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.
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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