How close can you get to a black hole?

Streams of gas fall to their dooms, plunging into black holes, locked away from the universe forever. In their final moments, these gassy shreds send out one last flare of light, some of the brightest emissions in the universe.

These death dives are too far away to be seen directly, but astronomers have devised a new technique for detecting their panicked cries for help. They're using the method to test our knowledge of gravity in the most extreme environments in the universe.

In a new study, physicists looked at specific features of that light to figure out the closest you can get to a black hole without having to work hard to prevent disaster — a threshold called the innermost stable circular orbit or ISCO. The researchers found their method could work with more sensitive X-ray telescopes coming online.

Over the waterfall

The event horizon of a black hole is the invisible line-in-the-sand across which you can never return. Once anything passes through the event horizon, even light itself, it can no longer return to the universe. The black hole's gravity is just too strong within that region.

Outside a black hole, however, everything is just dandy. A particular black hole will have a certain mass (anywhere from a few times the mass of the sun for the smaller ones in the galaxy up to billions of times heavier for the true monsters roaming the cosmos), and orbiting the black hole is just like orbiting anything else of identical mass. Gravity is just gravity, and orbits are orbits.

Indeed, lots of stuff in the universe finds itself orbiting around black holes. Once these foolhardy adventurers get caught in the black hole's gravitational embrace, they begin the journey toward the end. As material falls toward the black hole, it tends to get squeezed into a razor-thin band known as an accretion disk. That disk spins and spins, with heat, friction, and magnetic and electric forces energizing it, causing the material to glow brightly.

In the case of the most massive black holes, the accretion disks around them glow so intensely that they get a new name: active galactic nuclei (AGN), capable of outshining millions of individual galaxies.

In the accretion disk, individual bits of material rub up against other bits, draining them of rotational energy and driving them ever-inward to the gaping maw of the black hole's event horizon. But still, if it weren't for those frictional forces, the material would be able to orbit around the black hole in perpetuity, the same way that the planets can orbit around the sun for billions of years.

A call for help

As you get closer to the black hole's center, though, you reach a certain point where all hopes of stability are dashed against the rocks of gravity. Just outside the black hole, but before reaching the event horizon, the gravitational forces are so extreme that stable orbits become impossible. Once you reach this region, you cannot remain in placid orbit. You have only two choices: if you have rockets or some other source of energy, you can propel yourself away to safety. But if you're a hapless chunk of gas, you're doomed to fall freely toward the waiting dark nightmare below.

This boundary, the innermost stable circular orbit (or ISCO for the lovers of astronomical jargon), is a firm prediction of Einstein's general theory of relativity, the same theory that predicts the existence of black holes in the first place.

Despite the success of general relativity in predicting and explaining phenomena across the universe, and our sure knowledge that black holes are real, we've never been able to verify the existence of the ISCO and whether it conforms to the predictions of general relativity.

But the gas that falls to its doom may provide a way for us to verify that existence.

Dancing lights

A team of astronomers recently published an article in the journal Monthly Notices of the Royal Astronomical Society, which also was uploaded to the preprint journal arXiv, describing how to take advantage of that dying light to study the ISCO. Their technique relies on an astronomical trick known as reverberation mapping, which takes advantage of the fact that different regions around the black hole light up in different ways.

When gas flows from the accretion disk, past the ISCO — the innermost part of the accretion disk — and into the black hole itself, it becomes so hot that it emits a broad swath of high-energy X-ray radiation. That X-ray light shines in all directions away from the black hole. We can see this emission all the way from Earth, but the details of the accretion disk structure get lost in the blaze of X-ray glory. (Understanding more about the accretion disk will help astrophysicists get a handle on the ISCO, as well.)