What Powers a Black Hole's Jet? Three Suspects express
A branch falls into a whirlpool. It gets spun and broken, but sometimes a powerful fountain of water bursts from the center. Similarly, a black hole tears apart a passing star, and occasionally a blinding plasma jet, a jet, shoots into space.
Where does the energy for such a fountain come from? The prime suspect is the black hole and its curved spacetime. According to the Blandford–Znajek mechanism, the jet is powered by the hole's rotation and the magnetic field threading its horizon. But where does the field itself come from? The authors considered three possibilities. The first is the magnetic field of the shredded star. Alas, measurements show that a typical stellar field is too weak to accelerate the jet to observed speeds. It's like trying to light up a stadium with a pocket flashlight.
The second idea is more elegant: the field is carried in by the star's debris as it falls onto the hole from large distances (the lasso mechanism). But for the field not to dissipate along the way, it must be special, decaying slowly with distance, and the debris must neatly pack it near the horizon. The third idea is the most down-to-earth: the magnetic field was already there, in the remnants of an old accretion disk — the very "whirlpool" of gas and dust that swirled around the hole long before the catastrophe. Calculations show that such a "stored" magnetic flux can provide the needed power without extra tricks.
So, most likely, the dazzling jet is not a gift from the shredded star but a legacy of the old disk, suddenly getting a fresh supply of fuel. The fountain shoots not because the branch is electrified, but because the funnel was already charged.
Original abstract
as written by the authors arXiv:2607.20615
Tidal Disruption Events (TDEs) occur when a star approaches a black hole closely enough to be torn apart by tidal forces, after which the stellar debris begins to orbit the black hole. Over the past decade, hundreds of TDEs have been identified, with many more expected from upcoming surveys. A small subset of these events launch transient, highly luminous relativistic jets (10^47-10^48 erg/s isotropic-equivalent). These are generally interpreted in terms of the Blandford--Znajek mechanism, implying the presence of substantial magnetic flux near the black hole horizon. The question arises: What is the origin of this flux? In this paper, we investigate three candidate sources: stellar magnetic fields, magnetic flux from large radii around the black hole (the Lasso mechanism), and magnetic flux stored in the inner portion of a pre-existing accretion disk. We find that: observed stellar magnetic fields are insufficient to power these jets; the Lasso mechanism requires shallow radial magnetic field profiles and a mechanism to trap the magnetic flux brought by the debris in the vicinity of the black hole; pre-existing magnetic flux near the black hole in an accretion disk is the most plausible source.
Tidal disruption events (TDEs) are so rare that in a galaxy like ours they occur once every 10–100 thousand years. But since there are billions of galaxies, astronomers are seeing them more and more often.
In Peter Watts's novel Blindsight, alien probes use black holes' magnetic fields for interstellar communication — much like TDE jets, but artificial.