http://www.washingtonpost.com/wp-srv/aponline/20000114/aponline165828_000.htm
Black Hole Found Near Earth
By Paul Recer
AP Science Writer
Friday, Jan. 14, 2000; 4:58 p.m. EST
ATLANTA –– Four bursts of X-ray energy have alerted astronomers to a black
hole just 1,600 light years away from Earth, practically on the doorstep in
astronomical terms.
The black hole was discovered last September after an Australian amateur
astronomer noticed the suddenly brightening of a star and notified the pros.
Donald Smith, a Massachusetts Institute of Technology astronomer, and others
focused X-ray detectors on the target star and quickly got a surprise. On
Sept. 14, the black hole announced its presence with an eruption of X-rays
that was brief, but dramatic.
Three other eruptions followed, each one lasting less than two hours. But it
was enough for Smith and a team of radio astronomers to determine that the
energy was coming from a black hole.
"This is one of the fastest (bursts) we have ever seen," said Robert M.
Hjellming of the National Radio Astronomy Observatory.
Smith said that although the bursts were brief, at peak output they were the
brightest source of X-rays in the sky, except for the sun.
The black hole is located in constellation Sagittari and centers on a star
called V4641 Sgr.
Smith said the bursts come and go so rapidly that it may represent a new
subclass of X-ray-producing objects.
Radio telescopes detected twin jets of matter firing out of the black hole
region at nearly the speed of light. Three other similar X-ray sources have
been detected in distant parts of the universe and called by astronomers
"microquasars." They resemble quasars, but are much smaller.
But Hjellming said energy bursts from the other microquasars fade more
slowly, often lasting for weeks.
A black hole is an extremely dense object with a gravitation field so
powerful that not even light can escape. As a black hole pulls matter into
its center, gas and dust are heated to millions of degrees. This gives off
X-rays that can be detected by X-ray telescopes.
Many black holes acquire an envelope of gas and dust called an accretion
disk. Matter from the disk streams constantly into the black hole, triggering
a steady X-ray signal. Black holes are usually in the center of quasars and
characteristically have accretion disks.
The brief bursts from the V4641 Sgr black hole suggests it is not being fed
constantly by a reservoir of matter from an accretion disk. This puzzles
astronomers.
"Either the matter can flow into the black hole without forming an accretion
disk or the black hole is significantly different in its mass, spin or
charge," said Ronald Remmillard of MIT.
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