Blazars and Active Galactic Nuclei

Nature has provided us with spectacular particle accelerators called active galactic nuclei, or AGN. These are tremendously large black holes at the centers of galaxies, some of which are known to be a million times heavier than our sun. These distant celestial laboratories allow researchers to study physics at energies orders of magnitude greater than what can be generated by the most powerful man-made accelerators on Earth. Many—if not all—galaxies contain supermassive black holes in their centers; there is even a prominent one in the heart of our own Milky Way. So what makes a supermassive black hole an AGN?

Any black hole can grow by accreting, or swallowing, galactic gas in its vicinity. Accrete enough gas and a black hole can reach a mass equivalent to millions or even a billion times the mass of our Sun. Before such gas falls into the black hole it heats up and can generate a vast amount of electromagnetic radiation. This is an AGN. The most luminous AGNs can put out vastly more light than what's produced by all ordinary stars in the entire galaxy—even though the AGN's light is generated in a region comparable in size to our Solar system.

The accretion of material onto the black hole often leads to the formation of a collimated jet of plasma that travels outward along the axis of the accretion disk, although the formation, internal structure, and evolution of collimated jets is still not fully understood. Such jets produce additional radiation, usually seen as intense radio waves, X-rays, and gamma-rays. 

The majority of the gamma-ray emissions from an AGN are tightly focused along the jet axis, and when the jet points toward Earth, the AGN is called a blazar. At peak emission, observing a blazar feels a little like looking down the barrel of the most powerful gun in the universe. (In some cases, the material exiting a blazar appears to be moving faster than the speed of light, but this is an illusion caused by the geometry of a high-speed source that is not oriented exactly head-on with the observer.)

The light generated by a blazar has other unique properties, including an intensity that can vary dramatically with time. Blazars are strongly variable in all observable bands of the electromagnetic spectrum, so simultaneous observations with instruments sensitive to different wavelengths of radiation are critical for studying them.

But KIPAC scientists often focus on gamma rays; gamma-ray studies are crucial for understanding the jet energetics as well as the relation of the jet to other constituents of the nucleus such as the black hole and the accretion disk, as well as the contributions blazars have made to the evolution of the Universe as a whole.

For example, blazars comprise a significant fraction of the diffuse extragalactic gamma-ray background (EGB), an isotropic component of the gamma-ray sky. The diffuse X-ray background is known to be entirely made up of unresolved astrophysical sources, and in particular AGNs. However, by extrapolating the number density of the AGN population to undetectably faint fluxes, scientists at KIPAC have shown that their contribution to the gamma-ray background is limited to just 30 percent. This means that other gamma-ray sources must contribute to the EGB. But its brightness suggests that these mysterious sources, too faint to be detected individually, must be as numerous as normal galaxies, which are thought to contribute an additional 15-20 percent of the total diffuse flux.

This research uses data from the Fermi Gamma-ray Space Telescope's main instrument, the Large Area Telescope (LAT), which is one of the most efficient instruments ever designed to probe the Universe for blazars and other gamma-ray sources. It provides the Stanford community with streams of new data in the effort to understand the physics of these rare and fascinating cosmological objects.