BE Lyncis is not a Black Hole Binary: Lessons From Gaia and Hipparcos Astrometry

TL;DR

BE Lyncis had been proposed as a nearby dormant black hole binary based on LTTE with $e \approx 0.9989$ and a companion mass $M_2 \gtrsim 17.5\,M_{\odot}$. The authors use forward-modeled Gaia/Hipparcos astrometry (via gaiamock) to predict the 25-year PMA and Gaia DR3 RUWE under that scenario, then compare to observations. They find a predicted Hipparcos–Gaia PMA of $|\Delta\mu| \sim 20$–$45$ mas yr$^{-1}$ and DR3 RUWE $\sim 2.5$–$4.0$, both in strong conflict with the measurements ($|\Delta\mu| \approx 1.7 \pm 0.8$ mas yr$^{-1}$ and RUWE $=1.073$), and BE Lyn is absent from the Gaia DR3 non-single star catalog. The work argues that the LTTE-based orbit is likely incorrect, highlights biases in high-eccentricity, near-face-on BH claims, and recommends astrometric cross-checks and Gaia DR4 epoch data for robust vetting of dormant BH candidates.

Abstract

BE Lyncis (BE Lyn) is a well-studied high-amplitude $δ$ Scuti variable star (HADS). Recently, Niu et al. (2026) analyzed a 39-year baseline of times of maximum light of BE Lyn, reporting that it is the most eccentric binary known ($e \approx 0.9989$) and hosts the nearest black hole (BH) known to date. We analyze Hipparcos and Gaia astrometry of BE Lyn, predicting what the observed proper motion anomaly (PMA) over the 25 year baseline between the two missions would be were the companion really a $\gtrsim 17.5\,M_{\odot}$ BH. We find that the predicted PMA is at least an order of magnitude larger than the observed value of $\approx 1.7 \pm 0.8$ mas yr$^{-1}$, regardless of the assumed orientation of the orbit. We predict the expected Gaia DR3 RUWE for different orientations of the putative BH binary, finding that it ranges from $\approx 2.5$-$4.0$, much larger than the reported value of $1.073$. The observed value is instead consistent with a low-mass secondary or a single star. We find that BE Lyncis would have received a 7-parameter acceleration solution if it were a BH binary, in contradiction with its absence from the Gaia DR3 non-single star catalogs. Finally, we show that the reported orbit is impossible because the luminous star would overflow its Roche lobe at periastron, irrespective of inclination. We recommend caution in interpreting light-travel time effect (LTTE) models that require very high eccentricities, face-on inclinations, or large companion masses. The observed pulsation timing variations are most likely simply a result of red noise or pulsation phase evolution.

Figures (1)

• Figure 1: Predicted instantaneous proper motions for BE Lyn, assuming the best-fit orbital parameters reported by be_lyn_2026. We adopt $\Omega = 60^{\circ}$. We show the predicted Hipparcos, Gaia DR2, and Gaia DR3 proper motions as well, with the predicted individual epoch measurements plotted with lower opacity. We zoom in on these astrometric measurements in the lower panels. In these panels, we denote the measured Hipparcos proper motions with dashed lines; the shaded regions represent the corresponding uncertainties. Clearly, the observed and predicted Hipparcos proper motions are quite different. We predict that we would observe a proper motion anomaly of $\Delta \mu_{\alpha*} \approx 31$ mas yr$^{-1}$ and $\Delta \mu_{\delta} \approx 15$ mas yr$^{-1}$ over a 25-year baseline, far greater than the $1$--$2$ mas yr$^{-1}$ differences that are actually observed.