The opportunity arose in September 2012, when 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 away in the constellation Triangulum. These powerful outbursts in a known gravitational lens provided the key to making the measurement.
Astronomers classify B0218+357 as a blazar, a type of active galaxy noted for intense outbursts. 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 this black hole, some of it blasts outward as jets of particles traveling near the speed of light in opposite directions.
Long before light from B0218+357 reaches us, it passes directly through a spiral galaxy – one much like our own – located 4.03 billion light-years away. The galaxy's gravity bends the light into different paths, so astronomers see the background blazar as dual images. But these paths aren't the same length, which means that when one image flares, there's a delay of many days before the other does.
While radio and optical telescopes can resolve and monitor the individual blazar images, Fermi's LAT cannot. Instead, the Fermi team exploited the playback delay between the images.
In September 2012, when the blazar's flaring activity made it the brightest gamma-ray source outside of our own galaxy, Fermi scientists took advantage of the opportunity by using a week of dedicated LAT time to hunt for delayed flares.
Three episodes of flares showing playback delays of 11.46 days were found, with the strongest evidence in a sequence of flares captured during the week-long LAT observations.