"Sleeping" Black Hole Briefly Awoke in 1700s

Ker Than
for National Geographic News
April 18, 2008
The explosion of a dying star eons ago may have briefly awakened the supermassive black hole at the heart of the Milky Way, triggering an x-ray flare that ricocheted across the galaxy.

Known as Sagittarius A*, or Sgr A*, the Milky Way's central black hole is located about 26,000 light-years from Earth and has an estimated mass of about four million suns.

Supermassive black holes in other galaxies are typically a hundred to a thousand times more massive still.

(Related news: "Hundreds of 'Rogue' Black Holes May Roam Milky Way" [January 10, 2008].)

Despite its runt status, Sgr A* should be emitting about one billion times more radiation than can be detected, and this lower luminosity has long puzzled scientists.

Using NASA, Japanese, and European x-ray satellites, Japanese astronomers found lingering traces of a Sgr A* outburst in an enormous gas cloud located about 300 light-years away from the black hole.

"We have wondered why the Milky Way's black hole appears to be a slumbering giant," said study team member Tatsuya Inui of the Japan Aerospace Exploration Agency (JAXA).

"But now we realize that the black hole was far more active in the past. Perhaps it's just resting after a major outburst."

(See photos of black holes.)

The finding will appear in an upcoming issue of the journal Publications of the Astronomical Society of Japan.

Light Echo

Observations gathered between 1994 and 2005 show the gas cloud, called Sagittarius B2, brightened and dimmed quickly as it responded to powerful x-ray pulses.

The researchers suspect the pulses emanated from just outside the black hole's event horizon—the boundary beyond which matter and light are irrevocably lost to a black hole's gravity.

If the team is correct, then Sgr A* unleashed a powerful burst of x-rays about 26,300 years ago.

X-ray photons from the black hole traveled 26,000 light-years and reached Earth about 300 years ago.

The flare also created a "light echo" in Sagittarius B2 that was delayed 300 years, only recently reaching Earth.

As the black-hole x-rays zipped through the cloud, they collided with iron atoms, releasing additional x-rays. It is these iron atom x-rays that the Japanese team detected.

The researchers speculate that a supernova explosion at the galactic center several thousand years ago shoved gas onto the spinning ring of material that surrounds Sgr A* called an accretion disk.

This could have roused the black hole and triggered a feeding frenzy that resulted in an explosive burst of x-rays.

(See a gallery of colorful supernovae.)

The supernova blast would have gone unnoticed by humans, because any visible light from the blast would have long faded during the journey from the galactic center, said study leader Katsuji Koyama of Kyoto University.

(Related photo: "Gamma-Ray Burst Visible to Naked Eye" [March 21, 2008].)

No Threat to Earth

Geoffrey Bower, an astronomer at the University of California, Berkeley, was not involved in the study.

If the Japanese team's supernova-trigger hypothesis is correct, then scientists can expect to see similar outbursts from Sgr A* in the future, Bower said.

"If that's the case, then over the next million years we should see a lot more outbursts of this kind, because there is a cluster of massive stars close to Sgr A*."

If Sgr A* were to suddenly rouse itself again, however, life on Earth would not be in danger.

"The galactic center is too far away from us," study leader Koyama said. "We are entirely safe."

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