Glimpse at black hole's edge provides new insights

Chandra X-ray Observatory image of the spectrum of a black hole

By Richard Stenger
CNN

(CNN) -- Using a fleet of orbiting observatories, scientists have peered close to the edge of a black hole and come up with an unexpected discovery.

The observations from the satellites, which include the Chandra X-ray and Hubble space telescopes, revealed that the accretion disk around a black hole stopped much further out than predicted.

"The Chandra data indicate that this accretion disk gets no closer to the event horizon than about 600 miles, a far cry from the 25 miles that some had expected," said Chandra astronomer Jeffrey McClintock, who announced this findings this week along with an international team of colleagues.

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In binary star systems composed of a conventional star and a black hole, vast oceans of gas and matter from the star can swirl into the flattened accretion disk.

In this case, the star, comparable to our sun, is locked in close orbit with the black hole, which is about seven times the mass of our sun.

Black holes are super-massive stars that collapse in on themselves, creating gravity sinks so powerful that nothing can escape their grasp. The event horizon of a black hole is the point of no return for falling matter or light.

Stellar material from a companion star, swirling toward the edge of a black hole, is compressed and superheated, causing it to radiate in intense X-ray streams, a final distress call that space observatories can detect and measure.

Scientists speculate that an accretion disk halts before the edge of a black hole because the material bubbles into a hot gas before its final dive into oblivion.

An accretion disk edge can reach as close as 25 miles to an event horizon when it provides a bountiful fuel supply to a black hole, most astronomers agree.

But until comparing observations from Chandra with Hubble and other space telescopes of this black hole, which feeds from a disk offering only paltry rations, they had little idea that the edge could be so distant.

"This presents a fundamental problem for models in which the disk extends close to the event horizon," mused Ann Esin, an astrophysicist who took part in the study.

The binary system, known as XTE J1118+480, comprises an unusual "X-ray nova," which undergoes occasional eruptions followed by long periods of dormancy. It is located near Ursa Major, commonly called the Big Dipper.

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The Chandra X-ray Observatory Center
Space Telescope Science Institute
NASA

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