Chandra and XMM-Newton: Direct measurement of distant black hole's spin

Multiple images of a distant quasar known as RX J1131-1231 are visible in this combined view from Chandra (pink) and Hubble (red, green, and blue). 

Credit: NASA/CXC/Univ of Michigan/R.C.Reis et al; Optical: NASA/STSc

Astronomers have used NASA's Chandra X-ray Observatory and the European Space Agency's (ESA) XMM-Newton to show a supermassive black hole six billion light years from Earth is spinning extremely rapidly.

This first direct measurement of the spin of such a distant black hole is an important advance for understanding how black holes grow over time.

Chandra X-ray Observatory

Black holes are defined by just two simple characteristics: mass and spin.

While astronomers have long been able to measure black hole masses very effectively, determining their spins has been much more difficult.

In the past decade, astronomers have devised ways of estimating spins for black holes at distances greater than several billion light-years away, meaning we see the region around black holes as they were billions of years ago.

However, determining the spins of these remote black holes involves several steps that rely on one another.

Rubens Reis

"We want to be able to cut out the middle man, so to speak, of determining the spins of black holes across the universe," said Rubens Reis of the University of Michigan in Ann Arbor, who led a paper describing this result that was published online Wednesday in the journal Nature.

Reis and his colleagues determined the spin of the supermassive black hole that is pulling in surrounding gas, producing an extremely luminous quasar known as RX J1131-1231 (RX J1131 for short).

ESA XMM-Newton

Because of fortuitous alignment, the distortion of space-time by the gravitational field of a giant elliptical galaxy along the line of sight to the quasar acts as a gravitational lens that magnifies the light from the quasar.

Gravitational lensing, first predicted by Einstein, offers a rare opportunity to study the innermost region in distant quasars by acting as a natural telescope and magnifying the light from these sources.

Mark Reynolds

"Because of this gravitational lens, we were able to get very detailed information on the X-ray spectrum – that is, the amount of X-rays seen at different energies – from RX J1131," said co-author Mark Reynolds also of Michigan.

"This in turn allowed us to get a very accurate value for how fast the black hole is spinning."

The X-rays are produced when a swirling accretion disk of gas and dust that surrounds the black hole creates a multimillion-degree cloud, or corona near the black hole.

X-rays from this corona reflect off the inner edge of the accretion disk.

The strong gravitational forces near the black hole alter the reflected X-ray spectrum. The larger the change in the spectrum, the closer the inner edge of the disk must be to the black hole.

More information: Paper: dx.doi.org/10.1038/nature13031