Sometimes, you get lucky. There's a galaxy cluster that's been a frequent target of X-ray observatories due to the galaxies' interactions with the relatively dense gas in the vicinity. So, when a bright source of X-rays suddenly appeared behind this cluster, that brightness was imaged relatively quickly, and follow-up images have been captured regularly since then. Now, researchers are reporting that, a decade after the source of the X-rays first appeared, it's still around. That leaves just one likely explanation: the source is a supermassive black hole that's been devouring a star for all this time.

The source, 3XMM J150052.0+015452 (we'll stick with "the source" or "the object"), wasn't present when the region was first imaged in 2005. But it appeared in data obtained only four months later and has shown up in at least eight further observations since. Even at its first appearance, it was emitting energy the equivalent of over 1019 megatons every second, and it actually intensified considerably after that. Obviously, that implies something big.

Looking at visual images, the X-ray source appears to be centered on a small galaxy, roughly the size of the Large Magellanic Cloud that orbits the Milky Way. The researchers analyzing the data focus on the supermassive black hole at the center of this galaxy. Based on the galaxy's size, it would be a relatively "small" one, at about a million solar masses.

The area around black holes releases large amounts of energy when the black hole is drawing in matter. In the past, we've seen this power outbursts that last as long as a year. But this has been going for a decade, and is rather intense. As a result, the models we have that describe black hole feeding suggest that it's been exceeding a physical limit called the Eddington limit for the entire decade.

The Eddington Limit actually has something in common with the solar sails we discussed recently: photons that strike matter can impart a tiny bit of momentum to that matter. If the light is intense enough, it can actually repel the matter. As the matter falling into a black hole emits a lot of photons, it can create a pressure that keeps further matter from falling in. As more matter falls in, things eventually reach a point where the repulsive force starts to cut off the black hole's food supply.

So there's a limit to how much matter a black hole can swallow at once, called the Eddington limit (named after Arthur Eddington, the first man who tried to calculate its value). Given the mass of a black hole, we can calculate the Eddington limit and the amount of energy released as that matter falls in.

Doing so for our X-ray-emitting black hole suggests that it may have been exceeding the Eddington limit for at least a decade. We have seen things like this before, coming from black holes with masses similar to our Sun. But the amount of energy released by those black holes was much lower, and it generally tailed off within a year.

What could possibly supply a black hole with enough matter to last a decade? The researchers suggest that the object's gravity has torn a star apart and has been feeding on its remains. A star the size of the Sun, for example, would provide enough matter to feed a black hole of this size for up to two years. Scaling this up would imply that the star being swallowed by this object is massive, about 10 times the Sun's mass.

But it's also possible for different parts of the former star to interact as the black hole draws them in, extending out the period of high-level feeding. In this case, you don't need a star to be quite that big for feeding to last a decade. The researchers calculate that a star just twice the Sun's size would work for providing a decade of feeding, during which time the black hole would have swallowed the equivalent of the Sun's mass.

How can we tell the difference? If the culprit is a bigger star, the feeding could continue for quite some time, whereas a star twice the mass of the Sun would be largely digested by now. Recent observations suggest that the intensity of the X-ray emissions may be dropping off, which would be consistent with this. But the dimming is also consistent with a moving mass of gas somewhere between us and the black hole. We'll have to keep watching for a few more years to see what's going on with this object.

Nature Astronomy, 2017. DOI: 10.1038/s41550-016-0033 (About DOIs).