(Phys.org)—An international team of
researchers has spotted the first instance of a high-energy neutrino
collision from a source outside of the Milky Way, marking what they
describe as a significant discovery. In their paper published in the
journal Nature Physics, the team describes their work at the
South Pole Neutrino Observatory, the details pertaining to the sighting
and why they believe their discovery may lead to a new era in neutrino
astrophysics.
Neutrino's are massless and have no charge and
very seldom interact with other matter—the exception is when they
collide head on with another particle. Scientists have been studying
neutrinos for several years, believing that they may hold the key to
understanding many parts of the universe that remain otherwise hidden
from our view. To see evidence of them, researchers fill large
underground tanks with different types of fluids and then use extremely
sensitive sensors to capture very brief flashes of light which are
emitted when a neutrino collides with something in the fluid. The team
with this latest effort has taken a different approach, they have placed
sensors around a kilometer sized ice cube 2.5 kilometers beneath the
surface, in a location near the South Pole. The sensors capture the
brief flashes that occur when neutrinos collide with particles in the ice.
Capturing evidence of collisions does not happen very often,
but when it does, it sets off a chain of events that center around
trying to ascertain where the neutrino came from—most come from the sun
or cosmic rays striking our atmosphere. But back in 2012, the team
captured evidence of what they described as the most powerful yet,
registering two petavolts. Following that discovery, the team used data
from radio telescopes, and in particular data from a galaxy that has
been named KS B1424-418—astrophysicists have been studying it for
several decades and it had been observed to undergo a change in shape
during the time period 2011 to 2014. After much analysis, the team
confirmed that the neutrino collision they observed was due to an
emission from that very galaxy, making it the first neutrino collision
to be traced back to a source outside of the Milky Way.
Explore further:
X-ray telescopes find black hole may be a neutrino factory
More information:
M. Kadler et al. Coincidence of a high-fluence blazar outburst with a PeV-energy neutrino event, Nature Physics (2016). DOI: 10.1038/nphys3715
Abstract
The astrophysical sources of the extraterrestrial, very high-energy neutrinos detected by the IceCube collaboration remain to be identified. Gamma-ray (γ-ray) blazars have been predicted to yield a cumulative neutrino signal exceeding the atmospheric background above energies of 100 TeV, assuming that both the neutrinos and the γ-ray photons are produced by accelerated protons in relativistic jets. As the background spectrum falls steeply with increasing energy, the individual events with the clearest signature of being of extraterrestrial origin are those at petaelectronvolt energies. Inside the large positional-uncertainty fields of the first two petaelectronvolt neutrinos detected by IceCube, the integrated emission of the blazar population has a sufficiently high electromagnetic flux to explain the detected IceCube events, but fluences of individual objects are too low to make an unambiguous source association. Here, we report that a major outburst of the blazar PKS B1424–418 occurred in temporal and positional coincidence with a third petaelectronvolt-energy neutrino event (HESE-35) detected by IceCube. On the basis of an analysis of the full sample of γ-ray blazars in the HESE-35 field, we show that the long-term average γ-ray emission of blazars as a class is in agreement with both the measured all-sky flux of petaelectronvolt neutrinos and the spectral slope of the IceCube signal. The outburst of PKS B1424–418 provides an energy output high enough to explain the observed petaelectronvolt event, suggestive of a direct physical association.
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