Black Hole Jets Dance to Magnetic Music

Particle Escapees
In this artist's rendition, particles
mixed with outflowing gas can be seen as brown wisps to the north and south of
the central yellow spot, which represents a supermassive black hole. Though
invisible, black holes can be studied by watching the energy they beam out as
they gorge on nearby matter.
April 25, 2008 -- When it comes to getting information about
a black hole, indirect paths are scientists' only option.
There's just no getting around the fact that these regions of space, so dense
with matter, have gravity clamps so strong that nothing, not even light, can
escape.
But as black holes fly through space consuming everything within reach, they do not go unnoticed.
As a black hole's prey is ripped apart, it beams out energy in patterns as
distinctive as an SOS.
Scientists have wondered about one telltale sign in particular: jets of
highly energized particles that spiral out from the centers of galaxies containing black holes. They suspected the phenomena
were sculpted by twisted and coiled magnetic field lines.
Research published this week provides evidence they are right.
Astronomers trained an array of radio telescopes on a blazer, the most
energetic type of black hole, and were able to image the throat of a spiraling
jet. The target was in a galaxy called BL Lacertae, located about 950 million
light-years from Earth.
The theory was that magnetic fields, which are generated as material is
sucked inside a black hole, would become twisted, setting up a high-powered
launch system for some particles to escape.
A black hole lines up its dinner on what are called accretion disks. These
flat, spinning disks serve as departure platforms for material about to
disappear -- from the perspective of Earth anyway. (Physicists have many ideas about
what happens to matter inside a black hole.)
As material skates from the outer to inner edge of the disk, magnetic field
lines perpendicular to the disk get twisted, propelling and confining particles
of matter. The situation gets really extreme at the black hole's mouth, which
tugs at the magnetic fields along with everything else at the precipice.
Astrophysicists predicted that material zipping around in this region would
follow corkscrew-shaped paths inside the bundle of twisted magnetic fields.
They also guessed that the spinning, coiled mass would brighten when its path
of rotation was directly aligned with Earth and that there would be a final
flare when it blasted into a shock wave.
"That is exactly what we saw," said Boston University astronomer Alan
Marscher, who led an international team of scientists that announced their
findings in this week's issue of the journal Nature.
Between late 2005 and early 2006, Marscher and colleagues watched as a knot
of material was ejected outward from the blazer through a jet. Using the
National Radio Astronomy Observatory's Very Long Baseline Array and other
telescopes, the astronomers discovered bright bursts of light, X-rays and gamma
rays exactly when and where predicted.
"We got an unprecedented view," Marscher said, adding that the observations
are important to understanding how these natural particle accelerators work.