When a star wanders too close to a black hole, the black hole starts pulling it in. If the star doesn't fall in entirely but only loses some material, it can survive several such 'bites.' Each time, a chunk of gas is ripped off, heats up, and flares — astronomers see a bright spot in the sky.
Such repeated flares are rare, but they are exactly what we need to test how well we can weigh black holes. The new study compared three such events. For each one, the black hole's mass was estimated in several ways — from the infall speed of the gas, from the flare's brightness, and from its temperature. It turned out that all methods give similar numbers: the spread is only a factor of about two to three. That's not bad for objects sitting millions of light-years away.
But there's a catch. The models that describe how gas falls onto a black hole sometimes produce nonsensical parameters — for example, a star that's too light or an impact at an impossible angle. That means the models themselves still need work. It's especially important to observe flares in ultraviolet light: without those data, the parameters are hard to pin down.
In the coming years, telescopes will spot thousands of such flares. If we don't learn to interpret them correctly, black hole masses will be systematically underestimated — by a factor of about three. And that affects our understanding of how black holes grow and evolve in the centers of galaxies.
as written by the authors arXiv:2601.04406
Tidal disruption event (TDE) light curves are increasingly used to infer the masses of quiescent supermassive black holes ($$M_{\rm{BH}}$$), offering a powerful probe of low-mass black hole demographics independent of host-galaxy scaling relations. However, the reliability of most semi-analytic TDE models assume full stellar disruption, despite theoretical expectations that partial disruptions dominate the TDE population. In this work we test the robustness of current TDE models using three repeating partial TDEs (rpTDEs), in which the multiple flares produced by the same surviving stellar core must yield consistent black hole masses. We present spectroscopic observations establishing AT 2023adr as a rpTDE, making it the third such spectroscopically confirmed event. We independently model the flares of the three rpTDEs; 2020vdq, 2022dbl, and 2023adr, applying fallback-accretion fits, stream-stream collision scaling relations, luminosity-based empirical relations, and cooling-envelope fits. After accounting for statistical and model-specific systematics, we find that all TDE models generally return self-consistent $$M_{\rm{BH}}$$ values between flares, and are broadly consistent with host-galaxy $$M_{\rm{BH}}$$ proxies, recovering $$M_{\rm{BH}}$$ to within 0.3-0.5 dex. However, the convergence of fallback models towards unphysical stellar masses and impact parameters reveals limitations in the existing fallback model grids. We also show that light curve coverage, particularly in the near-UV, is critical for constraining model parameters. This has direct implications for interpreting the thousands of TDE light curves expected from upcoming surveys such as the Rubin Observatory's Legacy Survey of Space and Time, where from simulations, we find that $$M_{\rm{BH}}$$ may be underestimated on average by 0.5 dex without additional follow-up.
A star that survives several close encounters with a black hole can lose up to a few percent of its mass per 'bite'—and still keep shining.