Using data from ESA’s XMM-Newton satellite, astronomers have detected a long-sought X-ray echo that promises a new way to probe supermassive black holes in distant galaxies.
This artwork compares the environment around NGC 4151's supermassive black hole with the orbits of the planets in our solar system; the planets themselves are not shown to scale. Echoes of X-ray flares detected in XMM-Newton data demonstrate that the X-ray source - blue sphere, center - is located above the black hole's accretion disk (NASA's Goddard Space Flight Center)
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.
“Our analysis allows us to probe black holes through a different window. It confirms some long-held ideas about AGN and gives us a sense of what we can expect when a new generation of space-based X-ray telescopes eventually becomes available,” said Dr Abderahmen Zoghbi, a postdoctoral research associate at the University of Maryland and lead author of the study published in Monthly Notices of the Royal Astronomical Society.
One of the most important tools for astronomers studying AGN is an X-ray feature known as the broad iron line, now regarded as the signature of a rotating black hole. Excited iron atoms produce characteristic X-rays with energies around 6,000 to 7,000 electron volts — several thousand times the energy in visible light – and this emission is known as the iron K line.
The team using XMM-Newton has observed a galaxy that has one of the brightest AGN in X-rays. The galaxy known as NGC 4151 is located about 45 million light-years away in the constellation Canes Venatici. Astronomers think that the galaxy’s active nucleus is powered by a black hole weighing 50 million solar masses, which suggested the presence of a large accretion disk capable of producing especially long-lived and easily detectable echoes.
Since 2000, XMM-Newton has observed the galaxy with an accumulated exposure of about four days. By analyzing this data, the researchers uncovered numerous X-ray echoes, demonstrating for the first time the reality of relativistic reverberation.
The team found that echoes lagged behind the AGN flares by a little more than 30 minutes. 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.
“This tells us that the mysterious X-ray source in AGN hovers at some height above the accretion disk,” said co-author Prof Chris Reynolds of the University of Maryland. “Jets of accelerated particles often are associated with AGN, and this finding meshes with recent suggestions that the X-ray source may be located near the bases of these jets.”
“The data show that the earliest echo comes from the most broadened iron line emission. This originates from closest to the black hole and fits well with expectations,” added co-author Dr Andy Fabian, an astrophysicist at the University of Cambridge.
The detection of X-ray echoes in AGN provides a new way of studying black holes and their accretion disks. Astronomers envision the next generation of X-ray telescopes with collecting areas large enough to detect the echo of a single AGN flare in many different objects, thereby providing astronomers with a new tool for testing relativity and probing the immediate surroundings of massive black holes.
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Bibliographic information: Zoghbi, A., Fabian, A. C., Reynolds, C. S. and Cackett, E. M. 2012. Relativistic iron K X-ray reverberation in NGC 4151. Monthly Notices of the Royal Astronomical Society, 422: 129–134. doi: 10.1111/j.1365-2966.2012.20587.x
