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Web Extra: Gamma rays’ long road toward Fermi

Watch a gamma ray travel from its source in a faraway galaxy, through the extragalactic background, and finally to Earth, where the Fermi telescope can measure it.
RELATED TOPICS: GAMMA-RAY BURSTS
This NASA animation shows a gamma ray traveling from its blazar source, encountering the extragalactic background, and being absorbed by the Fermi telescope.
There are many ways to measure the extragalactic background (EBL), the universe’s glow from myriad sources. One of these measurement techniques uses the light from faraway blazars — active galaxies with supermassive black holes at their centers, blasting jets of high-speed particles and radiation out into space. Scientists study the gamma rays streaming from these high-powered jets. The gamma rays have a very long journey between their blazars and Earth, where astronomers can measure them. Along the way, some of these gamma rays will strike an intervening EBL photon and annihilate into a positron and an electron.

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