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A Hungry Black Hole

Astronomers have found a black hole that is consuming gas from a nearby star ten times faster than previously thought.

The black hole, known as P13, is situated near the galaxy NGC7793, about 12 million light years from Earth, and is ingesting a weight “equivalent to 100 billion billion hot dogs every minute”.

Dr Roberto Soria of the International Centre for Radio Astronomy Research at Curtin University said that as gas fell towards a black hole it becomes very hot and bright. “Scientists first noticed P13 because it was a lot more luminous than other black holes, but it was initially assumed that it was simply bigger,” Soria said.

“It was generally believed the maximum speed at which a black hole could swallow gas and produce light was tightly determined by its size. 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.”

When Soria and his colleagues from the University of Strasbourg measured the mass of P13 they found it was actually on the small side, despite being at least a million times brighter than the Sun. It was only then that they realised just how much material it was consuming.

“P13 rotates around a supergiant ‘donor’ star 20 times heavier than our own Sun,” Soria said. The scientists observed that one side of the donor star was always brighter than the other because it was illuminated by X-rays coming from near the black hole, so the star appeared brighter or fainter as it went around P13.

“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. From this we worked out that the black hole must be less than 15 times the mass of our Sun.”

Dr Soria said P13 is a member of a selected group of black holes known as ultra-luminous X-ray sources, which are “capable of swallowing their donor star in less than a million years, which is a very short time on the cosmic scales”.

The discovery was published in Nature.