A dramatic explosion occurred from a galaxy known as PKS B1424-418. Light from this blast began arriving at Earth in 2012. On Dec. 4, 2012, the IceCube Neutrino Observatory at the South Pole detected an event known as Big Bird – a neutrino gamma ray blazer with an energy exceeding 2 quadrillion electron volts (PeV). Now, an international team of astronomers, led by Matthias Kadler, professor for astrophysics at the University of Würzburg, has published their results in the scientific journal Nature Physics.
Starting in the summer of 2012, NASA’s Fermi satellite witnessed 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. The excess luminosity of the central region 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 then so are many other active galaxies detected by Fermi.
The scientists searching for the neutrino source then turned to data from a long-term observing program named TANAMI. Since 2007, TANAMI has routinely monitored nearly 100 active galaxies in the southern sky, including many flaring sources detected by Fermi. Three radio observations between 2011 and 2013 cover the period of the Fermi outburst. They reveal that the core of the galaxy’s jet had been brightening by about four times. No other galaxy observed by TANAMI over the life of the program has exhibited such a dramatic change.
“Within their jets, blazars are capable of accelerating protons to relativistic energies. Interactions of these protons with light in the central regions of the blazar can create pions. When these pions decay, both gamma rays and neutrinos are produced,” explains Karl Mannheim, a coauthor of the study and astronomy professor in Würzburg, Germany. “We combed through the field where Big Bird must have originated looking for astrophysical objects capable of producing high-energy particles and light,” adds coauthor Felicia Kraub from the University of Erlangen-Nürnberg in Germany.
In published report, the team suggests the PKS B1424-418 outburst and Big Bird are linked, 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.