A spinning top on a table wobbles: its axis is tilted, and the motion becomes unsteady. Black holes surrounded by a disk of gas and dust behave similarly. If the rotation axes of the black hole and the disk do not align, the whole system begins to vibrate, producing ripples in spacetime — gravitational waves.
In a new study, scientists for the first time used supercomputers to model such tilted systems, taking into account both magnetism and the disk's own gravity. The disk was massive — up to 28% of the black hole's mass — and the hole itself spun at nearly its maximum rate. The magnetic fields threading the disk act as a damper or an accelerator depending on the direction of rotation. When the disk spins in the same direction as the hole, the fields dampen the instability. When it spins in the opposite direction, they instead fuel it, making gravitational waves noticeably stronger.
In all models, narrow jets of plasma — accelerated by magnetic fields — erupted from the black hole's poles. Their direction and shape depended on the tilt of the rotation axis. Such systems are candidates for sources of two signals at once: gravitational waves and electromagnetic radiation from the jets. This makes them ideal targets for multimessenger astronomy, where a single event is observed simultaneously with different instruments.
The collision of two such "tops" — a black hole with a tilted disk — leaves a characteristic imprint in gravitational waves: asymmetric, with one dominant mode. Next-generation detectors will be able to catch this signature and distinguish tilted systems from neat, aligned ones.
Magnetic fields in a disk around a black hole can be billions of times stronger than that of a refrigerator magnet — they are literally twisted up by the disk's rotation.
In the movie Interstellar, the black hole Gargantua is surrounded by a glowing accretion disk, but its axis aligns with the hole's rotation axis — a tilted disk would look completely different and would generate noticeable gravitational waves.