Fermi space telescope links cosmic neutrino to blazar blast
Almost 10 billion years ago, a supermassive black hole at the center of a galaxy named PKS B1424-418 produced a powerful outburst. Light from the blast began arriving at Earth in 2012. Researchers using data from NASA’s Fermi Gamma-ray Space Telescope and other space and ground-based observatories recently discovered a link between that event and a record-breaking cosmic neutrino observed in December 2012.
“Neutrinos are the fastest, lightest, most unsociable and least understood fundamental particles, and we are just now capable of detecting high-energy ones arriving from beyond our galaxy,” said Roopesh Ojha, a Fermi team member at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and a co-author of the study. “Our work provides the first plausible association between a single extragalactic object and one of these cosmic neutrinos.”
Neutrinos far outnumber all of the atoms in the universe but rarely interact with matter, which makes them extremely difficult to detect. The same property that makes them hard to find also allows them to exit from places where light cannot escape, such as the core of a collapsing star. Neutrinos can provide scientists with information about processes and environments that is not available through the study of the electromagnetic spectrum alone.
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. Image & Caption Credit: NASA/DOE/LAT Collaboration
Built into a cubic kilometer of clear glacial ice at the South Pole, the IceCube Neutrino Observatory detects neutrinos when they interact with atoms in the ice. This interaction triggers a cascade of fast-moving charged particles that emit a faint glow – called Cherenkov radiation – as they travel, which is picked by an array of thousands of optical sensors strung throughout IceCube. Researchers determine the energy of a passing neutrino by the amount of light emitted by its particle cascade.
So far, the IceCube research team has detected about a hundred very high-energy neutrinos, and they nicknamed some of the most extreme events after characters from the children’s TV series Sesame Street. On December 4, 2012, IceCube detected a neutrino dubbed “Big Bird”. It is the second-highest-energy neutrino ever detected.
Exactly where the neutrino came from was unknown. The best IceCube location only narrowed its source to a patch of the southern sky about 32 degrees across, equivalent to the apparent size of 64 full moons.
Radio images from the TANAMI project reveal the 2012-2013 eruption of PKS B1424-418 at a wavelength of 8.4 GHz. The core of the blazar’s jet brightened by four times, producing the most dramatic blazar outburst TANAMI has observed to date. Image & Caption Credit: TANAMI
Data from the Fermi Gamma-ray Space Telescope helped to narrow the search. Beginning in the summer of 2012, the satellite’s Large Area Telescope witnessed a dramatic brightening of PKS B1424-418, an active galaxy classified as a gamma-ray blazar. An active galaxy was a compact and unusually bright core. The excessive brightness is produced by matter falling toward a supermassive black hole. As it approaches the black hole, some of the material becomes channeled into particle jets moving in opposite directions at nearly the speed of light. In blazars, one of these jets happens to be pointed almost directly at the Earth.
Throughout the year-long outburst, PKS B1424-418 radiated 15 to 30 times brighter in gamma rays than it normally did before the eruption. The location of the blazar is within the Big Bird source region, but that is also the case with a multitude of other active galaxies detected by Fermi.
The scientists then utilized data from a long-term observing program named TANAMI to locate the source of the neutrinos. TANAMI has been routinely observing nearly 100 active galaxies in the southern sky since 2007, including various flaring sources detected by Fermi. The program involves regular radio frequency observations utilizing the Australian Long Baseline Array (LBA) and associated telescopes in Chile, South Africa, New Zealand, and Antarctica. When combined in a network, they operate as a single radio telescope more than 6,000 miles (9,656 km) across, providing a unique high-resolution look into the jets of active galaxies.
Three radio observations of PKS B1424-418, taken between 2011 and 2013, cover the duration of the Fermi outburst. The observations show that the core of the galaxy’s jet had intensified by about four times. During the life of the TANAMI program, no other galaxy has been observed that has displayed such a dramatic change.
“We combed through the field where Big Bird must have originated looking for astrophysical objects capable of producing high-energy particles and light,” said co-author Felicia Krauss, a doctoral student at the University of Erlangen-Nuremberg in Germany. “There was a moment of wonder and awe when we realized that the most dramatic outburst we had ever seen in a blazar happened in just the right place at just the right time.”
In a paper published on Monday in Nature Physics, the research team suggests the PKS B1424-418 outburst and Big Bird are linked, calculating only a 5-percent chance that the two events occurred by chance alone. Using data from Fermi and NASA’s Swift and WISE satellites, the scientists determined how the energy of the outburst was spread across the electromagnetic spectrum and showed that it was sufficiently powerful to produce the Big Bird neutrino.
“Taking into account all of the observations, the blazar seems to have had means, motive and opportunity to fire off the Big Bird neutrino, which makes it our prime suspect,” said lead author Matthias Kadler, a professor of astrophysics at the University of Wuerzburg in Germany.
Video courtesy of NASA
Jim Sharkey
Jim Sharkey is a lab assistant, writer and general science enthusiast who grew up in Enid, Oklahoma, the hometown of Skylab and Shuttle astronaut Owen K. Garriott. As a young Star Trek fan he participated in the letter-writing campaign which resulted in the space shuttle prototype being named Enterprise. While his academic studies have ranged from psychology and archaeology to biology, he has never lost his passion for space exploration. Jim began blogging about science, science fiction and futurism in 2004. Jim resides in the San Francisco Bay area and has attended NASA Socials for the Mars Science Laboratory Curiosity rover landing and the NASA LADEE lunar orbiter launch.
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