Astronomers using data from the European Space Agency's XMM-Newton satellite have found a long-sought X-ray signal from NGC 4151, a galaxy that contains a supermassive black hole. The discovery promises a new way to unravel what's happening in the neighborhood of these powerful objects. Most big galaxies host a big central black hole containing millions of times the sun's mass. When matter streams toward one of these supermassive black holes, the galaxy's center lights up, emitting billions of times more energy than the sun. For years, astronomers have been monitoring such "active galactic nuclei" (AGN) to better understand what happens on the brink of a monster black hole.
Matter falling toward a black hole collects into a rotating disk, where it becomes compressed and heated before eventually spilling over the black hole's event horizon, the point beyond which nothing can escape and astronomers cannot observe. A mysterious and intense X-ray source near the black hole shines onto the surface layers of the accretion disk, causing iron atoms to radiate characteristic emission -- what astronomers call the iron K line -- at about 6,000 to 7,000 electron volts. The inner part of the disk is orbiting the black hole so fast that the effects of Einstein's relativity come into play -- most notably, how time slows down close to the black hole. These relativistic effects broaden and distort the X-ray signal in a unique way. When the X-ray source near NGC 4151's black hole flares up, the accretion disk reflects the emission about half an hour later. Moving at the speed of light, the X-rays associated with the echo must have traveled an additional 400 million miles -- equivalent to about four times Earth's average distance from the sun -- than those that came to us directly from the flare.
Since 2000, XMM-Newton has observed NGC 4151 -- among the brightest AGN in X-rays -- with an accumulated exposure of about four days. By analyzing all of this data, a team led by Abderahmen Zoghbi at the University of Maryland at College Park uncovered numerous X-ray echoes, demonstrating for the first time that what was expected from theory really occurred in nature. Amazingly, the extreme environment at the heart of NGC 4151 is built on a scale comparable to our own solar system. If we replaced the sun with the black hole, the event horizon would extend less than halfway to Earth if the black hole spins rapidly; slower spin would result in a larger horizon. The X-ray source would hover above the black hole and its accretion disk at a distance similar to that between the sun and the middle of the asteroid belt.
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