A blazar is a supermassive black hole—with material whirling around
it, and huge jets of material bursting from it—at the center of an
elliptical galaxy. Millions of times the mass of the sun, the black hole
sucks in matter when it is actively accreting, and twin jets of light
and subatomic particles blast out from the poles along the axis of the
black hole’s rotation. One of the jets of blazar TXS 0506+056 points at
“The connection of these jets to high-energy cosmic rays is of great
interest to us,” said Gaisser, who comes from a distinguished line of
particle physicists at the Bartol Research Institute in the UD
Department of Physics and Astronomy.
The late Martin Pomerantz, Gaisser’s mentor and former Bartol
Institute director, pioneered astronomy and astrophysics research in
Antarctica. This harsh environment is an ideal place for such studies
because cosmic rays can enter at the poles unimpeded by Earth’s magnetic
Since they were first detected over one hundred years ago, cosmic
rays—highly energetic particles that continuously rain down on Earth
from space—have posed an enduring mystery: Where do they come from?
Because cosmic rays are charged particles, their paths cannot be
traced directly back to their sources due to the strong magnetic fields
that fill space and warp their trajectories. But the powerful cosmic
accelerators that produce them may also produce neutrinos.
Neutrinos are uncharged particles, unaffected by even the most
powerful magnetic field. Because they rarely interact with matter and
have almost no mass—hence their sobriquet “ghost particle”—neutrinos
travel nearly undisturbed from their accelerators, giving scientists an
almost direct pointer to their source.
“The era of multimessenger astrophysics is here,” said NSF Director
France Córdova. “Each messenger—from electromagnetic radiation,
gravitational waves and now neutrinos—gives us a more complete
understanding of the universe, and important new insights into the most
powerful objects and events in the sky. Such breakthroughs are only
possible through a long-term commitment to fundamental research and
investment in superb research facilities.”
World’s largest particle detector
Spotting the highest energy neutrinos requires a massive particle
detector, and IceCube is by volume the world’s largest. Encompassing a
cubic kilometer of deep, pristine ice a mile beneath the surface at the
South Pole, the detector, which UD scientists helped build, is composed
of more than 5,000 light sensors—on 86 cables on a grid with a spacing
more than the length of a football field.
When a neutrino interacts with the nucleus of an atom, it creates a
secondary charged particle, which, in turn, produces a characteristic
cone of blue light that is detected by IceCube and mapped through the
detector’s grid of photomultiplier tubes. Because the charged particle
and light it creates stay essentially true to the neutrino’s direction,
they give scientists a path to follow back to the source.
Following the Sept. 22 detection, the IceCube team quickly scoured
the detector’s archival data and discovered a flare of over a dozen
astrophysical neutrinos detected in late 2014 and early 2015, coincident
with the same blazar, TXS 0506+056. This independent observation adds
to a growing body of data that indicates TXS 0506+056 is the first known
accelerator of the highest energy neutrinos and cosmic rays.
The IceCube Collaboration, with more than 300 scientists in 49
institutions from around the world, runs an extensive scientific program
that has established the foundations of neutrino astronomy. Their
research efforts, including critical contributions to the detector
operation, are funded by agencies in Australia, Belgium, Canada,
Denmark, Germany, Japan, New Zealand, Republic of Korea, Sweden,
Switzerland, the United Kingdom, and the United States.
by UD communications staff with material from the IceCube
Collaboration; photos and illustrations courtesy of South Pole Group,
Science magazine, IceCube/NASA