Small Black Hole Has Huge Appetite That Defies Theory

Artist's rendering of what P13 would look like close up. Credit: Image created by Tom Russell (ICRAR) using software created by Rob Hynes (Louisiana State University).

Astronomers have recently discovered that P13, a small black hole about 12 million light years away on the outskirts of galaxy NGC7793, eats very quickly. In fact, P13 is stripping material away from a nearby star about ten times faster than was previously believed to be physically possible. The research was led by Christian Motch from the University of Strasbourg, and the paper was published in Nature.

As black holes consume stars, the gas heats up and gives off light. P13 was giving off quite a bit of light, giving astronomers the impression that it was a very large black hole. Because they initially believed it was much bigger than it is, it wasn’t viewed as odd at first, 

"It was generally believed the maximum speed at which a black hole could swallow gas and produce light was tightly determined by its size," said co-author Roberto Soria of the International Centre for Radio Astronomy Research (ICRAR) in a press release. "So it made sense to assume that P13 was bigger than the ordinary, less bright black holes we see in our own galaxy, the Milky Way.”

Further analysis revealed that though P13 is a million times brighter than our Sun, it really isn’t much more massive. Soria’s team revealed flaws in the theory that a black hole’s mass and rate of consumption is a fixed relationship.

They found that the black hole is in a binary system with a star 20 times more massive than our Sun. As P13 orbits and strips away material, the heated gas creates bright spots around the star that move as the black hole moves. These bright X-ray patches allowed astronomers to track P13’s movements and learn more about its size and healthy appetite.

"This allowed us to measure the time it takes for the black hole and the donor star to rotate around each other, which is 64 days, and to model the velocity of the two objects and the shape of the orbit,” Soria explained. "From this, we worked out that the black hole must be less than 15 times the mass of our Sun.”

So much light can be seen shining from P13 that it joins an elite group known as ultra-luminous X-ray sources (ULXs). These bodies are brighter than typical stellar or black hole activity, though not quite as bright as an active galactic nucleus, which is a dense region with radiation spanning almost all of the electromagnetic spectrum. Though P13 isn’t as massive as other ULXs, its appetite contributed to its incredible luminosity.

“As hotdog-eating legend Takeru Kobayashi famously showed us, size does not always matter in the world of competitive eating and even small black holes can sometimes eat gas at an exceptional rate," he mused. "These are the champions of competitive gas eating in the universe, capable of swallowing their donor star in less than a million years, which is a very short time on cosmic scales.” 

If P13’s eating habits were compared to Kobayashi, the black hole eats the equivalent of 100 billion billion hot dogs, while Kobayashi’s title sits closer to 11 per minute. That’s fast enough to consume the mass of the Earth in less than a year.

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