When a black hole interacts with surrounding gas and dust, it can create jets of material that fly away from the black hole at nearly the speed of light. When those jets interact with the cosmic microwave background, something brilliant can occur.
The cosmic microwave background (CMB) is an afterglow of the big bang. We see it as a faint glow of microwaves in all directions, and that’s because the entire universe is filled with the remnant light of the big bang. Photons from the CMB are streaming through the universe in all directions. When electrons in a black hole jet collide with these photons, they can give the photons an energy boost. Since the electrons collide at nearly the speed of light, the photons are boosted into x-rays.
Recently the Chandra x-ray observatory found an example of such a CMB enhanced jet. The x-rays from this jet began their journey more than 11 billion years ago. At this time the CMB was stronger, and so the x-rays emitted by this jet are about 150 times brighter than a jet forming in the present universe would be. It’s a great example of how the background light of the universe can be rekindled by black holes.
Interestingly, this active black hole isn’t bright at radio frequencies. Often the electrons of a black hole jet interact with magnetic fields to create strong radio emissions, and usually such black hole jets are detected by radio waves first. If more black holes can be x-ray loud but radio quiet, it could mean that there are many more distant x-ray jets that have simply been overlooked.
Paper: A. Simionescu, et al. Serendipitous discovery of an extended X-ray jet without a radio counterpart in a high-redshift quasar. arXiv:1509.04822 [astro-ph.HE]