ESA XMM-Newton: X-ray satellite's breakthrough in mapping black holes

A veteran space telescope observing with X-ray eyes has made a breakthrough in helping to understand the supermassive black holes at the centre of galaxies.

Astronomers using the European Space Agency's (ESA) XMM-Newton satellite identified an "echo" within the galaxy NGC 4151, 45 million light-years away in the constellation of Canes Venatici, that they had long been searching for.

Galaxies such as NGC 4151 are known as active galaxies because they have brilliant centres which emit billions of times more energy than the Sun. These are lit up by material streaming in to a spinning monster black hole containing the mass of millions of stars.

A signature of these cosmic cannibals is a broad line in the X-ray region of the spectrum caused by excited and highly energetic iron atoms. This radiation is produced when a mystery X-ray source shines onto the whirling disk of matter spilling into the black hole, known as the accretion disk.

Astronomers know that these X-ray sources flare and they predicted that when this happened the broad iron line would brighten too after an interval depending on how long the X-rays took to light up the accretion disk in that strange physical environment.

Sadly these echoes of the flares were each too weak for XMM-Newton or NASA's powerful X-ray telescope Chandra to see individually. But an international team led by Dr Abderahmen Zoghbi, of the University of Maryland in the US, realised that if they could combine the echoes from several flares, then these might then become detectable.

They tested their theory using data from NGC 4151 which has an active nucleus powered by a black hole 50 million times the mass of the Sun. The galaxy's accretion disk was thought to be capable of producing more easily detectable echoes that would reverberate for a particularly long time.

An artist's impression of XMM-Newton in orbit. Credit: ESA

XMM-Newton has observed the galaxy extensively since the year 2000, building up the X-ray equivalent of a four-day photographic exposure. This has revealed numerous echoes, proving the astronomers' technique.

It allowed them to calculate that the X-ray source must lie around 640,000 km (400 million miles) above the supermassive black hole's accretion disk. Despite the black hole's incredible mass, that event horizon lies only half as far from it as the Earth's distance from the Sun. The team's work is published in the Monthly Notices of the Royal Astronomical Society.

Dr Zoghbi said: "Our analysis confirms some long-held ideas about active galactic nuclei and gives us a sense of what we can expect when a new generation of space-based X-ray telescopes eventually becomes available."

NASA's newest X-ray telescope, NuSTAR, is due to launch later this month from the Kwajalein Atoll in the central Pacific Ocean and is expected to study black holes in greater detail than ever.