For decades, astronomers have sought to detect high-energy cosmic neutrinos, and to learn where and how these ghostly subatomic particles are generated with energies thousands to millions of times greater than those attained by the Large Hadron Collider and other particle collider here on Earth.
"The primary challenge in detecting these neutrinos and studying their sources is that they interact very weakly with matter," said Derek Fox, associate professor of astronomy and astrophysics at Penn State and a coauthor of the paper. "The IceCube collaboration overcame this challenge by instrumenting a cubic kilometer of Antarctic ice, more than a kilometer below the South Pole, and watching the ice closely, for years, with the sensitive detectors."
In this manner, IceCube achieved the first-ever detection of high-energy cosmic neutrinos in 2013, and began distributing prompt alerts of new neutrinos in April 2016 in a collaborative effort with the Astrophysical Multimessenger Observatory Network (AMON) at Penn State. Over the course of the next 16 months, 11 IceCube-AMON neutrino alerts were distributed.
"These alerts stimulated an impressive array of follow-up observations, but no interesting sources ... until IceCube-170922A came along," Fox said.
"IceCube-170922A — a high-energy neutrino detected by IceCube on Sept. 22, 2017 — had an energy of 300 trillion electron volts and a trajectory pointing back to a small patch of sky in the constellation Orion," said Azadeh Keivani, a postdoctoral scholar at Penn State and coauthor of the paper. "The IceCube-AMON alert we distributed within seconds of the neutrino detection triggered an automated sequence of X-ray and ultraviolet observations with NASA's Neil Gehrels Swift Observatory and led to further studies with NASA's Fermi Gamma-Ray Space Telescope, its Nuclear Spectroscopic Telescope Array (NuSTAR), and 13 other observatories around the world."
Together with a new analysis of the IceCube team's full dataset for this region of the sky, these observations provide strong evidence that the neutrino was generated by a flaring supermassive black hole or "blazar," 3.7 billion light years from Earth, known to astronomers as TXS 0506+056. The high-energy neutrino detection and source identification are the focus of two Science papers and a July 12 press conference at the National Science Foundation in Washington, D.C.
Swift was the first facility to identify the flaring blazar as a possible counterpart to the neutrino event.
"Thanks to the automated trigger from AMON, Swift was observing within four hours of the neutrino detection, and noticed this intriguing blazar right away," said Jamie Kennea, science operations team lead for the Swift Observatory. Swift's science and flight operations are controlled by Penn State from the Mission Operations Center at the University Park Campus.
"This identification launches the new field of high-energy neutrino astronomy, which we expect will yield exciting breakthroughs in our understanding of the universe and fundamental physics, including how and where these ultra-high-energy particles are produced," said Doug Cowen, professor of physics and astronomy and astrophysics at Penn State; founding member of the IceCube collaboration; and coauthor of the Science paper. "For 20 years, one of our dreams as a collaboration was to identify the sources of high-energy cosmic neutrinos, and it looks like we've finally done it!"
Keivani, who led the Penn State team that first identified the flaring blazar as a prominent high-energy source near the neutrino's arrival direction, is also the lead author of a companion paper on the blazar's properties that has been submitted to the Astrophysical Journal and is posted today on the ArXiv.org scientific preprint website. "We decided to take a close look at the physical conditions near this supermassive black hole, which might allow it to generate high-energy neutrinos," said Keivani. "I am pleased to report that according to our best current theories, it's definitely possible ... not easy, but possible."
In addition to their appointments in the Eberly College of Science, Fox, Keivani and Cowen also are affiliated with Penn State's Institute for Gravitation and the Cosmos.