For the first time, astronomers followed cosmic neutrinos into the fire-spitting heart of a supermassive blazar.
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It
was the smallest bullet you could possibly imagine, a subatomic
particle weighing barely more than a thought, and a cosmic blunderbuss, a
supermassive black hole, squirting fire down a gravitational gun
barrel.
On Sept. 22, 2017, a particle
known as a neutrino zinged down from the sky and through the ice of
Antarctica at nearly the speed of light, setting off a cascade of alarms
in an array of detectors called IceCube.
Within
seconds IceCube had alerted an armada of astronomical satellites,
including the Fermi Gamma-ray Space Telescope. That spacecraft traced
the neutrino back to an obscure dot in the sky, a distant galaxy known
as TXS 0506+056, just off the left shoulder of the constellation Orion,
which was having a high-energy outburst of X-rays and gamma-rays.
While
astronomers around the world scrambled to their telescopes to get in on
the fun, the IceCube scientists scoured their previous data and found
that there had been previous outbursts of neutrinos from the galaxy,
which they nicknamed the “Texas source,” including an enormous neutrino
outburst in 2014 and 2015.
Astronomers
said the discovery could provide a long sought clue to one of the
enduring mysteries of physics and the cosmos. Where does the rain of
high-energy particles from space known as cosmic rays come from?
The
leading suspects have long been quasars. They are supermassive black
holes in the centers of galaxies where matter and energy get squeezed
like toothpaste out of the top and bottom of a doughnut of doomed
swirling material in a violent jet.
Now
they know at least one in which that seems to be the case. TXS 0506+056
is a type of quasar known as a blazar, in which our line of sight from
Earth is along the jet — right down the gun barrel. The term blazar
comes partly from BL Lacertae, a starlike object that turned out to be
the first of these objects ever recognized.
“We
have found the first source of cosmic rays,” said Francis Halzen, of
the University of Wisconsin, Madison, and IceCube’s director, in an
interview.
“Where
exactly in the active galaxy, the neutrinos are produced will be a
matter of debate,” he added in an email. “It is clear that the
supermassive black hole provides the accelerator power,” he said, but
how is a mystery.
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The discovery is being announced in a series of papers by an international array of physicists and astronomers in Science
and the Astrophysical Journal, and in a news conference sponsored by
the National Science Foundation, which funds the IceCube Neutrino
Observatory at the Amundsen-Scott South Pole Station.
“I
think this is the real thing,” said John Learned, a neutrino expert at
the University of Hawaii who is not part of IceCube, in an email, “the
true beginning of high energy neutrino astronomy, of which we have
dreamed for many decades.” Now, he added, “we will start seeing into the guts of the most energetic objects in the universe.”
Neutrinos
are among the most plentiful particles in the universe — far
outnumbering the protons and electrons out of which we are composed.
They have no electrical charge and so little mass that it has not been
accurately measured yet. They interact with other matter only by gravity
and the so-called weak nuclear force and thus flow through us, Earth
and even miles of lead like ghosts.
Yet
in theory they are all over. Produced by radioactive decays of other
particles, they are flooding us from nuclear reactions in the sun,
distant supernova explosions and even the Big Bang. The previous great
moment in neutrino astronomy happened in 1987, when some 25 neutrinos
were recorded in three detectors on Earth coincident with a supernova
explosion in the Large Magellanic Cloud, a nearby galaxy.
The
lure of neutrinos for astronomy is that it is possible to trace them
back to their origins. Not only do they fly long distances and from
otherwise impenetrable spots like the cores of stars at virtually the
speed of light, but by not having an electrical charge they are not
affected by interstellar and intergalactic magnetic fields and other
influences that scramble the paths of other types of cosmic particles,
like protons and electrons. Neutrinos go as straight through the
universe as Einsteinian gravity will allow.
IceCube,
an international observatory run by 300 scientists from 12 countries,
consists of more than 5,000 sensitive photomultiplier tubes embedded in
grid encompassing a cubic kilometer of ice at the South Pole. When a
neutrino very, very, very, very, very rarely hits an atomic nucleus in
the ice, it produces a cone of blue light called Cerenkov radiation that
spreads through the ice and is picked up by the photomultipliers.
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IceCube
was built, Dr. Halzen said, to find the source of cosmic rays, and the
observatory has been recording neutrinos ever since it started working
in 2011, but had not been able to pinpoint the sources of any of them
until now. One reason, he said, was that the scientists had assumed the
sources would be nearby, perhaps even in our own Milky Way galaxy.
But TXS 0506+056, the
Texas source, is very far away, some 4 billion light-years. It is one
of the brightest objects in the universe, said Dr. Halzen.
The
neutrino that set off the alarm in 2017 had an energy of some 300
trillion electron volts, by the units of energy and mass that physicists
prefer. Which means it had been produced by a proton that had been a
booster to that energy, nearly 50 times the energy delivered by the
Large Hadron Collider at CERN, the biggest particle accelerator on
Earth.
Call it the Large Hadron
Collider in the sky. Presumably it is some kind of supermassive black
hole rumbling in the heart of that distant galaxy. For now, how this
cosmic accelerator works in detail is a mystery
Azadeh Keivani, of Pennsylvania State University and lead author of the Astrophysical Journal paper that tried to model it, wrote that “typically the mass of blazars are about 1 billion solar masses.”
Why
is this source so special? Why is it so far away? Those are the
questions that need to be answered, Dr. Halzen said. Do such blazars
produce all the neutrinos and all the cosmic rays we see?
Luckily
an enormous amount of data has been collected from the world’s
telescopes over the last few months in what astronomers like to call
“multi-messenger astronomy” to give hope of making progress on these and
other questions. And the inventory of cosmic neutrinos is only
beginning. IceCube has a large and long agenda.
Noting
that the Texas source has only erupted twice in the last nine years,
Dr. Halzen said, “This is not going to be an everyday event.”
IceCube
cost about $250 million to build and almost nothing to operate, because
it is all frozen in the ice. Dr. Halzen said he could now operate it
from his laptop.
They keep two people on site at the South Pole, he said. “Ideally they have nothing to do.”
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