Astronomers using data from NASA's Fermi Gamma-ray Space Telescope have detected hints of periodic changes in the brightness of a so-called "active" galaxy, whose emissions are powered by a supersized black hole.If confirmed, the discovery would mark the first years-long cyclic gamma-ray emission ever detected from any galaxy, providing new insights into physical processes near the black hole."Looking at many years of data from Fermi's Large Area Telescope (LAT), we picked up indications of a roughly two-year-long variation of gamma rays from a galaxy known as PG 1553+113," said Stefano Ciprini, who coordinates the Fermi team at the Italian Space Agency's Science Data Center (ASDC) in Rome.
"This signal is subtle and has been seen over less than four cycles, so while this is tantalising we need more observations," he added.Supermassive black holes lie at the hearts of most large galaxies, including our own Milky Way.In about one 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.More than half of the gamma-ray sources seen by Fermi's LAT are active galaxies called blazars.If the gamma-ray cycle of PG 1553+113 is in fact real, the scientists predict it will peak again in 2017 and 2019, well within Fermi's expected operational lifetime.PG 1553+113 lies in the direction of the constellation Serpens, and its light takes about five billion years to reach Earth.NASA's Fermi Gamma-ray Space Telescope was launched in June 2008.The findings were published in The Astrophysical Journal Letters.