Long Ago, In a Galaxy Far Away

In September, 2012, NASA’s Fermi Gamma Ray Telescope detected a series of bright gamma-ray flares from a source known as B0218+357, located 4.35 billion light-years from Earth in the direction of a constellation called Triangulum. These powerful flares in a known gravitational lens system provided the key to making the lens measurement. Long before gamma-ray bursts from B0218+357 reached us, they passed through a face-on spiral galaxy very much like our own about 4 billion light-years away. That galaxy’s gravity bent the light into different paths, so we see the background blazar as dual images. With just a third of an arcsecond (less than 0.0001 degree) between them, the B0218+357 images hold the record for the smallest separation of any lensed system known.

While radio and optical telescopes can resolve and monitor the individual blazar images, Fermi cannot. Instead, the Fermi team exploited a “delayed playback” effect. This movie illustrates the components of the gravitational lens system. Different sight lines to a background blazar resulted in two images that arrived at slightly different times. Fermi made the first gamma-ray measurements of this sort of delay in a lens system

Video Credit: NASA

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