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.