First Gamma Ray Study Gravitational Lens
January 6, 2014

NASA Conducts First Ever Gamma-Ray Study Of A Gravitational Lens

[ Watch the Video: Fermi Provides New Details About Gravitational Lenses ]

Brett Smith for - Your Universe Online

A gravitational lens is created when a galaxy or other massive object bends and amplifies light heading toward an observer from a more distant source and a new paper from an international team of researchers has reported the first-ever gamma ray study of the cosmic phenomenon.

The paper, which will be published in an upcoming edition of Astrophysical Journal Letters, is based on data from NASA’s Fermi gamma-ray observatory in low-Earth orbit.

"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 study author Teddy Cheung, an astrophysicist at the Naval Research Laboratory in Washington.

In September 2012, the orbiter’s Large Area Telescope (LAT) picked up a sequence of bright gamma-ray flares from a source known as B0218+357, located 4.35 billion light-years from Earth. Realizing the flares could be a major opportunity for observations, Cheung applied for and was granted a week of LAT target-of-opportunity observing time.

B0218+357 is a type of unpredictable, active galaxy called a blazar. With a super-massive black hole in the middle, a blazar’s matter spirals inward and occasionally blasts out in opposite directions as brilliant jets of particles traveling close to the speed of light. When one of these jets points almost directly at Earth, astronomers effectively look down the barrel of the jet, to observe the emissions.

[ Watch The Video: Gravitational Lens System ]

Before light from B0218+357 reaches Earth, it travels through an Earth-facing spiral galaxy about 4 billion light-years away. Like a lens in your typical pair of glasses, the galaxy's gravity bends the light into different paths, causing Earth astronomers see the background blazar as two images.

Radio and optical telescopes are able to monitor these dual images, but Fermi's LAT cannot. So to make their observations, the study team took advantage of what's know as the 'delayed playback' effect.

"One light path is slightly longer than the other, so when we detect flares in one image we can try to catch them days later when they replay in the other image," explained team member Jeff Scargle, an astrophysicist at NASA's Ames Research Center in California.

At the American Astronomical Society meeting in National Harbor, Md. this week, Cheung announced that the team had recorded three incidents of flares showing playback delays of over 11 days. The team noted that the gamma-ray delay lasts one day longer than the one seen for radio observations. Also, the gamma-ray flares and their playback show brightness, unlike radio wavelength flares that differ four-fold in brightness.

The team theorized that the gamma rays arise from the same regions in the blazar as the radio waves, meaning these types of emissions take slightly different paths with correspondingly different delays and amplifications as they are affected by the gravitational lens.

"Over the course of a day, one of these flares can brighten the blazar by 10 times in gamma rays but only 10 percent in visible light and radio, which tells us that the region emitting gamma rays is very small compared to those emitting at lower energies," said team member Stefan Larsson, an astrophysicist at Stockholm University in Sweden.

The study team said their findings could provide new details on the workings of black hole jets. They could also establish new guides on important cosmological measures like the Hubble constant, which is used to explain the universe's rate of expansion.