed as a Be star + black hole (BH) binary based on large-amplitude radial velocity (RV) variations of a Be star and non-detection of spectroscopic features from a luminous companion. We reanalyze low- and medium-resolution LAMOST spectra of ALS 8814 and show that the system's spectroscopic variability is considerably more complex than previously recognized. Inspection of the system's trailed spectra reveals the presence of a second set of absorption lines that move in anti-phase with the Be star. In addition, the emission and absorption lines exhibit inconsistent RV variability, suggesting that they trace different stars. Using spectral disentangling, we recover the spectrum of a rapidly rotating companion whose broad, shallow lines were previously undetected. Time-variability in the emission lines complicates interpretation of the disentangled spectrum, such that the physical parameters of the components are still uncertain, but we find with simulations that emission line variability alone is unlikely to explain all signatures of the companion. The system's high Gaia RUWE value suggests a third luminous companion, making ALS 8814 a likely hierarchical triple. Although it is unlikely to contain a BH, the system is unusual, with the largest RV semi-amplitude observed in any known classical Be star and a companion that does not appear to be stripped. More extensive spectroscopic monitoring and high-resolution imaging will be necessary to fully characterize the system's orbital architecture, stellar parameters, and evolutionary status. Show Chinese abstract

Abstract: 基于Be星和黑洞（BH）双星系统的大型振幅径向速度（RV）变化以及对明亮伴星光谱特征的非检测，最近提出AL8814作为Be星+黑洞双星系统。我们重新分析了AL8814的低分辨率和中分辨率LAMOST光谱，并表明该系统的光谱变异性比之前认识到的要复杂得多。检查该系统的拖尾光谱显示存在一组与Be星反相移动的吸收线。此外，发射线和吸收线表现出不一致的RV变化，表明它们追踪不同的恒星。使用光谱分离，我们恢复了一个快速旋转伴星的光谱，其宽而浅的线条此前未被检测到。发射线的时间变化使分离后的光谱解释变得复杂，因此各组分的物理参数仍然不确定，但通过模拟我们发现仅发射线的变化不太可能解释伴星的所有特征。该系统的高Gaia RUWE值表明有一个第三明亮伴星，使AL8814成为一个可能的分级三重系统。尽管它不太可能包含黑洞，但该系统是不寻常的，具有在任何已知的经典Be星中观察到的最大RV半振幅，并且伴星似乎没有被剥离。需要更广泛的光谱监测和高分辨率成像来完全表征该系统的轨道结构、恒星参数和演化状态。