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 ≳2.5 M ⊙ . By combining TESS photometry with an extensive set of times of maximum light spanning 39 yr, we identify BE Lyn as a high-amplitude δ Scuti star in a binary with an orbital period of ≈15.9 yr and an extraordinary eccentricity of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>e</mml:mi> <mml:mo>=</mml:mo> <mml:mn>0.998</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>9</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.0021</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.0008</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> (&gt;0.9968 at 95% confidence)—the most extreme eccentricity reliably measured for any binary system. Dynamical constraints limit the orbital inclination to i ≲ 10 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover> <mml:mrow> <mml:mo>.</mml:mo> </mml:mrow> <mml:mrow> <mml:mtext>°</mml:mtext> </mml:mrow> </mml:mover> </mml:math> 1, implying a companion mass M 2 ≳ 2.5 M ⊙ , 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.