Fermi observations suggest possible years-long cyclic changes in gamma-ray emission from the blazar PG 1553+113. The graph shows Fermi Large Area Telescope data from August 2008 to July 2015 for gamma rays with energies above 100 million electron volts (MeV). For comparison, visible light ranges between 2 and 3 electron volts. Vertical lines on data points are error bars. Background: One possible explanation for the gamma-ray cycle is an oscillation of the jet produced by the gravitational pull of a second massive black hole, seen at top left in this artist's rendering.