Astronomers using NASA's Fermi observatory have made the first-ever gamma-ray measurements of a gravitational lens, thanks to B0218+357, located 4.35 billion light-years from Earth in the direction of a constellation Triangulum.
It's not really what it was designed to do. While radio and optical telescopes can resolve and monitor individual blazar images, Fermi's LAT cannot. Instead, the Fermi team exploited a "delayed playback" effect, which
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.
A gravitational lens is what it sounds like - a kind of natural telescope formed when the gravity of a massive object causes light from a distant source to bend and amplify. 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, which was classified as a blazar. The powerful flares in a known gravitational lens system provided the key to making the lens measurement.
At the heart of B0218+357 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.
"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," said team member Jeff Scargle, an astrophysicist at NASA's Ames Research Center in Moffett Field, Calif.
In September 2012, when the blazar's flaring activity made it the brightest gamma-ray source outside of our own galaxy, Teddy Cheung, leader of the study and an astrophysicist at the Naval Research Laboratory in Washington, got a golden opportunity: a week of LAT target-of-opportunity observing time, from Sept. 24 to Oct. 1, to hunt for delayed flares.
Cheung said the team had identified three episodes of flares showing playback delays of 11.46 days, with the strongest evidence found in a sequence of flares captured during the week-long LAT observations. Intriguingly, the gamma-ray delay is about a day longer than radio observations report for this system. And while the flares and their playback show similar gamma-ray brightness, in radio wavelengths one blazar image is about four times brighter than the other.
Astronomers don't think the gamma rays arise from the same regions as the radio waves, so these emissions likely take slightly different paths, with correspondingly different delays and amplifications, as they travel through the 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.
As a result, the gravity of small concentrations of matter in the lensing galaxy may deflect and amplify gamma rays more significantly than lower-energy light. Disentangling these so-called microlensing effects poses a challenge to taking further advantage of high-energy lens observations.
The scientists say that comparing radio and gamma-ray observations of additional lens systems could help provide new insights into the workings of powerful black-hole jets and establish new constraints on important cosmological quantities like the Hubble constant, which describes the universe's rate of expansion.
The most exciting result, the team said, would be the LAT's detection of a playback delay in a flaring gamma-ray source not yet identified as a gravitational lens in other wavelengths.
Presented at the American Astronomical Society meeting in National Harbor, Md., Upcoming in the Astrophysical Journal Letters. Source: NASA/Goddard Space Flight Center