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Giant Blast of X-Rays Traced to Dead Star

Ultraluminous x-rays found spurting from dead stars may overturn the idea that only actively feeding black holes have the power to fire up these displays.

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Intense x-rays (pink) emanate in pulses from one spot in the M82 galaxy, suggesting their source is a pulsar. Such ultrabright x-rays had previously been thought to originate only in actively feeding black holes.

Astronomers have pinned down the origins of an especially spectacular fountain of x-rays from a distant galaxy. The surprising source of all those x-rays: a dead star known as a pulsar, researchers report in this week's Nature.

The finding overturns the received wisdom that black holes, rather than their less glamorous relatives, deserve the credit for every ultrabright x-ray display.

Astronomers who thought all these x-ray shows came from black holes "have to swallow hard and say, 'Yes, they're not all black holes,'" says University of Iowa astrophysicist Philip Kaaret, who was not involved in the new study and says the finding leaves him "kind of shocked."

Also shocked is Matteo Bachetti of Italy's Cagliari Observatory, who led the team that identified the pulsar. When Bachetti began studying a patch of ultrabright x-rays in the nearby M82 galaxy, he was convinced the x-rays were coming from a black hole. But then he began to pore over data from NASA's NuSTAR telescope and realized that the x-rays were blinking in and out. Black holes don't pulse that way.

The pulsating x-rays were the signature of a pulsar, a rapidly whirling fossil star. Because of the pulsar's spin, its radiation is seen in spurts. It's like a lighthouse whose rotating beam sweeps past a particular spot with a precise rhythm.

The pulsar is also circling a much bigger companion star and, in not-very-neighborly fashion, yanking off some of its companion's outer materials and pulling them toward itself. This purloined material accelerates to nearly the speed of light and heats up to temperatures of tens of millions of degrees as it approaches the pulsar.

Radiation in the form of x-rays bursts away from the dead star and streams into space. By Kaaret's estimate, the pulsar pours out as much radiation in a single second as the sun flames out in an entire month. If the Earth were in orbit around such an object, "we would be fried," Bachetti says.

Apparently no one told the pulsar that it's emitting a hundred times more radiation, in the form of x-rays, than it can. By well-established theory, matter can't fall into the pulsar fast enough to cause the release of such an extraordinary quantity of x-rays.

Perhaps there's another explanation. Maybe the radiation is streaming from the pulsar in a neat beam rather than in a more diffuse form, says University of Michigan astrophysicist Joel Bregman, who was not involved with the new study. That would allow more matter to fall toward the pulsar.

"The only problem is that no one quite understands how to do that," Bregman says.

If this dead star does indeed violate theories about how fast material can fall into a pulsar, researchers may also need to revise their thinking on how fast material can fall into a black hole. That in turn could change theories of how fast black holes grow, a key underpinning of our understanding of the evolution of the universe, Bachetti says.

In the meantime, Bachetti and his colleagues will try to understand how their new pulsar blasts out enough x-rays to draw the attention of humans 12 million light-years away.