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

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.

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