First ever gamma-ray study of gravitational lens conducted

Astronomers using NASA's Fermi observatory, has made the first-ever gamma-ray measurements of a gravitational lens - a 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.

In September 2012, Fermi's Large Area Telescope (LAT) 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 light from B0218+357 reaches us, it passes directly through a face-on spiral galaxy-one very much like our own-about 4 billion light-years away.

The galaxy's gravity bends the light into different paths, so astronomers 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's LAT cannot. Instead, the Fermi team exploited a "delayed playback" effect.

Team member Jeff Scargle, an astrophysicist at NASA's Ames Research Center in Moffett Field, California, said that one light path is slightly longer than the other, so when they detect flares in one image they can try to catch them days later when they replay in the other image.

A paper is set to be published in The Astrophysical Journal Letters.