for National Geographic News
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.
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.
SOURCES AND RELATED WEB SITES