SEATTLE--An ancient
X-ray outburst from the supermassive
black hole at the center of our Milky
Way galaxy caused surrounding gas clouds to glow brightly in a cosmic light
show that is only now being detected.
The output
likely involved the consumption of a snack equal in mass to the planet Mercury, researchers said here
yesterday at the 209th meeting of the American Astronomical Society.
Called Sagittarius
A*, the supermassive black hole at the Milky Way's core is located some 27,000
light-years away and has an estimated mass of about three million suns. It is surrounded by several massive
iron-rich gas clouds that glow and emit their own X-rays when struck by photons
or electrons [image].
No X-ray telescopes were in place when light
from the black hole outburst first began to reach Earth about 60 years ago, but
astronomers deduced the event based on "light echoes" from the clouds
recently recorded by NASA's Chandra
X-ray Observatory.
Echoes
of a meal
Light
echoes are reflections of light from a primary source--in this case a black hole
outburst--bounced off another object such as dust or gas.
Observations
by Chandra over several years reveal different areas of the gas clouds lighting
up, as if a wave of energy were passing through them, illuminating different
areas at a time.
Sagittarius
A* is known to have produced smaller and more recent outbursts, but this latest
one is estimated to have been about 1,000 times brighter and lasted about 1,000
times longer than the others. Evidence from other studies looking at light
echoes originating outside the galactic plane suggests Sagittarius A*
experiences an outburst every 100 years or so.
The
researchers think the outburst was triggered by a small amount of matter
falling into the black hole.
"It
doesn't actually require a lot of mass falling into the black hole to produce
this outburst," said Michael Muno of Caltech. "The mass would be
equivalent to the planet Mercury."
As matter
swirls around a black hole, it generates a large amount of friction, producing X-rays.
Nobody
knows what the black hole actually devoured. Like other galactic black holes,
Sagittarius A* is thought to be surrounded by a swirling disk of matter blown
in by stellar winds. Muno speculates that every once in a while, something
upsets the swirling disk, causing a hiccup in its motions that makes it dump
its material onto the black hole, triggering an outburst.
"Another
option is that there may be small clouds of gas and dust drifting around the
galactic center and every once in awhile, one of those get close enough to the
black hole," he said.
Strangely
calm
For all its
activity, however, Sagittarius A* is unusually faint for a galactic
supermassive black hole. The answer may lie in its maturity. Our galaxy is several billion years old,
and one idea is that most of the galaxy-building that can feed a black hole has
already taken place.
A quasar, on
the other hand, is typically a relatively young galactic setup involving a black
hole that can outshine the combined starlight of its entire host galaxy.
The energy
output of Sagittarius A* is nowhere close to this. Why our black hole is so dim
is not entirely understood.
"This
faintness implies that stars and gas rarely get close enough to the black hole
to be in any danger," said study team member Frederick Baganoff of the
Massachusetts Institute of Technology. "The huge appetite is there, but
it's not being satisfied."
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