Detection of Single ‘Ghost Particle’ Yields Solution of Decade-Old Cosmic Ray Mystery

An international team of scientists, with key contributions from researchers at the University of Maryland, for the first time have pinpointed a supermassive black hole as the source of high-energy cosmic neutrinos—ghostly subatomic particles that are among the most abundant known particles in the universe and among the hardest to detect.

For more than 100 years scientists have been searching for the source of cosmic rays, high energy charged particles (atoms) that move through space at nearly the speed of light. Within cosmic rays there also are neutrinos and other subatomic particles, thus the new finding points at supermassive black holes, called blazars, as generators of neutrinos and cosmic rays.

The finding began with the detection of a single neutrino flashing through the IceCube Neutrino Observatory, a sophisticated array of sensors suspended in the ice thousands of feet deep at the South Pole. The observatory is equipped with a nearly real-time alert system—developed with leadership by UMD scientists—that on Sept. 22, 2017, notified ground- and space-based telescopes around the globe capable of detecting different “messenger” signals: electromagnetic radiation, gravitational waves, neutrinos, and cosmic rays. The coordinated observation and interpretation of data from these different telescopes indicated the source of that neutrino was a blazar, designated TXS 0506+056 by astronomers.

“This result really highlights the importance of taking a multimessenger approach to these searches,” said Erik Blaufuss, a research scientist in the UMD Department of Physics who led the effort over the past several years to create and deploy IceCube’s high-energy event alert system. “Any one observation made alone would likely not have let us piece together what is actually going on inside this source.”

Work on the IceCube alert system by Blaufuss, astrophysicist Gregory Sullivan and other UMD researchers, is part of a long history of Maryland neutrino science that also includes the design of the IceCube data collection system and its software—called IceTray.

The findings that resulted from the coordinated observations of many different observatories were published in two papers in the July 13 issue of the journal Science.

“The era of multimessenger astrophysics is here,” said France Córdova, director of the National Science Foundation, which funds the IceCube Neutrino Observatory. “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.”

Detecting the highest energy neutrinos requires a massive particle detector, and IceCube is the world’s largest by volume. Encompassing a cubic kilometer of deep, pristine ice a mile beneath the surface at the South Pole, the detector is composed of more than 5,000 light sensors arranged in a grid. 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 sensitive cameras. Because a charged particle and the 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 more than a dozen astrophysical neutrinos detected in late 2014 and early 2015, coincident with the same blazar, TXS 0506+056. This independent observation greatly strengthens the initial detection of a single high-energy neutrino and 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 from 49 institutions 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.

The IceCube Neutrino Observatory is funded primarily by the U.S. National Science Foundation and is operated by a team headquartered at the University of Wisconsin–Madison. IceCube construction was also funded with significant contributions from the National Fund for Scientific Research (FNRS & FWO) in Belgium; the Federal Ministry of Education and Research (BMBF) and the German Research Foundation (DFG) in Germany; the Knut and Alice Wallenberg Foundation, the Swedish Polar Research Secretariat, and the Swedish Research Council in Sweden; and the Department of Energy and the University of Wisconsin–Madison Research Fund in the U.S.