Super-massive black holes in the centers of some active galaxies create powerful jets of radiation and particles travelling close to the speed of light. Attracted by strong gravity, matter falls towards the central black hole as it feeds on the surrounding gas and dust. But instead of falling into the black hole, a small fraction of particles get accelerated to speed almost as great as the speed of light and spewn out in two narrow beams along the axis of rotation of the black hole. These jets are believed to be the sources of the fastest-travelling particles in the Universe -- cosmic rays.
In some cases these jets can reach outside of the galaxy itself, ending in giant radio lobes far from the active galaxy center. Observed with radio telescopes these galaxies can have a variety of shapes, mostly resembling dumbbells. We call these objects either radio galaxies or quasars, depending on how bright they are and how fast they consume the surrounding matter. As these monster black holes grow to become a billion times more massive than our Sun, their jets eventually get strong enough to blow gas out of the galaxy and shut off the formation of new stars!
A small fraction of active galaxies with jets are oriented so that their jet is pointed straight at Earth. In those cases we observe radiation across the electromagnetic spectrum enhanced by the enormous speed of the jet and call such sources blazars. By combining NuSTAR X-ray observations with observations in the radio, visible light and extremely energetic gamma-rays, we are learning about the physics of how powerful jets are formed and sustained. One of the remaining mysteries is how do jets create radiation of such extraordinary broad spectrum up to very high energies.