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,” said Abderahmen Zoghbi from the University of Maryland, College Park. “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.”
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.
Matter falling toward a black hole collects into a rotating accretion 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 disk’s surface layers, causing iron atoms to radiate K-line emission. The inner part of the disk is orbiting the black hole so rapidly that the effects of Einstein’s relativity come into play — most notably, how time slows down close to the black hole. These relativistic effects skew or broaden the signal in a distinctive way.
Astronomers predicted that when the X-ray source near the black hole flared, the broad iron K line would brighten after a delay corresponding to how long the X-rays took to reach and illuminate the accretion disk. Astronomers call the process relativistic reverberation. With each flare from the X-ray source, a light echo sweeps across the disk and the iron line brightens accordingly.
Unfortunately, neither ESA’s XMM-Newton satellite nor NASA’s Chandra X-ray Observatory possess telescopes powerful enough to spot reverberations from individual flares.
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 (540 million kilometers) — 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 Chris Reynolds from 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,” said Andy Fabian from the University of Cambridge in England.
Amazingly, the extreme environment at the heart of NGC 4151 is built on a scale comparable to our 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; a 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.
“Teasing out the echo of X-ray light in NGC 4151 is a remarkable achievement. This work propels the science of AGN into a fundamental new area of mapping the neighborhoods of supermassive black holes,” said Kimberly Weaver from NASA’s Goddard Space Flight Center in Greenbelt, Maryland. NASA Goddard hosts the XMM-Newton Guest Observer Facility, which supports U.S. astronomers who request observing time on the satellite.
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.