The engines of the most famous vessel in the "Star Trek" universe, the USS Enterprise, are powered by the annihilation of matter and antimatter, a process that produces energy in the form of gamma rays. More than half the gamma-ray sources cataloged by the Fermi mission come from a different type of engine — supermassive black holes in the cores of distant galaxies.
Most large galaxies harbor monster black holes millions to billions of times more massive than the Sun. When matter falls toward a supermassive black hole, the center of the host galaxy emits far more light than normal and may flare up unpredictably. Astronomers say such galaxies possess active galactic nuclei, or AGN for short.
Fermi sees the universe in gamma rays, the most energetic form of light. In its first four years, Fermi found more than 1,500 gamma-ray AGN, and it continues to find more.
The rotating supermassive black hole at the center of an AGN works like an engine that converts the gravitational energy of matter falling toward the black hole into other forms, such as light. Gas falling toward the black hole is compressed and heated to millions of degrees and glows brightly near the black hole. Strong magnetic fields, combined with the black hole's rotation, can accelerate some of these charged particles to velocities approaching the speed of light.
Particles accelerated near the black hole's event horizon, the point of no return, can escape along the black hole's spin axis, forming a pair of particle jets moving in opposite directions. Working with other missions, Fermi has shown that the fastest jets are the best at producing gamma-rays. When fast-moving particles run into lower-energy light, they can impart energy to it and boost it up to gamma-ray energies. In the coming years, Fermi scientists hope to determine which types of particles are responsible for these "kicks" — lighter ones like electrons and positrons, heavier particles like protons, or some combination.
Fermi data illustrate how the gamma-ray brightness of AGN can vary. On June 14, 2015, an AGN called 3C 279 increased in brightness by 10 times over the course of a few hours. These rapid changes are associated with "blazars," a kind of AGN where one of the particle jets happens to point almost directly toward us. This orientation, together with near-light-speed motions in the jet, amplify the object's brightness and variability, which helps astronomers explore the workings of its central engine.