Fermi Science Standoff
Gamma-ray Cycle in AGN

Fermi Finds Hints of Gamma Ray Cycle in an Active Galaxy
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In data from the Fermi Large Area Telescope (LAT) there are indications of a roughly two-year-long variation of gamma-rays from a galaxy known as PG 1553+113. It takes light about 5 billion years to reach the Earth from PG 1553+113. If confirmed, the discovery would mark the first years-long cyclic gamma-ray emission ever detected from any galaxy, which could provide new insights into physical processes near the black hole.

Supermassive black holes weighing millions of times the Sun's mass lie at the hearts of most large galaxies, including our own Milky Way. In about 1 percent of these galaxies, the monster black hole radiates billions of times as much energy as the Sun, emission that can vary unpredictably on timescales ranging from minutes to years. Astronomers refer to these as active galaxies (AGN). This figure shows an artist's impression of the region close to the central black hole of an active galaxy with the variation in gamma-ray brightness over time shown in the inset.

More than half the gamma-ray sources seen by the LAT are active galaxies called blazars, like PG 1553+113. As matter falls toward its supermassive black hole, some subatomic particles escape at nearly the speed of light along a pair of jets pointed in opposite directions. What makes a blazar so bright is that one of these particle jets happens to be aimed almost directly toward us.

Scientists have identified several scenarios that could drive periodic emission, including different mechanisms that could produce a years-long wobble in the jet of high-energy particles emanating from the black hole. The most exciting scenario involves the presence of a second supermassive black hole closely orbiting the one producing the jet we observe. The gravitational pull of the neighboring black hole would periodically tilt the inner part of its companion's accretion disk, where gas falling toward the black hole accumulates and heats up. The result would be a slow oscillation of the jet much like that of a lawn sprinkler, which could produce the cyclic gamma-ray changes we observe.

» https://phys.org/news/2015-11-fermi-mission-hints-gamma-ray-galaxy.html