BE Lyncis: A Pulsating Star in the Most Eccentric Binary with a Massive Unseen Companion

Abstract

We report the discovery of an exceptionally eccentric binary system, BE Lyncis (BE Lyn), which might host a compact companion with mass $\gtrsim 2.5M_{\odot }$. By combining TESS photometry with an extensive set of times of maximum light spanning 39 years, we identify BE Lyn as a high-amplitude $\delta$ Scuti star in a binary with an orbital period of $\approx 15.9$ years and an extraordinary eccentricity of $e=0.9989_{-0.0021}^{+0.0008}$ ($>0.9968$ at 95% confidence) -- the most extreme eccentricity reliably measured for any binary system. Dynamical constraints limit the orbital inclination to $i\lesssim 10^{\circ }.1$, implying a companion mass $M_2\gtrsim 2.5M_{\odot }$, which identifies the companion as a compact object. This mass points to it most likely being a black hole; if instead it is a rapidly rotating neutron star, it would be the most massive known. If the black hole interpretation holds, it would be the closest such object to Earth. This system provides a unique laboratory for studying asteroseismology in strong gravitational fields, as well as the formation and evolution of extremely eccentric binaries. Our work demonstrates the use of the light-travel time effect in a pulsating star to reveal a compact companion, offering a novel method for detecting black holes in noninteracting binaries.

17 pages, 5 figures, 4 tables. ApJL accepted

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