While we may never know what it looks like inside a black
hole, astronomers recently obtained one of the closest views yet. The sighting
allowed scientists to confirm theories about how these giant cosmic sinkholes
spew out jets of particles travelling at nearly the speed of light.
Ever since the first observations of these powerful jets,
which are among the brightest objects seen in the universe, astronomers have wondered
what causes the particles to accelerate to such great speeds. A leading hypothesis
suggested the black
hole's gigantic mass distorts space and time around it, twisting magnetic
field lines into a coil that propels material outward.
Now researchers have observed a jet during a period of extreme
outburst and found evidence that streams of particles wind a corkscrew path
away from the black hole, as the leading hypothesis predicts.
"We got an unprecedented view of the inner portion of
one of these jets and gained information that's very important to understanding
how these tremendous particle accelerators work," said Boston University astronomer Alan Marscher, who led the research team. The results of the study
are detailed in the April 24 issue of the journal Nature.
The team studied a galaxy called BL Lacertae (BL Lac), about
950 million light years from Earth, with a central black hole containing 200
million times the mass of our Sun. Since this supermassive black hole's jets
are pointing nearly straight at us, it is called a blazar (a quasar is often
thought to be the same as a blazar, except its jets are pointed away from us).
The new observations, taken by the National Science
Foundation's Very Long Baseline Array (VLBA) radio telescope, along with NASA's
Rossi X-ray Timing Explorer and a number of optical telescopes, show material
moving outward along a spiral channel, as the scientists expected.
These data support the suggestion that twisted magnetic
field lines are creating the jet plumes. Material in the center of the
galaxy, such as nearby stars and gas, gets pulled in by the black hole's
overwhelming gravity and forms a disk orbiting around the core (the material's
inertia keeps it spiraling in a disk rather than falling straight into the
black hole). The distorted magnetic field lines seem to pull charged particles
off the disk and cause them to gush outward at nearly the speed of light.
"We knew that material was falling in to these regions,
and we knew that there were outbursts coming out," said University of Michigan astronomer Hugh Aller, who worked on the new study. "What's really
been a mystery was that we could see there were these really high-energy
particles, but we didn't know how they were created, how they were accelerated.
It turns out that the model matches the data. We can actually see the particles
gaining velocity as they are accelerated along this magnetic field."
The astronomers also observed evidence of another phenomenon
predicted by the leading hypothesis — that a flare would be produced when
material spewing out in the jets hit a shock wave beyond the core of the black
hole.
"That behavior is exactly what we saw," Marscher
said.
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