Mysterious high-energy particles could come from black hole jets

It’s three for the price of one. A trio of mysterious high-energy particles could all have the same source: active black holes embedded in galaxy clusters, researchers suggest January 22 in Nature Physics.

Scientists have been unable to figure out the origins of the three types of particles — gamma rays that give a background glow to the universe, cosmic neutrinos and ultrahigh energy cosmic rays. Each carries a huge amount of energy, from about a billion electron volts for a gamma ray to 100 billion billion electron volts for some cosmic rays.

Strangely, each particle type seems to contribute the same total amount of energy to the universe as the other two. That’s a clue that all three may be powered by the same engine, says physicist Kohta Murase of Penn State.

“We can explain the data of these three messengers with one single picture,” Murase says.

First, a black hole accelerates charged particles to extreme energies in a powerful jet (SN: 9/16/17, p. 16). These jets “are one of the most promising candidate sources of ultrahigh energy cosmic rays,” Murase says. The most energetic cosmic rays escape the jet and immediately plow through a sea of magnetized gas within the galaxy cluster.

Some rays escape the gas as well and zip towards Earth. But less energetic rays are trapped in the cluster for up to a billion years. There, they interact with the gas and create high-energy neutrinos that then escape the cluster.

Meanwhile, the cosmic rays that escaped travel through intergalactic space and interact with photons to produce the glow of gamma rays.

Murase and astrophysicist Ke Fang of the University of Maryland in College Park found that computer simulations of this scenario lined up with observations of how many cosmic rays, neutrinos and gamma rays reached Earth.

“It’s a nice piece of unification of many ideas,” says physicist Francis Halzen of the IceCube Neutrino Observatory in Antarctica, where the highest energy neutrinos have been observed.

There are other possible sources for the particles — for one, IceCube has already traced an especially high-energy neutrino to a single active black hole that may not be in a cluster (SN Online: 4/7/16). The observatory could eventually trace neutrinos back to galaxy clusters. “That’s the ultimate test,” Halzen says. “This could be tomorrow, could be God knows when.”