BE Lyncis: An Extremely Eccentric Binary with the Nearest Known Black Hole

Abstract

We report the discovery of an exceptionally eccentric binary system, BE Lyncis (BE Lyn), which hosts the nearest known black hole (BH) to Earth. Through the analysis of $\textit{TESS}$ photometry combined with an extensive set of times of maximum light spanning 39 years, we identify BE Lyn as a high-amplitude $δ$ Scuti star in a binary with an orbital period of $\approx15.9$ years and an extraordinary orbital eccentricity of $e=0.9989^{+0.0008}_{-0.0021}$ ($>0.9968$ at 95% confidence) -- the highest reliably measured for any binary system. Dynamical constraints impose an upper limit on the orbital inclination of $i \lesssim 4.0^{\circ}$, corresponding to a companion mass of $M_2 \gtrsim 17.5~M_{\odot}$, which unequivocally favors a black hole. This system provides a unique laboratory for studying asteroseismology in strong gravitational fields, the formation of black holes via asymmetric supernovae, and the evolution of extreme binary systems. Our work demonstrates, for the first time, the successful application of the light-travel time effect in a pulsating variable to unveil a dormant black hole, establishing a novel method for BH detection in non-interacting binaries.

Figures (5)

• Figure 1: $O-C$ diagram of BE Lyn (with the linear ephemeris subtracted). The top panel shows the data and the best-fit model (magenta solid line), with the quadratic component indicated by the green dashed line. The bottom panel displays the residuals. Data sources are color-coded: Boonyarak2011 (blue), historical literature Hubscher_2013Hubscher_2014Hubscher_2017Pena2015Pagel_2021Pagel_2022Pagel_2023 (gray), AAVSO (red), and TESS (green).
• Figure 2: Hertzsprung--Russell diagram showing evolutionary tracks (colored solid lines from the zero-age main sequence, ZAMS, to post-MS) and the best-fit seismic model for BE Lyn's primary (red star). Black rectangles highlight zoomed-in views around the best-fit solutions. Colored dashed lines indicate tracks with a $0.01 M_{\odot}$ mass step.
• Figure 3: Dynamical constraints on the unseen companion. Top: Companion mass $M_2$ versus orbital inclination $i$, derived from the mass function (red curve). Bottom: Primary-companion separation at periastron versus $i$ (green curve). The intersection of the primary's Roche lobe radius $R_{1,\mathrm{R}}$ (blue curve) and its physical radius $R_1$ (grey curve) sets the stringent upper limit $i \lesssim 4.0^\circ$ (dark blue dot), corresponding to the lower mass limit $M_2 \gtrsim 17.5 M_{\odot}$ (dark red dot). The gray hatched region denotes parameter space excluded.
• Figure A1: Light curve of BE Lyn from TESS Sector 21. The lower panel shows a one-day zoom for clarity.
• Figure A2: Frequency spectrum of BE Lyn. The main panel ($0$--$100\ \mathrm{c/d}$) marks the fundamental ($f_0$, red) and first-overtone ($f_1$, green) modes. The inset highlights the low-amplitude, non-radial mode region ($\sim 9-17\ \mathrm{c/d}$) with a linear y-axis.