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

TL;DR

This study uses Gaia and Hipparcos astrometry to challenge claims that BE Lyncis hosts a black hole binary, showing the data is inconsistent with such a scenario and highlighting the importance of careful interpretation of orbital models.

Contribution

The paper demonstrates that astrometric data contradict the black hole binary hypothesis for BE Lyncis, emphasizing the need for cautious analysis of high-eccentricity orbital models.

Findings

Predicted proper motion anomaly is much larger than observed.

Gaia DR3 RUWE value is inconsistent with a black hole companion.

Roche lobe overflow would occur at periastron, making the orbit impossible.

Abstract

BE Lyncis (BE Lyn) is a well-studied high-amplitude $\delta$ 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.