Researchers working with the Fermi observatory have used gamma radiation coming from a distant supermassive black hole (SMBH) to measure the amplifying effect of a gravitational lens. When distant galaxies are lined up just right with even more distant ultra-bright objects, the galaxy’s gravitational field bends the distant object’s light and curves it back into our line of sight, just like a gigantic lens. Astronomers have used these chance alignments for years to study things that would otherwise be too far for even our best telescopes to see, but it’s only recently that they began comparing radio observations of gravitational lenses with gamma ray images. To their surprise, they found that gamma ray signals seem to take longer to arrive than the radio images.
4.35 billion light years away, in the constellation Triangulum, lies a blazar called B0218+357. Blazars are young galaxies with highly active SMBH’s at their core. As the SMBH consumes passing nebulae and stars, it belches out enormously focused blasts of high energy radiation, some of which happens to be pointed exactly in our direction. This lucky alignment allows us to observe these unpredictable high energy emissions as they occur (Astronomers often describe events like this by the the time they appear to happen and ignoring the speed of light. This saves them the confusion of having to remember to repeatedly add and subtract billions of years when speaking to lay-persons). However, B0218+357 is so far away that even these signals become too faint to detect. Fortunately, there is a spiral galaxy relatively near the blazar — about 4 billion light years from Earth — which is in just the right position to work as a gravitational lens pointing at the blazar.
A burst of activty from B0218+357 was discovered in September 2012 by astronomers working with Fermi, which led them to consider the possibility of using gamma ray observations to study the gravitational lens itself. “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.
An interesting feature of gravitational lenses is that they frequently bend more than one beam of light, so that what astronomers actually see is several highly distorted images of the source object, arranged in different patterns around the lens depending on how perfectly the system aligns with Earth. In this case, we get two images, and they’re positioned closer together than any other lensed image discovered so far. In fact, they’re close enough together that Fermi cannot distinguish them as separate objects. Fortunately, the paths travelled by the two images differ in length, so that if the blazar flares up, one image brightens before the other. Cheung’s team knew that by measuring the delay between the two images, they could calculate the lengths of the two light paths and so learn more about the spiral galaxy causing the lens effect.
The big surprise came when they compared their measurements with those made by radio telescopes observing the same flares. The gamma ray observations showed that the two images were separated by 11.46 days. However, this was longer than the same observations made by radio telescopes by a full day. And that was not the only discrepancy: Two images of any articular event are the same brightness when viewed in gamma rays, but radio observations have the second image appearing four time brighter than the first.
How do we explain this? Astronomers believe that different wavelength emissions must be coming from different parts of the blazar. This would affect the path travelled by the light of different wavelengths, which in turn affects how the radiation is delayed or amplified. “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.
Light coming from a small region is more concentrated than the diffuse radio emissions of a large region. This means that the visible and gamma radition can be more sharply affected, while changes to radio wavelengths will be smeared out and less obvious. Scientists hope to use this effect to learn more about the internal structure of the blazar. By comparing the radio and gamma ray data, they might be able to separate the lensing effects of the spiral galaxy from the smaller microlensing effects of the blazar’s own galaxy. And learning more about the behaviour of these ancient beasts will in turn teach them more about the scale of the universe and how it has changed over the past 4.5 billion years.
For more information read a pre-release copy of the science paper describing their work here: arXiv:1401.0548 [astro-ph.HE]