XMM-Newton takes astronomers to a black hole's edge
Observations of iron emissions reveal that the black hole is spinning very rapidly and eating matter quickly.
Provided by ESA, Noordwijk, Netherlands
May 27, 2009
Astronomers have probed closer than ever to a supermassive black hole lying deep at the core of a distant active galaxy using new data from the European Space Agency's (ESA) XMM-Newton spaceborne observatory,. The galaxy — known as 1H0707-495 — was observed during four 48-hour-long orbits of XMM-Newton around Earth, starting in January 2008. The black hole at the galaxy's center was thought to be partially obscured from view by intervening clouds of gas and dust, but these current observations have revealed the innermost depths of the galaxy.
"We can now start to map out the region immediately around the black hole," said Andrew Fabian, of the University of Cambridge in England, who headed the observations and analysis.
X rays are produced as matter swirls into a supermassive black hole. The X rays illuminate and are reflected from the matter before its eventual accretion. Iron atoms in the flow imprint characteristic iron lines on the reflected light. The iron lines are distorted in a number of characteristic ways: by the speed of the orbiting iron atoms, by the energy required for the X rays to escape the black hole's gravitational field, and by the spin of the black hole. All these features show that the astronomers are tracking matter to within twice the radius of the black hole itself.
XMM-Newton detected two bright features of iron emission in the reflected X rays that astronomers had never seen together in an active galaxy. These bright features are known as the iron L and K lines, and they can be so bright only if there is a high abundance of iron. Seeing both in this galaxy suggests that the core is much richer in iron than the rest of the galaxy. The direct X-ray emission varies in brightness with time. During the observation, the iron L line was bright enough for XXM-Newton to follow its variations.
A painstaking statistical analysis of the data revealed a time lag of 30 seconds between changes in the X-ray light observed directly and those seen in its reflection from the disk. This delay in the echo enabled scientists to measure the size of the reflecting region, which lead to an estimate of black hole's mass to be about 3 to 5 million solar masses.
The observations of the iron lines also reveal that the black hole is spinning very rapidly and eating matter so quickly that it verges on the theoretical limit of its eating ability, swallowing the equivalent of two Earths per hour.
The team is continuing to track the galaxy using its new technique. Far from being a steady process, a feeding black hole is a messy eater. "Accretion is a very messy process because of the magnetic fields that are involved," Fabian said.
The team's new technique will enable the astronomers to map out the process in all its complexity, taking them to previously unseen regions at the very edges of this and other supermassive black holes.