Last updated February 21, 2018 at 11:03 am
Scientists still do not fully understand the accretion process around black holes, but a new study that shows how much material is expelled outwards with enormous force, gives us some more clues.
Credit: iStock
Black holes are not the all devouring bottomless pits Hollywood would have us believe. For one thing they’re messy eaters, with barely a fifth of what falls towards them actually making it in. The rest is blasted across space in a truly staggering display of wind, and a recent study has caught a dozen of them in the act.
The ultimate prison
A black hole is defined as a region where the gravity of an object is so great that not even light can escape, resulting in a perfectly black area.
And since Einstein tells us mass can never travel as fast as light, then the blackness is the ultimate gravitational prison.
As any signals of events occurring travel outwards at the speed of light, anything happening within this region is lost to the outside world, which is why the boundary is known as the Event Horizon.
So it may come as a surprise that most material falling towards a black hole does not, in fact, become trapped within it. Why? For the same reason that the Earth doesn’t fall into the Sun or the Moon onto the Earth.
It is because they have angular momentum, their motion around an object, and that they must lose this first before they can fall inwards.
And the same occurs around the black hole, and to understand why 80 percent of its meal doesn’t make it to the Event Horizon we first have to understand how it feeds.
Spinning meals
As material falls towards a black hole it will have some sideways motion meaning it doesn’t fall straight in like a ball in a billiards / pool pocket. Instead will swirl around the black hole to form a disc – what is known as an accretion disc – as if it were water forming a vortex as it spins down the gravitational plughole.
This disc spins ever faster and faster, the closer it comes to the Event Horizon – ultimately reaching close to the speed of light itself.
But friction between particles moving at high speeds heats them up. It’s similar to the way a heat shield on a spacecraft re-entering the atmosphere glows red hot from friction with air.
Related: NASA takes us on a virtual journey to the centre of the galaxy
But an accretion disc is moving thousands of times faster than a spacecraft and, as a result, the temperatures can reach millions of Kelvin. This glow can be seen across the Universe as quasars around feeding supermassive black holes.
To summarise, friction in the accretion disc transfers angular momentum outwards, lets mass fall inwards and all the while light radiates the orbital energy away.
On a smaller scale, black holes with masses similar to our Sun can appear in our galaxy as X-ray binaries when they rip material from a companion star.
But it’s here that things can get a little different.
Have you tried turning it on-off again?
Material that reaches the X-ray binary is relatively cool, meaning atoms can hold on to their electrons and the entire disc is neutral.
As gas piles up in the disc, the temperature rises until eventually at some point in the disc, known as the ignition radius, it becomes hot enough to liberate the electron from the hydrogen atom and the gas becomes ionised.
At this point the ionised gas interacts with the enormously strong magnetic field around the black hole and it is funneled inwards dramatically.
During this phase distant telescopes on Earth can see this hot gas as a dramatic brightening, or flaring, of the X-ray binary.
Eventually the disc loses so much mass that the temperature cools enough for a neutral disc to once again form, and the process repeats.
The speed at which the mass is lost, and the flare fades, is known as the viscosity parameter; the higher the viscosity the greater the mass is lost inside the black hole and quicker the flare fades returning to the calm, neutral state.
It is this parameter which the investigators searched for in 21 outbursts from 12 known X-ray binaries.
Sticky situation
This was the first time that the viscosity parameter had been directly measured for low mass X-ray binaries in their irradiated state and it was shockingly high – five times more than any theoretical model had suggested possible.
To drag so much material into the black hole would entail new and exciting physics between the flowing gas and intense magnetic fields, a branch of notoriously challenging physics known as magneto-hydrodynamics.
The other explanation is that the material wasn’t dragged in but instead expelled outwards. In that case, the researchers were witnessing a wind in action.
This wind would in some cases be enormous, removing 80 per cent of all of the mass in the disc to sufficiently shorten the X-ray binary flare. In the process of material falling towards the black hole almost all of it appears to be heated and blasted away.
In fact there is so much material lost from the black hole’s diet that it has ramifications for how rapidly they can grow, as this lost material could otherwise feed them.
It also gives support to theoretical models of galaxy formation that require their larger cousins, supermassive black holes, to expel most of their potential accretion material wind outwards.
These winds are so energetic that they blast the gas in galaxies apart, throttling down the rate at which the galaxies can turn gas into stars, and conserving the material for many more billions of years.
Simulations show that our Milky Way may well have used all of its gas reserves before our Sun had a chance to form without the wind from black holes.
The old adage that it’s better out than in appears to be true for black holes, just don’t follow their example in polite company.
The research was published in Nature.
