Evrim Yazgin
Cosmos science journalist
Astrophysicists at Melbourne’s Monash University have generated the first simulation which accurately depicts what happens when a star ventures too close to a supermassive black hole.
The research, published in Astrophysical Journal Letters, is a technical milestone in our attempts to understand these mysterious cosmic giants.
First author Daniel Price, a professor at Monash, tells Cosmos that there are about 100 events which have been observed over the past decade-and-a-half which astronomers believe fit the bill to be a star being destroyed by a supermassive black hole, also called a tidal disruption event (TDE).
Not X-ray vision
But these observations have thrown up some odd measurements which haven’t been explained until now.
“If you dump a bunch of material close to black hole and form an accretion disk around that black hole, there’s a prediction for where the material should land,” Price says. “The material at that location should be more than a million degrees in temperature. It should generate X-rays.
“So, if you have unobscured stuff feeding a black hole, you get X-ray emission. For example, the black hole sources in the galaxy, they’re all X-ray emitters.”
Stars falling into supermassive black holes, however, do not result in emission of X-rays. They emit light in the visible, or optical, spectrum.
Current theories can only speculate why such events lead to material being flung toward us at 20,000km per second – about one-fifteenth the speed of light.
An eating analogy – but not in the way you think
Price explains that the simulation illuminates why it is optical light, not X-rays, which we observe when our telescopes pick up stars falling into supermassive black holes.
“The analogy with me eating is that you don’t see my stomach. You’re not seeing the thing that’s generating the energy, you’re seeing it reprocessed through my skin,” Price says. “If you look at my light curve, you see that I’m a constant temperature of 38°C all day.
“My light curve is very much like a disruption event. The temperatures are pretty much constant. Luminosity changes a bit, but you infer that’s because the size of the objects changing, but the temperature evolution is very flat. So, it looks like exactly like me, just a lot warmer and a lot bigger.”
In fact, this size of the photosphere – the object which emits the optical rays – itself is surprising, says Price.
The photosphere in the simulation, which matches observations, is about 100 astronomical units (AU), where 1 AU is the distance from the Earth to the Sun (roughly 150 million kilometres).
“No one knows what it is,” Price laughs.
What we see is muffled
Price says the simulations confirm a theoretical explanation for these unexpected observations called the Eddington envelope.
“That’s the concept that you’re stuffing material down towards the black hole faster than it can process it,” Price says. “By process, I mean like the sun processes the energy from its core – it just kind of gently radiates it away. So the black hole can’t radiate away the stuff that you’re trying to feed it. And, so, it has to literally blow it away.”
This material “smothers” the black hole, absorbing the X-rays that the black hole emits and re-emitting it as optical light.
Price extends the eating analogy to an unpleasant place.
“Basically, it’s like stuffing your stomach. You’re going to vomit eventually. That’s pretty much what happens.”
The power of a simulation
“That’s the exciting thing in simulations. People have speculated for a long time and drawn illustrations and this kind of thing, but there’s no physics in that. That’s just what we call phenomenology. That’s how it must be to explain this phenomena. But we don’t know what produces that kind of envelope or layer, or reprocessing layer,” Price says.
The simulation, Price says, just requires the initial conditions – the star – the fluid mechanics governing the star, and the rules of general relativity.
“Then it’s just a technical challenge,” he says.
“In a lot of simulation work, you’re kind of guessing what might have happened,” he adds. “But in this case, we’re pretty sure what happens. It’s really nice to get that connection to the observations of transients from just chucking a star at a computer.”
Price explains that the simulation will set astrophysicists and astronomers up to be able to understand such phenomena much better as more observations are expected to be made soon.
“The first optical transient was only detected in 2010, but what’s coming is the Rubins observatory being built in Chile. That’s expected to boost the population of these things into the thousands.
“Having a good theoretical understanding of what the kind of phenomena is sets us up really well for that future flood of observations. It’s not just some theoretical speculation. There’s really something we can go after and understand by looking at it.”
