Black hole Discovered to be spinning Close to the relativistic limit

A composite X-ray image of the galaxy NGC1365 taken by NuSTAR and XMM-Newton.

Courtesy: Guido Risaliti

The best evidence yet that some supermassive black holes (SMBH) rotate at extremely high rates has been found by an international team of astronomers.

Made using the recently launched NuStar space telescope, the study suggests that a huge black hole at the centre of a distant galaxy acquired a huge amount of rotational energy as it formed.

The discovery could provide important information about how SMBHs and their associated galaxies form and evolve.

Astronomers know that black holes that are as large as a billion solar masses can be found at the heart of most galaxies.

Because these gravitational behemoths are created at the same time as their host galaxies, understanding how they formed could provide important information about galaxy formation and evolution.

Knowing the spin of an SMBH can provide important clues about how it formed. If the black hole grew slowly, by sucking in small amounts of matter from all directions, then it isn't expected to have much spin.

However, if the formation process involves the black hole gorging rapidly on matter from a specific direction, conservation of angular momentum would leave it with an extremely large spin.

Redshifted X-rays
The spin of a supermassive black hole can be measured by looking at the effect that the spin has on material that is being sucked in to the black hole.

This material forms an accretion disc that swirls around the black hole before disappearing from sight. The faster the black hole is spinning, the closer the inner edge of the disc is to the centre of the black hole.

As a result, the X-rays emanating from the inner edge are affected by the black hole's gravity more when the black hole is spinning.

Astronomers see this as a "stretching" of the wavelength (redshift) of characteristic X-rays emanating from iron and other elements in the accretion disc. By measuring the redshift, the spin of the black hole can be deduced.

The problem, however, is that these X-rays must first travel through fast-moving clouds of gas that surround the accretion disc.

The absorption of X-rays by the gas could mimic the effect of a spinning black hole. As a result, astronomers have not been that confident about their estimates of black-hole spin.