At night, a spotlight beam cuts through fog. Some of the light comes straight from the lamp, and some from water droplets that scatter that light. Quasar PKS 0637-752 has a similar jet, except instead of fog there is the cosmic microwave background left over from the Big Bang, and instead of a spotlight there is a stream of electrons traveling at nearly the speed of light.

Previously, it was unclear why the faint inner part of the jet, located close to the core, shines in X-rays. The scientists took archival observations from the Chandra telescope from 1999 and 2017, removed the light of the bright core, and built a model. It turned out that the X-ray glow consists of two parts. About a fifth is produced when fast electrons collide with photons of the cosmic microwave background and transfer energy to them. The rest comes from synchrotron radiation from another group of electrons that spiral in the magnetic field. The model agrees with all observations, including the absence of overly strong gamma-ray emission that the Fermi telescope would have seen.