Astronomers using a special NASA telescope say they've discovered an "impossibly" bright dead star pulsating with so much energy they at first thought it must be a black hole.

The astronomers used NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) to detect the distant object emitting beams displaying the energy equal to around 10 million of our suns.

The powerful object is a pulsar, made up of extraordinarily dense remains of a dead star that have begun to rapidly rotate.

"This compact little stellar remnant is a real powerhouse," says Fiona Harrison, NuSTAR investigator and a physics professor at Caltech. "We've never seen anything quite like it. We all thought an object with that much energy had to be a black hole."

The extreme energy of this pulsar makes in an oddity in the cosmic zoo, researchers say. Pulsars are usually about one or two times as massive as our sun, but newly discovered pulsar is around 100 times as bright as current theories on pulsars suggest something with its mass ought to be.

"We've never seen a pulsar even close to being this bright," says Caltech postdoctoral scholar Dom Walton. "Honestly, we don't know how this happens, and theorists will be chewing on it for a long time."

The researchers have published their NuSTAR discovery in the journal Nature.

What makes the discovery all the more exciting is that the scientists were making observations of a different cosmic target entirely.

Astronomers around the world had turned their telescopes on a relatively close galaxy known as M82 after a once-in-100-years supernova was detected there earlier this year.

Matteo Bachetti of the University of Toulouse in France, lead author of the Nature paper, was looking for bright X-ray sources, or ULXs, within the galaxy when he detected an object pulsing with light.

"That was a big surprise," Harrison says. "For decades everybody has thought these ultraluminous X-ray sources had to be black holes. But black holes don't have a way to create this pulsing."

That's the feature trick of pulsars, which act like giant rotating magnets that emit streams of radiation from their opposite magnetic poles.

As pulsars rotate, they seem to pulse as the powerful beams of light sweep through the cosmos.

Attempting to pin down the exact source, the researchers looked at 25 X-ray sources in the M82 galaxy, finally identifying a ULX dubbed M82X-2 as the pinpoint source of the immensely powerful flashes.

The object's brightness is several times above what is known as the Eddington limit, a primary tenet of physics that holds there's upper limits to how bright an object with a given mass can be.

"This is the most extreme violation of that limit that we've ever seen," says Walton. "We have known that things can go above that by a small amount, but this blows that limit away."

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