Neutrinos are the most elusive particles in the Universe, capable of traveling nearly unimpeded across it. Despite the vast amount of data collected, a long-standing and unsolved issue is still the association of high-energy neutrinos with the sources that originate them. Among the candidate sources of neutrinos, there are blazars, a class of extragalactic sources powered by supermassive black holes that feed highly relativistic jets, pointed toward Earth. In a new study, researchers from Clemson University, the Universität Würzburg, and the University of Geneva show that blazars are unambiguously associated with high-energy astrophysical neutrinos at an unprecedented level of confidence, i.e., a chance probability of 6*10−7.
An artist’s illustration of neutrinos originating from a high-energy blazar. Image credit: Benjamin Amend, Clemson University.
Cosmic rays are charged particles of energies up to 1020 electronvolts, far higher than the most powerful human-attained particle accelerator, i.e., CERN’s Large Hadron Collider.
The nature and origin of these particles arriving from deep outer space remain elusive and represent a foremost challenge for the astroparticle and astrophysics fields.
Cosmic rays’ birthplaces generate other particles, neutrinos and gamma-rays among them. Unlike gamma-rays, astrophysical neutrinos are solely created in processes involving cosmic-ray acceleration, making them unique smoking-gun signatures of a cosmic-ray source.
Among the candidate sources of high-energy neutrinos, there are blazars, a class of extragalactic sources powered by supermassive black holes harbored at the center of their host galaxies.
Blazars efficiently convert the gravitational energy of accreting gas into kinetic energy of highly relativistic jets, pointed toward Earth.
In 2017, the IceCube Neutrino Observatory, buried deep in the ice at the South Pole, detected a neutrino; astrophysicists traced it back to blazar TXS 0506+056.
“We had a hint back then, and now we have evidence,” said Dr. Marco Ajello, a researcher in the Department of Physics and Astronomy at Clemson University.
“The results provide, for the first time, incontrovertible observational evidence that the sub-sample of PeVatron blazars are extragalactic neutrino sources and thus cosmic ray accelerators,” said Dr. Sara Buson, a researcher at the Universität Würzburg.
Using neutrino data obtained by the IceCube Neutrino Observatory between 2008 and 2015 and a catalog of blazars, the scientists found powerful evidence that blazars originated the observed high-energy neutrinos.
They found that 10 out of the 19 IceCube hotspots located in the southern sky likely originated from blazars.
“The discovery of these high-energy neutrino factories represents a major milestone for astrophysics,” said Dr. Andrea Tramacere, a researcher in the Department of Astronomy at the University of Geneva.
“It places us a step forward in solving the century-old mystery of the origin of cosmic rays.”
The authors now plan to study the detected blazars to understand what makes them good accelerators.
“The statistical analysis has focused only on the most promising sets of IceCube neutrino data,” Dr. Buson said.
“Further sophisticated analytical techniques may bring more discoveries.”
The team’s work was published in the Astrophysical Journal Letters.
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Sara Buson et al. 2022. Beginning a Journey Across the Universe: The Discovery of Extragalactic Neutrino Factories. ApJL 933, L43; doi: 10.3847/2041-8213/ac7d5b
