Photons with a trillion times more energy than visible
light are flying out of a relatively nearby galaxy.
Until now, scientists didn't understand this light's origin,
but a new study shows that its source is a giant black
hole inside the M87 galaxy.
The radiation takes the form of high-frequency
gamma rays.
"We detect it in roughly 25 galaxies so far but we
never knew where exactly it was coming from," said study team member
Matthias Beilicke, an astrophysicist at Washington University in St. Louis. "Only
in the case of M87 were we able to narrow it down to the black hole vicinity."
Matter funnels into the supermassive black hole from a ring
of material circling around it and called an accretion disk. Some matter is
also propelled out in very
bright jets that stream across space.
One hypothesis suggests the gamma-ray flares occur when an
extra amount of matter is pulled out of the accretion disk and injected into
the jets. Either electrons or protons in the jet could interact with light photons
or matter to create the extremely high-energy gamma rays.
"This acceleration mechanism requires strong
magnetic fields which can be found around the black hole," Beilicke
said. "We need charged particles accelerated to very high energies in order to emit gamma rays at these energies."
The observations could help scientists understand the
environment around gigantic black holes like this one, which contains the mass
of more than six billion suns.
"The fact that there are particles accelerated up to
such high speeds gives us input for the modeling of these systems,"
Beilicke told SPACE.com. "M87 is so far more or less the only source which
really allows this particular kind of study."
The researchers were able to pin down the radiation's origination
point by combining measurements of the gamma-rays with radio-wave observations.
Even though gamma rays cannot usually pierce through Earth's atmosphere,
ground-based telescopes can detect them by looking for telltale flashes of
faint blue light that result when gamma rays hit the air. These observations
also reflect the direction the gamma rays came from, though they are not very
precise.
Astronomers also observed M87 with the National Science
Foundation's Very Long Baseline Array (VLBA), a group of radio telescopes
spread out around the United States. These observations revealed a burst of
radio light that coincided with the gamma-ray flares, which usually lasted a
few days.
The radio flares, imaged with much more precision than gamma-ray
light, could be traced back to the location of the supermassive black hole in
the center of M87.
"Combining the gamma-ray observations with the
supersharp radio 'vision' of the VLBA allowed us to see that the gamma rays are
coming from a region very near the black hole itself," said team member
Craig Walker of the National Radio Astronomy Observatory (NRAO).
M87, at 50 million light-years from Earth, is much closer
than any similar gamma-ray flare sources, so it could be scientists best bet to
study these systems for a while.
The new study is detailed in the July 2 issue of the journal
Science.
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