“Echo” of light augurs new era in study of black holes

The findings open up new opportunities for scientists trying to map and understand what happens on the brink of “active galactic nuclei”, or AGNs; monster black holes that exist at the heart of most big galaxies.

These black holes contain millions of times the sun’s mass. As matter streams towards them, the centre of the galaxy lights up, emitting billions of times more energy than the sun and illuminating the disk of matter that forms at the hole’s edge.

One of the most important tools for astronomers studying these black holes is something called the “iron K line”. This is a shape which appears as very distinctive X-rays created by iron atoms, which, when excited, emit energies of around 6,000 to 7,000 electron volts – several thousand times the energy in visible light.

The line brightens as the result of a mysterious and intense X-ray source near to the black hole. This source shines on to the accumulated matter, causing the iron atoms to radiate their K-line energy. In effect, when the source flares, a light “echo” sweeps across the disk of matter and the iron K line lights up accordingly, after a delay corresponding to how long the X-rays took to reach the disk. The process is called relativistic reverberation.

Although observing this process carries the promise of a much better understanding of what is happening around supersized black holes, neither the European Space Agency, nor NASA, have telescopes powerful enough to spot the reverberations of single flares coming from the source.

To get round this problem, the researchers, from the Universities of Maryland and Cambridge, reasoned that it might be possible to detect the combined echoes from several flares, if a large amount of data from a particular object in space could be analysed.

This object turned out to be the galaxy NGC 4151, which is located about 45 million light-years away in the constellation Canes Venatici. The galaxy has one of the brightest AGNs in X-rays and astronomers think that it is powered by a black hole weighing 50 million solar masses. The sheer scale of this black hole suggests that it also has a large accretion disk of matter, capable of producing particularly long-lived and detectable echoes of light coming from the X-ray source.

The data needed to identify the light echoes came from observations carried out using the European Space Agency’s XMM-Newton satellite. By analysing the data, the researchers were able to uncover numerous X-ray echoes, demonstrating the reality of relativistic reverberation for the first time. Their findings are published in the May 8 issue of the Monthly Notices of the Royal Astronomical Society.

The study shows that the echoes lagged behind the original flares by a little more than 30 minutes. Moving at the speed of light, the X-rays associated with the echo must have travelled an additional 400 million miles – about four times the Earth’s average distance from the sun, to reach the accretion disk.

“This tells us that the mysterious X-ray source in the AGN hovers at some height above the accretion disk,” Chris Reynolds, a professor of astronomy at the University of Maryland at College Park and a co-author on the study, said. Jets of accelerated particles are often found around AGNs, and the finding appears to endorse an idea which has already been postulated that whatever the X-ray source is, it may be located near to the bases of these jets.

“The data show that the earliest echo comes from the most broadened iron line emission,” Andy Fabian, from the Institute of Astronomy at the University of Cambridge, and another co-author, added. “This line originates from closest to the black hole, and fits well with what we were expecting.”

The detection of X-ray echoes in the AGN opens up a new way of studying black holes and the disks of matter that accrete around them. Astronomers anticipate that the next generation of X-ray telescopes will have collecting areas large enough to detect the echo produced by a single flare from the X-ray source, giving them an even better tool for testing relativity and probing the immediate surroundings of massive black holes.

“Our analysis allows us to probe black holes through a different window,” Abderahmen Zoghbi, a postdoctoral research associate at the University of Maryland at College Park and the report’s lead author, said. “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.”