Blazars and Active Galaxies
GLAST will study a wide variety of astronomical objects and phenomena, but according to GLAST Project Scientist Steve Ritz of NASA's Goddard Space Flight Center in Greenbelt, Md., "Active galactic nuclei will be GLAST's bread and butter. There are guaranteed results."
Active galactic nuclei, or AGN for short, are galaxies with extraordinarily luminous cores powered by black holes containing millions or even billions of times more material than our Sun. As gas is trapped by a monster black hole's gravity, it settles into an accretion disk and starts to spiral down the Universe's ultimate drain. Before the gas crosses the black hole's outer boundary (the event horizon) — beyond which nothing can escape — the material generates a vast outpouring of electromagnetic radiation. In the most luminous AGN, the visible light exceeds the combined output of an entire galaxy's worth of stars, even though the light-emitting area is only about the size of our solar system.
Even more amazing, radio, optical, and X-ray telescopes have resolved jets shooting away from galactic cores in opposite directions. The material in these jets can rip across space at more than 99% the speed of light, and some jets remain tightly collimated for hundreds of thousands of light-years. When a jet points almost directly toward Earth, the material can appear to be moving faster than the speed of light. This superluminal motion is an illusion caused by the geometry of a source moving at high speed that is nearly but not perfectly head-on.
But despite the staggering scale and speed of these jets, astronomers haven't been able to answer the most basic questions about them, such as how matter is accelerated to within a whisker of the speed of light. "We don’t know what the jets are made of or how they are produced. It is one of the biggest unsolved mysteries of astrophysics. But jets are the link between the activity of the supermassive black hole and the AGN's surrounding environment in intergalactic space," says Peter Michelson of Stanford University in California, who is the Principal Investigator of GLAST's primary science instrument: the Large Area Telescope (LAT).
The LAT will probably detect gamma rays from different types of AGN, such as radio galaxies, Seyfert galaxies, quasars, and blazars. But the biggest contribution may come from blazars, which are thought to be AGN whose black holes aim their jets almost directly at Earth. Whereas the Energetic Gamma-Ray Experiment Telescope (EGRET) on NASA's Compton Gamma-ray Observatory identified 66 blazars during the mission, GLAST should see thousands. By studying the energy spectra and variability of gamma rays and other wavelengths of light coming from blazars, the LAT instrument should be able to determine the composition of the jets, establishing whether they are dominated by electrons and positrons (the antimatter counterpart of electrons), or by protons.
In this radio image, two jets shoot out of the center of active galaxy Cygnus A. GLAST may solve the mystery of how these jets are produced and what they are made of. Credit: NRAO
+ High resolution image
"When GLAST detects a blazar, it is monitoring violent activity from a black hole taking place in the distant past," says GLAST Interdisciplinary Scientist Charles Dermer of the Naval Research Laboratory in Washington, D.C. "Understanding gamma rays from these sources is a form of black hole archeology that reveals the high-energy history of our Universe."
The LAT may also detect AGN that do not produce jets, or whose jets are not aimed directly at Earth. EGRET saw hints of gamma rays from at least two radio galaxies. The High Energy Stereoscopic System (H.E.S.S.), an array of four telescopes currently operating in Namibia, has discovered that gamma rays are coming from the giant elliptical galaxy M87, whose jets do not point toward Earth. These gamma-ray photons may originate from a region of the accretion disk very near the central black hole. By observing these and other galaxies, the LAT should provide precious insights into the mechanism that powers AGN activity.
Moreover, the LAT will investigate a curious discrepancy between EGRET and results from several ground-based observatories, including the Whipple Observatory in Arizona. EGRET detected low-energy gamma rays from blazars, whereas Whipple has discovered high-energy TeV-level gamma rays. "Ground- and space-based telescopes have detected blazars, but there is almost no overlap in the blazars they detect," notes GLAST Deputy Project Scientist Julie McEnery of NASA Goddard. "Clearly, each type of telescope is seeing a different type of object."
With its ability to survey the entire sky every three hours, the LAT will undoubtedly catch many AGN giving off giant flares of energy, and this flaring is one of the most important tools for studying AGN. Blazars in particular are extremely variable at all wavelengths, changing both their total energy output and spectra on timescales ranging from less than an hour to many years. The relationship of variability at different wavelengths is a crucial test for models attempting to explain these outbursts and to identify the nature of the jet particles. Obtaining measurements across the spectrum is challenging, especially on short timescales, so GLAST team members will communicate with other astronomers, who can point various ground- and space-based telescopes at flaring blazars.
by Robert Naeye