National Aeronautics and Space Administration

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A Gravitational Lens is a kind of natural telescope formed when a rare cosmic alignment allows the gravity of a massive object to bend and amplify light from a more distant source. This figure is an artist's impression of Fermi observations of a 'gravitational lens'.

In September 2012, the Fermi Large Area Telescope detected a series of bright gamma-ray outbursts from a powerful blazar called B0218+357 located 4.35 billion light-years away. Blazars are a type of active galaxy with a supersized black hole that blasts out some of the matter falling toward it as a pair of 'jets' traveling in opposite directions almost at the speed of light.

On its way to us, light from this blazar passed through a spiral galaxy located 4.03 billion light years away. The gravity of this spiral galaxy bends the light into different paths, so astronomers see the background blazar as dual images. Since these different paths are of different length, there is a delay between flares of the two images.

At that time, this blazar was briefly the brightest gamma-ray source outside of our own galaxy, and LAT observations of three flaring episodes showed a 'playback' delay of 11.46 days.

» http://svs.gsfc.nasa.gov/11437