This is a guest article by Ariana Vlad, senior at the International Computers High School of Bucharest, Romania, where she focuses on studying Physics and Mathematics.
Active Galactic Nuclei
Through observations from the early 20th century, astrophysicists gathered convincing evidence for a new theory regarding the Universe: they linked some of the larger galaxies to emission lines from the central, active nucleus. It was only after the perfectionism of radio detectors in the 1950s that the study of the active galactic nuclei became a major astronomy topic. In time, following the discovery of some black holes acting as sources of X-ray radiation, a theory started to develop. It is now accepted that the engines of these active galactic nuclei are supermassive black holes found in the center of large galaxies.
For most of the galaxies observed by scientists, the black holes are not visually observed since they don’t reflect any amount of light. Sometimes, when the galactic nuclei go through a “radio-loud” phase, they become active, producing bright radiation and making the rest of the galaxy very hard to observe directly. This happens because black holes interact with their surroundings due to their rapid rotation and great tidal forces, therefore “feeding” their accretion disk with interstellar gas and matter from disrupted stars. Heated to incredibly high temperatures due to numerous high-energy collisions, it starts to emit huge amounts of energy above and below the black hole in the form of jets of radiation and particles. When the jets stop, the galaxy goes back to its initial “radio-quiet” phase.
The supermassive black hole from the center of the Milky Way, Sagittarius A, went through this process once and will enter a new “radio-loud” phase in a few millions or billions of years.
Nature of Jets
These jets contain plasma spanning over distances as long as several mega parsecs and they emit X-ray radiation due to synchrotron emission from relativistic electrons rotating in strong magnetic fields. The power of this emission is proportional to the fourth power of the electron’s energy. In a frame where the electron is at rest at any moment, the radiation is isotropic. When an observer on Earth looks towards the galaxy, he will register pulses, as the radiation is concentrated on a forward cone with a small angle. From the period of the pulses one can determine the period of the electron’s motion and further the characteristics of the active galactic nucleus.
How an observer sees the active nucleus is very important as the direction of the line of sight is related to the variation of its properties. When viewed edge-on, some infrared radiation from the accretion disk is detected, but not much emission from the jets. On the contrary, looking directly into the jets makes any radiation from the accretion disk or the galaxy itself insignificant. In the first case, we detect radio galaxies or quasar, while in the second case we observe blazars.