A Supermassive Black Hole Becomes More Energetic When It Should Have Died Out: A Still Unexplained Mystery

In 2018, a team of astronomers from the Harvard & Smithsonian Observatory witnessed the feast of a black hole, taking advantage of a star that had gotten a little too close to it. It is therefore swallowed up by AT2018hyz, and it should normally have followed a decreasing brightness curve until its visible signal goes out. In short, he should have disappeared into the limbo of the cosmos, but the monster decided to wake up, even though it had not shown any signs of activity for three years.

In a study published on Thursday February 5, 2026 in the journal The Astrophysical Journalthe same researchers demonstrated that the intensity of its radio radiation has been multiplied by 50 compared to the first measurements of 2018. An electromagnetic explosion that should never have happened, since it goes completely against the life cycle of black holes. After IGR J17091-3624 which flashes like a beacon, we are faced with an ogre who has confused the dessert with the starter: a phenomenon which has never before been observed.

A late electric wake-up call

Normally, when a black hole shreds a star, it expels a jet of relativistic matter (at a speed close to that of light) almost instantly. This is the conventional outcome of a TDE (Tidal Disruption Event), or « tidal rupture event “. At that moment, the gravitational forces of the black hole are so heterogeneous that they stretch the star until break it, transforming it into a ribbon of plasma several million kilometers long.

Part of its debris is then violently propelled outwards, while the rest clumps together into an ultra-hot accretion disk before being engulfed. It is during this accretion phase that the radio energy is supposed to peak, before quickly fading away.

During AT2018hyz’s three years of silence, the researchers who had observed it then considered that the TDE was over. But without warning, his signal jumped, although nothing suggested such a burst of energy after such a long period of dormancy. “ It’s really unusual “, says Yvette Cendes, research associate at Harvard & Smithsonian and lead author of the study. “ I would be hard-pressed to name another object whose luminosity increases like this over such a long period of time. ».

The energy it emitted rivals gamma-ray bursts, the most powerful explosions in the Universe ; not even our Sun could produce such a quantity of energy during its 10 billion years of existence.

Researchers have two theories to explain this resurgence. The first would be caused by a phenomenon called Lense-Thirring precession : the axis of rotation of the black hole pivots on itself. The jet, like a beam rotating in slow motion, would not have ended up sweeping our line of sight until years later, which would explain why we only detected it now.

The second concerns the accretion disk. Normally, stellar remnants quickly lose their momentum and fall into the abyss. Here, the gas (hydrogen and helium) would have remained “stuck” in orbit, unable to escape towards the center. This disk would have remained dormant for three years, until the internal friction (viscosity) became sufficient to slow down the gas and cause it to fall suddenly. This sudden movement awakened the monster, expelling accumulated debris in the form of an ultra-powerful radio jet.

Neither theory cannot, however, explain what really happened. without calling into question our digital simulation models. Either they are incomplete (which is highly probable)where the temporality of TDE needs to be completely reviewed. In both cases, nHere we reach the limits of our knowledge mechanics inherent to the accretion of black holes and the action of gravitational fields. By following the evolution of AT2018hyz until its broadcast reaches its peak in 2027maybe the Cendes team will have elucidated the mystery of this late awakening.