Fermi Helps Link a Cosmic Neutrino to a Blazar Outburst
Nearly 10 billion years ago, the black hole at the center of a distant galaxy produced a powerful outburst, and light from this blast began arriving at Earth in 2012. Astronomers using data from NASA's Fermi Gamma-ray Space Telescope and other space- and ground-based observatories have shown that a record-breaking neutrino seen around the same time likely was born in the same event.
Neutrinos are the fastest, lightest, most unsociable and least understood fundamental particles. The study provides the first plausible association between a single extragalactic object and a high-energy cosmic neutrino.
Although neutrinos far outnumber all the atoms in the universe, they rarely interact with matter. While this property makes them hard to detect, it lets neutrinos make a fast exit from places where light cannot easily escape -- such as the core of a collapsing star -- and zip across the universe almost completely unimpeded. Neutrinos can provide information about processes and environments that simply aren't available through a study of light alone.
The IceCube Neutrino Observatory, built into a cubic kilometer of clear glacial ice at the South Pole, detects neutrinos when they interact with atoms in the ice. Some of the most extreme particles detected by IceCube receive nicknames based on characters on the children's TV series "Sesame Street." On Dec. 4, 2012, IceCube detected an event known as Big Bird, a neutrino with an energy exceeding 2 quadrillion electron volts (PeV), the highest-energy neutrino ever detected at the time. But the best IceCube position only narrowed the source to a patch of the southern sky about 32 degrees across, equivalent to the apparent size of 64 full moons.
In the summer of 2012, Fermi's Large Area Telescope (LAT) witnessed the onset of a dramatic brightening of PKS B1424-418, an active galaxy classified as a gamma-ray blazar. An active galaxy is an otherwise typical galaxy with a compact and unusually bright core; this excess luminosity is produced by matter falling toward a supermassive black hole weighing millions of times the mass of our sun. As it approaches the black hole, some of the material becomes channeled into particle jets moving outward in opposite directions at nearly the speed of light. In blazars, one of these jets happens to point almost directly toward Earth. During the year-long outburst, PKS B1424-418 shone between 15 and 30 times brighter in gamma rays than its average before the eruption. The blazar is located within the Big Bird source region, but so are many other active galaxies detected by Fermi. Was it the culprit?
Astronomers investigating the link turned to the TANAMI project, which since 2007 has routinely monitored dozens of active galaxies in the southern sky. The program includes regular radio observations using the Australian Long Baseline Array (LBA) and associated telescopes in Chile, South Africa, New Zealand and Antarctica. When networked together, they operate as a single radio telescope more than 6,000 miles across and provide a unique high-resolution look into the jets of active galaxies.
Three radio observations of PKS B1424-418 between 2011 and 2013 cover the period of the Fermi outburst. They reveal that the core of the galaxy's jet had brightened by about four times. No other galaxy observed by TANAMI over the life of the program has exhibited such a dramatic change.
The team suggests the PKS B1424-418 outburst and Big Bird are connected, calculating only a 5-percent probability the two events occurred by chance alone. Using data from Fermi, NASA’s Swift and WISE satellites, the LBA and other facilities, the researchers determined how the energy of the eruption was distributed across the electromagnetic spectrum and showed that it was sufficiently powerful to produce a neutrino at PeV energies.
Labeled version. Fermi LAT images showing the gamma-ray sky around the blazar PKS B1424-418. Brighter colors indicate greater numbers of gamma rays. The dashed arc marks part of the source region established by IceCube for the Big Bird neutrino (50-percent confidence level). Left: An average of LAT data centered on July 8, 2011, and covering 300 days when the blazar was inactive. Right: An average of 300 active days centered on Feb. 27, 2013, when PKS B1424-418 was the brightest blazar in this part of the sky.
Credit: NASA/DOE/LAT Collaboration
Unlabeled version. Fermi LAT images showing the gamma-ray sky around the blazar PKS B1424-418. Brighter colors indicate greater numbers of gamma rays. The dashed arc marks part of the source region established by IceCube for the Big Bird neutrino (50-percent confidence level). Left: An average of LAT data centered on July 8, 2011, and covering 300 days when the blazar was inactive. Right: An average of 300 active days centered on Feb. 27, 2013, when PKS B1424-418 was the brightest blazar in this part of the sky.
Credit: NASA/DOE/LAT Collaboration
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NASA's Goddard Space Flight Center. However, individual images should be credited as indicated above.
- Walt Feimer (KBR Wyle Services, LLC)
- Francis Reddy (University of Maryland College Park)
- Francis Reddy (University of Maryland College Park) [Lead]
- Scott Wiessinger (KBR Wyle Services, LLC) [Lead]
- Roopesh Ojha (University of Maryland Baltimore County)
- Aaron E. Lepsch (ADNET Systems, Inc.)
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