Black Hole at Milky Way's Core? 
Satellite Detects Telltale X-Ray Flare
 
By William Harwood
Special to The Washington Post
Thursday, September 6, 2001; Page A03 


Astronomers announced yesterday that they had captured the most compelling
evidence yet that a massive black hole is lurking in the cloud-shrouded
heart of our Milky Way galaxy.

By a stroke of good fortune, NASA's Chandra X-ray Observatory satellite
happened to be looking when the presumed black hole enjoyed a quick snack
of gas and dust. Analysis of the resulting belch -- a sudden X-ray flare
that changed in intensity over a span of just a few minutes -- provided
strong new support for the existence of the black hole.

The findings are consistent with the predictions of Einstein's theory of
general relativity. As a result, the discovery is "either a cosmic
coincidence or it's telling you there's something really right about the
theory," said Frederick Baganoff, a Massachusetts Institute of Technology
astronomer whose team announced the discovery at a news conference in
Washington. A paper detailing the observations appears in today's issue
of Nature magazine.

"This discovery seems to have taken us to the threshold of actually being
able to confirm one of the most important predictions that the theory
makes, and that's the existence of an event horizon, or black holes,"
said Fulvio Melia, a University of Arizona astronomer who wrote a
commentary for Nature. "Modern physics doesn't have any theory that
could adequately account for such an object if this is not a black hole."

Black holes are the end result of the stellar evolution of stars at
least 20 times more massive than the sun. When such a star exhausts its
nuclear fuel, fusion reactions in the core die out. Without a source of
energy to offset the inward pull of gravity, the star's core quickly
collapses and a black hole is born.

By definition, a black hole is invisible. Nothing, not even light, can
escape the crushing grip of the dead star's gravity. To an outside observer,
the black hole begins at what is known as the event horizon, an imaginary
sphere marking the point of no return. Anything that falls through the
event horizon effectively disappears from the universe. To get back out,
an object would have to exceed the speed of light and, according to Einstein,
that's impossible.

While a black hole might be invisible, its gravitational effects are not.
As matter is sucked into a black hole, it is accelerated to extreme
velocities. Radiation is emitted as the material spirals inward, increasing
in energy as it nears the event horizon.

Earth's solar system slowly orbits the core of the Milky Way at a distance
of about 27,000 light years. The core is in the constellation Sagittarius,
but intervening clouds of gas and dust block our view at visible wavelengths.
In 1974, however, astronomers discovered a strong, compact source of radio
energy, dubbed Sagittarius A*, located at the center of the galaxy. About
10 million stars are now thought to orbit within a light year of the radio
source. To put that in perspective, the nearest star to Earth's solar
system is a bit more than four light years away.

Analysis of the orbital velocities of stars within a few light days -- a
hundred billion miles or so -- of the Sagittarius A* radio source indicated
that the object must have a mass of 2.6 million suns. Astronomers have long
suspected that the object is a super-massive black hole. But direct proof
has been difficult to obtain.

In 1999, Chandra's high-resolution mirrors were aimed at the galactic core,
and astronomers were surprised -- if not dismayed -- by the result. The
presumed black hole was radiating far less energy than one might expect,
the equivalent of a single sun's luminosity.

Then, last October, Chandra took a second look. This time around,
Baganoff's team caught an intense X-ray flare from start to finish
as the hole sucked in a comet-sized cloud of dusty gas. Over a period
of about three hours, X-ray emissions from the galactic core increased
to 45 times the sun's luminosity. More important, the intensity of the
flare suddenly decreased for about 10 minutes and then jumped back up again.

The rapidity of that change was a direct indication of the size of the
area where the emissions occurred. As it turns out, the region in question
could not be much larger than the diameter of Earth's orbit around the sun,
or about 20 times the size of the hole's event horizon. It could be much
smaller and, in any case, it is consistent with what could be expected of
a black hole with the mass of 2.6 million suns.

© 2001 The Washington Post Company