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Milky Way’s Giant Black Hole Awoke from Slumber 300 Years Ago
April
16 2008
Using NASA,
Japanese, and
European X-ray
satellites, a
team of Japanese
astronomers has
discovered that
our galaxy’s
central black
hole let loose a
powerful flare
three centuries
ago.
This Chandra
image shows our
Galaxy's center.
The location of
the black hole,
known as
Sagittarius A*,
or Sgr A* for
short, is
arrowed. Credit:
NASA/CXC/MIT/Frederick
K. Baganoff et
al.
The finding helps resolve a
long-standing mystery: why is the Milky Way’s black hole so
quiescent? The black hole, known as Sagittarius A* (pronounced
"A-star"), is a certified monster, containing about 4 million
times the mass of our Sun. Yet the energy radiated from its
surroundings is billions of times weaker than the radiation
emitted from central black holes in other galaxies.
"We
have wondered why the Milky Way’s black hole appears to be a
slumbering giant," says team leader Tatsuya Inui of Kyoto
University in Japan. "But now we realize that the black hole was
far more active in the past. Perhaps it’s just resting after a
major outburst."
The new study, which will
appear in the Publications
of the Astronomical Society of Japan, combines results from
Japan’s Suzaku and ASCA X-ray satellites, NASA’s Chandra X-ray
Observatory, and the European Space Agency’s XMM-Newton X-ray
Observatory.
The observations, collected between 1994 and 2005, revealed that
clouds of gas near the central black hole brightened and faded
quickly in X-ray light as they responded to X-ray pulses
emanating from just outside the black hole. When gas spirals
inward toward the black hole, it heats up to millions of degrees
and emits X-rays. As more and more matter piles up near the
black hole, the greater the X-ray output.
These X-ray pulses take
300 years to traverse the distance between the central black
hole and a large cloud known as Sagittarius B2, so the cloud
responds to events that occurred 300 years earlier. When the
X-rays reach the cloud, they collide with iron atoms, kicking
out electrons that are close to the atomic nucleus. When
electrons from farther out fill in these gaps, the iron atoms
emit X-rays. But after the X-ray pulse passes through, the cloud
fades to its normal brightness.
Amazingly, a region in Sagittarius B2 only 10 light-years across
varied considerably in brightness in just 5 years. These
brightenings are known as light echoes. By resolving the X-ray
spectral line from iron, Suzaku’s observations were crucial for
eliminating the possibility that subatomic particles caused the
light echoes.
"By observing how this cloud lit up and faded over 10 years, we
could trace back the black hole’s activity 300 years ago," says
team member Katsuji Koyama of Kyoto University. "The black hole
was a million times brighter three centuries ago. It must have
unleashed an incredibly powerful flare."
This new study builds upon research by several groups who
pioneered the light-echo technique. Last year, a team led by
Michael Muno, who now works at the California Institute of
Technology in Pasadena, Calif., used Chandra observations of
X-ray light echoes to show that Sagittarius A* generated a
powerful burst of X-rays about 50 years ago — about a dozen
years before astronomers had satellites that could detect X-rays
from outer space. "The outburst three centuries ago was 10 times
brighter than the one we detected," says Muno.
The galactic center is about 26,000 light-years from Earth,
meaning we see events as they occurred 26,000 years ago.
Astronomers still lack a detailed understanding of why
Sagittarius A* varies so much in its activity. One possibility,
says Koyama, is that a supernova a few centuries ago plowed up
gas and swept it into the black hole, leading to a temporary
feeding frenzy that awoke the black hole from its slumber and
produced the giant flare.
Launched in 2005, Suzaku is the fifth in a series of Japanese
satellites devoted to studying celestial X-ray sources and is
managed by the Japan Aerospace Exploration Agency (JAXA). This
mission is a collaborative effort between Japanese universities
and institutions and NASA Goddard.
Source: Goddard Space Flight Center
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