NASA Fermi Observatory Makes First-Ever Gamma-Ray Measurements of Gravitational Lens

Using NASA's Fermi observatory, a team of astronomers have made the first-ever gamma-ray measurements of a gravitational lens. The findings open new avenues for future research, including a novel way to probe emission regions near supermassive black holes.

A gravitational lens is 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. Yet the scientists detected this lens thanks to a blazar, a type of galaxy noted for its intense emissions and unpredictable behavior.

The researchers first discovered this blazar, known as B0218+357, in 2012. It's located about 4.35 billion light-years from Earth within a known gravitational lens system. In addition, it possesses a supersized black hole at its heart. This black hole has a mass millions to billions of times that of our sun.

The extreme brightness and variability of blazars result from a chance orientation that brings one jet almost directly in line with Earth. Astronomers look down the barrel of the jet, which greatly enhances its apparent emission. Yet long before light from B0218+357 reaches us, it passes directly through a face-on spiral galaxy about 4 billion light-years away. The gravity from this galaxy bends the light into different paths, which allows astronomers to see the background blazar as dual images.

"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 Jeff Scargle, one of the researchers, in a news release.

In order to examine this object and the lens a bit more closely, the researchers identified three episodes of flares showing playback delays of 11.46 days. Intriguingly, the gamma-ray delay is about a day longer than radio observations report for this system. While the flares and their playbacks show similar gamma-ray brightness, in radio wavelengths one blazar image is about four times brighter than the other.

What does this mean? It's possible that the gamma rays don't arise from the same regions as the radio waves, so these emissions take slightly different paths. This corresponds it different delays and amplifications.

The findings reveal a little more about these lenses. Currently, the scientists plan to compare radio and gamma-ray observations of additional lens systems, which could provide new insights into the workings of powerful black-hold jets.

The findings are published in the journal Astrophysical Journal Letters.