VERITAS supplies critical piece to neutrino discovery puzzle
Publication highlight: Associate Prof. John Quinn
The VERITAS collaboration, including UCD School of Physics members Assoc. Prof. John Quinn and Ph.D. student Ste O’Brien, has confirmed the detection of very-high-energy gamma-ray emission from the vicinity of a supermassive black hole, observations prompted by the detection of a spatially coincident high-energy neutrino event by the IceCube collaboration. This black hole is potentially the first known astrophysical source of high-energy cosmic neutrinos, a type of ghostly subatomic particle, and the observations represent a breakthrough in multi-messenger astrophysics.
This artist's conception shows a blazar -- the core of an active galaxy powered by a supermassive black hole. Scientists recently used VERITAS to detect gamma rays from the blazar TXS 0506+056, which is also thought to be a source of neutrinos. Image credit: M. Weiss/CfA
On September 22, 2017 the IceCube Neutrino Observatory, a cubic-kilometer neutrino telescope located at the South Pole, detected a high-energy neutrino of potential astrophysical origin. However, the observation of a single neutrino by itself is not enough for IceCube to claim the detection of a source. For that, scientists needed more information.
Very quickly after the detection by IceCube was announced, telescopes around the world including VERITAS (which stands for the "Very Energetic Radiation Imaging Telescope Array System") swung into action to identify the source. The VERITAS, MAGIC and H.E.S.S. gamma-ray observatories all looked at the neutrino position. In addition, two gamma-ray observatories that monitor much of the sky at lower and higher energies also provided coverage.
These follow-up observations of the rough IceCube neutrino position suggest that the source of the neutrino is a blazar, which is a supermassive black hole with powerful outflowing jets that can change dramatically in brightness over time. This blazar, known as TXS 0506+056, is located at the center of a galaxy about 4 billion light years from Earth. Initially, NASA's Fermi Gamma-ray Space Telescope observed that TXS 0506+056 was several times brighter than usually seen in its all-sky monitoring. Eventually, the MAGIC observatory made a detection of much higher-energy gamma rays within two weeks of the neutrino detection, while VERITAS, H.E.S.S. and HAWC did not see the blazar in any of their observations during the two weeks following the alert.
Given the importance of higher-energy gamma-ray detections in identifying the possible source of the neutrino, VERITAS continued to observe TXS 0506+056 over the following months, through February 2018, and revealed the source but at a dimmer state than what was detected by MAGIC. "The VERITAS detection shows us that the gamma-ray brightness of the source changes, which is a signature of a blazar," said Wystan Benbow of the Smithsonian Astrophysical Observatory (SAO) that operates and manages VERITAS, and the Principal Investigator of VERITAS operations. "Finding a link between an astrophysical source and a neutrino event could open yet another window of exploration to the extreme Universe."
The detection of gamma rays coincident with neutrinos is tantalizing, since both particles must be produced in the generation of cosmic rays. 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. What creates and launches these particles across such vast distances? Where do they come from?
"The potential association of this neutrino event with enhanced gamma-ray emission from TXS 0506+056 lends observational support to the theory that supermassive black holes at the centers of galaxies power nature’s most energetic particle accelerators and could be responsible for producing the highest-energy cosmic rays" said co-author and Deputy Spokesperson of the VERITAS Collaboration, John Quinn of University College Dublin, Ireland.
"The era of multi-messenger 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." “The detection of very-high-energy gamma-rays from TXS 0506+056 with VERITAS provides vital information to understand the powerful processes taking place in this and other potential neutrino sources," said co-author Marcos Santander of the University of Alabama in Tuscaloosa, who led the study. "The deep interconnection between neutrinos and gamma-rays is allowing us, for the first time, to study astrophysical objects using multimessenger observations in a way that would be impossible using single messengers."
A paper describing the deep VERITAS observations of TXS 0506+056 ("VERITAS Observations of the BL Lac Object TXS 0506+056") is accepted for publication in The Astrophysical Journal Letters and appears online on July 12, 2018. A paper on the IceCube and initial gamma-ray observations, including VERITAS's, appears in the latest issue of the journal Science. VERITAS is a ground-based facility located at the SAO's Fred Lawrence Whipple Observatory in southern Arizona. It consists of an array of four 12-meter optical telescopes that can detect gamma rays via the extremely brief flashes of blue "Cherenkov" light created when gamma rays are absorbed in the Earth's atmosphere. The VERITAS Collaboration consists of about 80 scientists from 20 institutions in the United States, Canada, Germany and Ireland. In July 2018 the VERITAS Summer 2018 Collaboration Meeting was held at University College Dublin.