Ever since they were discovered in 1962, quasars have intrigued astronomers for being the size of solar systems, but brighter than a hundred galaxies. Astronomers have theorized that only supermassive black holes, lurking at their center, are dense and powerful enough to keep such relatively small objects glowing as brightly as they do, but solid evidence was lacking. This week, however, astronomers presented the strongest evidence yet that black holes do indeed power quasars.
Because quasars lie billions of light-years from Earth, even the most advanced telescopes have not been able to probe them in much detail. Theorists think they are fountains of radiation streaming from disks of matter that heat up to many millions of degrees as they spiral in toward black holes, and preliminary data have supported this assumption. Astronomers Xinyu Dai and Christopher Kochanek, both of Ohio State University in Columbus, thought that a principle known as gravitational lensing might help shed more light on quasars. Gravitational lensing occurs when massive objects such as galaxies bend and focus incoming light from more distant quasars, acting like giant magnifying glasses and making it easier for astronomers to study them.
In a session yesterday at the American Astronomical Society meeting in San Francisco, California, Xinyu described how he and Kochanek used gravitational lensing to detect the presence of supermassive black holes by comparing both optical and x-ray images of two quasars. Over months, they gathered data for an hour a night first using SMARTS, the Small and Medium Aperture Research Telescope System, and then from the orbiting Chandra X-ray Observatory accessed at the Harvard Smithsonian Center for Astrophysics in Cambridge, Massachusetts. The combined data revealed unmistakable signatures of supermassives at work. The most energetic wavelengths of radiation appeared closest to the center of the quasars--the place where, if the models were correct, matter was being ripped apart most violently.
"We all fully expected that the source of the x-ray light comes from closer to the black hole, while the source of the optical light comes from much farther out, and that's what they see quite nicely in their lensing result," says astrophysicist Chris Reynolds of the University of Maryland, College Park. Reynolds called the technique used by Xinyu and Kochanek clever because of the way it allows a different view inside the super bright objects. "We can imagine making maps of where the light comes from around the black holes powering quasars, and this is the first step down that road."
For now, even a glimpse will give astronomers a big boost in understanding what is going on. "There are many models that try to describe what's happening inside a quasar," Xinyu says. "Before, none of them could be ruled out--now some of them can."
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