A 3D model of the irregular feeding pattern of a black hole reveals a potential explanation for the bizarre behaviour of dim-switched quasars.

It is highly unusual for galaxies to do anything in a hurry, but 'changing-looking' quasars are an exception.

They can switch radiation jets on and off in just years or even months. This disappearing act shocked astrophysicists when it was first observed in 2014 and continues to amaze scientists.

Located at the centre of baby galaxies, quasars (or 'quasi-stellar radio sources') typically burn 27 trillion times brighter than our Sun. Quasars consist of a ravenous supermassive black hole and its food: gas and dust swirling around it in an accretion disc.

Past models assumed that matter orbiting a black hole would cross the event horizon in a relatively orderly fashion, like water flowing through a sewer. This was assumed to be a slow process lasting 10,000 years or more.

"For decades, people made the very large assumption that accretion discs align with the rotation of the black hole," says lead author Nick Kaaz, an astrophysicist at Northwestern University in the US.

"But the gas that fuels these black holes doesn't necessarily know which way the black hole is spinning, so why would they automatically align? Changing the alignment changes the picture drastically."

When the team created a 3D computer model of a spinning black hole that was not aligned with the accretion disc, strange things started to happen.

Instead of behaving like a pizza base spinning around a chef's finger, the accretion disc began to twist and break apart.

It split into an inner sub-disc and an outer disc, and each disc spun at a different tilt, like a gyroscope.

This disc splitting was caused by the spinning black hole bending the spacetime around it, a general-relativistic process called frame dragging.

"When they [black holes] spin, they drag the space around them like a giant carousel, forcing it to spin as well," says Kaaz.

Frame dragging has a stronger pull on material orbiting closer to the black hole, while it has a weaker pull on material orbiting further away.

This difference means that the inner disc rotates faster than the outer disc.

At first, friction and pressure hold the disc together. Eventually, however, the bending of spacetime by the black hole causes the inner disc to separate into a separate disc that is smaller and therefore easier to move.

"The rupture zone is where the black hole wins," says Kaaz.

These two discs are at odd angles to each other and collide, causing shocks. 'Streams' from the outer disc rain down on the inner sub-disc and allow matter to feed into the black hole faster.

This additional mass pushes the inner disc towards the black hole to be engulfed, then the black hole's gravity pulls gas from the outer region to replenish the inner region. These processes may shed light on why quasars can suddenly switch off and on.

"The classical accretion disc theory cannot explain this violent change," says Kaaz.

"But the phenomena we see in our simulations can potentially explain it. The rapid brightening and dimming are consistent with the destruction of the inner regions of the disc."

 

Source: https://www.sciencealert.com/