Fermi observatory: First gamma-ray study of a gravitational lens

In the heart of an active galaxy, matter falling toward a supermassive black hole creates jets of particles traveling near the speed of light. 

For active galaxies classified as blazars, one of these jets beams almost directly toward Earth. 

Credit: NASA /Goddard Space Flight Center Conceptual Image Lab

An international team of astronomers, using NASA's Fermi observatory, has made the first-ever gamma-ray measurements of a gravitational lens, 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 accomplishment opens new avenues for research, including a novel way to probe emission regions near supermassive black holes.

It may even be possible to find other gravitational lenses with data from the Fermi Gamma-ray Space Telescope.

Teddy Cheung

"We began thinking about the possibility of making this observation a couple of years after Fermi launched, and all of the pieces finally came together in late 2012," said Teddy Cheung, lead scientist for the finding and an astrophysicist at the Naval Research Laboratory in Washington.

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.

Astronomers classify B0218+357 as a blazar—a type of active galaxy noted for its intense emissions and unpredictable behavior.

At the blazar's heart is a supersized black hole with a mass millions to billions of times that of the sun.

As matter spirals toward the black hole, some of it blasts outward as jets of particles traveling near the speed of light in opposite directions.

The extreme brightness and variability of blazars result from a chance orientation that brings one jet almost directly in line with Earth.

Astronomers effectively look down the barrel of the jet, which greatly enhances its apparent emission.

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.

More Information: Fermi Gravitational Study