A nearby black hole, hurtling through the plane of our galaxy
like a cannonball, has given what some astronomers say is their
best evidence yet that stellar-mass black holes are made in
supernova explosions. The black hole, called GRO J1655-40,
is streaking across space at a rate of 250,000 miles per hour.
That speed is four times faster than the average velocity of
the stars in that galactic neighborhood. The most likely
“cannon blast” is the explosive kick of a supernova, one
of the universe’s most titanic events.
Even though, by definition, black holes swallow light, the
runaway black hole has a companion star, allowing astronomers
to track it. NASA Hubble Space Telescope’s sharp view allowed
astronomers to measure the black hole’s motion across the sky
in images taken in 1995 and 2001. Combining the Hubble data
with separate measurements of its radial motion toward Earth
taken from ground-based telescopes yields the true “space
velocity” of the black hole, and shows that it is streaking
across the plane of our Milky Way in a highly elliptical orbit.
“This is the first black hole found to be moving fast through
the plane of our galaxy,” says Felix Mirabel of the French
Atomic Energy Commission and the Institute for Astronomy and
Space Physics of Argentina. “This discovery is exciting because
it shows the link of a black hole to a supernova,” aside from
observing gamma-ray busts from hypernovae (even more powerful
stellar explosions), which are believed to make black holes.
Mirabel’s results appear in the November 19 issue of Astronomy
and Astrophysics.
Though the black hole is roughly heading in our direction, it
is at a “safe” distance, 6,000 to 9,000 light-years away, in the
direction of the constellation Scorpius. Mirabel believes the
black hole may have been born in the inner disk of our galaxy,
where the highest rate of star formation is taking place.
An aging, evolved star whirls around the black hole, completing
one orbit just every 2.6 days. The hole is slowly devouring the
companion, which apparently survived the supernova that originally
created the black hole. This process makes blowtorch-like jets
that stream away from the black hole at a significant fraction of
the speed of light. It is the second “microquasar” discovered in
our galaxy (meaning that it is a scaled-down model of monster black
holes at the cores of extremely active galaxies, called quasars.)
Astronomers have known about stellar-mass black holes (ranging
anywhere from 3.5 to approximately 15 solar masses) since the
early 1970s. The only conceivable mechanism for making such black
holes would be the implosion of the core of a star when it dies.
The implosion sends out a shockwave that rips the rest of the star
to shreds as a supernova. If the surviving core is greater than
3.5 times our Sun’s mass, no forces can stop the collapse, and it
will shrink to an infinitely small and dense singularity.
Astronomers have catalogued even faster-moving neutron stars
catapulted by a supernova explosion. The black hole is moving
relatively slower because it has much more mass and so has more
resistance to being accelerated.
