A black hole under the gravitational lens

Sky chart (left) of the enlargement by factors of up to 20, caused by a star in the foreground galaxy. The gamma ray region of the blazar behind it moves across this pattern (white cross). The image this creates (right) is amplified to different degrees. The splitting into several images which can also be seen on the right cannot be resolved with gamma ray telescopes. At the top left of the image on the right is the blazar as it would appear without the lens effect. 

Blazars rewrite the thermal history of the universe

Many galaxies host a supermassive black hole at their center. Such black holes can emit high-energy gamma rays and are then called blazars. Whereas other radiation such as visible light and radio waves traverses the universe without problems, this is not the case for high-energy gamma rays. This particular radiation interacts with the optical light that is emitted by galaxies, transforming it into electrons and positrons. Initially, these particles move almost at the speed of light. But as they are slowed down by the ambient diffuse gas, their energy is converted into heat, just like in other braking processes. As a result, the surrounding gas is heated efficiently. In fact, the temperature of the gas at mean density becomes ten times higher, and in underdense regions more than one hundred times higher than previously thought.

A supermassive black hole is surrounded by a dust ring (torus). The collapse of gas onto the black hole launches an energetic jet of matter and radiation, which is transported over cosmological distances. A jet that is pointing into our direction is called a "blazar". Credit: ESA/NASA, the AVO project and Paolo Padovani