Milky Way Black Hole May Be A Colossal 'Particle Accelerator'
The graphic illustrates the idea that
the black hole at the center of the Milky Way is like an extremely powerful
particle accelerator, revving up protons in the surrounding magnetic plasma and
slinging them into lower-energy protons with such energy that high-energy gamma
rays result from the collision. The yellow line depicts a high-energy
proton flung into a lower-energy proton in the hydrogen gas cloud. The green
arrow represents the high-energy gamma ray that results from the proton
collision. (Credit: Artwork by Sarah Ballantyne)
ScienceDaily (Feb. 28, 2007) —
Scientists were startled when they discovered in 2004 that the center of our
galaxy is emitting gamma rays with energies in the tens of trillions of
electronvolts.
Now astrophysicists at The University of Arizona, Los Alamos National Laboratory
and the University of Adelaide (Australia) have discovered a mechanism that
might produce these high-energy gamma rays.
The black hole at the center of our
Milky Way could be working like a cosmic particle accelerator, revving up
protons that smash at incredible speeds into lower energy protons and creating
high-energy gamma rays, they report.
"It's similar to the same kind of
particle physics experiments that the Large Hadron Collider being built at CERN
will perform," UA astrophysicist David Ballantyne said.
When complete, the Large Hadron Collider
in Switzerland will be able to accelerate protons to seven trillion
electronvolts. Our galaxy's black hole whips protons to energies as much as 100
trillion electronvolts, according to the team's new study. That's all the
more impressive because "Our black hole is pretty inactive compared to massive
black holes sitting in other galaxies," Ballantyne noted.
Ballantyne collaborated with UA astrophysics Professor Fulvio Melia in the new
study published in Astrophysical Journal Letters.
For the last several years, Melia has been developing a theory of what may be
going on very close to the Milky Way's black hole.
Melia and his group find that powerful,
chaotic magnetic fields accelerate protons and other particles near the black
hole to extremely high energies.
"Our galaxy's central supermassive object has been a constant source of surprise
ever since it's discovery some 30 years ago," Melia said. "Slowly but surely it
has become the best studied and most compelling black hole in the universe. Now
we're even finding that its apparent quietness over much of the spectrum belies
the real power it generates a mere breath above its event horizon---the point of
no return."
The Milky Way black hole "is one of the most energetic particle accelerators in
the galaxy, but it does this by proxy, by cajoling the magnetized plasma
haplessly trapped within its clutches into slinging protons to unearthly
speeds," Melia said.
Ballantyne used detailed, realistic maps of interstellar gas extending 10 light
years beyond the black hole in modeling whether accelerated protons launched
from the galactic center would produce gamma rays.
"We calculated very exactly how the protons would travel in this medium, taking
into account specifically the magnetic force that changes the protons'
trajectories," he said. The team calculated 222,000 proton trajectories for a
statistically solid study.
Even though the protons move close to
the speed of light, their motion is so random that it takes several thousand
years for the particles to travel beyond 10 light years of the black hole. After
the high-energy protons escape the black hole environment, they fly off into the
interstellar medium, where they collide with low-energy protons (hydrogen gas)
in a smash-up so energetic that particles called 'pions' form. These particles
of matter quickly decay into high-energy gamma rays that, like other radiation,
travel in all directions.
Ballantyne, Melia and and their colleagues found that this process can explain
the energy spectrum and brightness of gamma-ray emission that astronomers
observe. Researchers detect the high-energy gamma-ray emission with ground-based
telescopes at Namibia, Africa, at Whipple Observatory in southeastern Arizona,
and elsewhere.
"Ironically, even though our galaxy's central black hole does not itself
abundantly eject hyper-relativistic plasma into the surrounding medium, this
discovery may indirectly explain how the most powerful black holes in the
universe, including quasars, produce their enormous jets extending over
intergalactic proportions. The same particle slinging almost certainly occurs in
all black-hole systems, though with much greater power earlier in the universe,"
Melia said.
Only 31 percent of the 222,000 proton trajectories in their sample produced
gamma rays within 10 light years of the black hole, Ballantyne said. The other
69 percent escape to greater distances, where presumably they, too, will
interact in gamma ray-generating collisions.
"Astronomers do, indeed, observe a glow of very-high energy gamma-rays from the
inner regions of the galaxy," Ballantyne said. "It's possible that this emission
is also caused by protons accelerated close to the central black hole."
Ballantyne holds UA's Theoretical Astrophysics Program Prize Postdoctoral
Fellowship. The university's Theoretical Astrophysics Program, organized in
1985, is an interdisciplinary program of the UA departments of physics,
astronomy and planetary sciences.