Measuring a Black Hole’s Spin

NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR) and the European Space Agency’s XMM-Newton are two X-ray space telescopes that have teamed up to measure the spin of a 2 million solar mass supermassive black hole at the centre of the galaxy NGC1365. The observations were conducted simultaneously on July 2012 and provided the first ever definitive measurement of a supermassive black hole’s spin. In the region near the supermassive black hole in NGC1365, there is a pair of bipolar jets consisting of energetic particles that travel at very close to the speed of light. At the base of each jet, X-rays is produced which reflects off the surrounding accretion disk and makes the accretion disk observable in X-rays. By measuring how fast matter is swirling around the black hole in the inner region of the accretion disk, the spin rate of the black hole was determined.

Figure 1: This artist’s concept illustrates a supermassive black hole with millions to billions times the mass of our sun. Supermassive black holes are enormously dense objects buried at the hearts of galaxies. Image credit: NASA/JPL-Caltech

The gravitational radius or the event horizon of a black hole is a region of space around a black hole where the gravity is sufficiently strong to prevent even light from escaping. This region can be seen as a “point of no return” for any object that enters. The innermost stable circular orbit around a black hole defines the inner edge of the black hole’s accretion disk. How close the inner edge of an accretion disk can get to a black hole’s gravitational radius depends on the black hole’s spin rate. For a black hole with a faster spin rate, the inner edge of its accretion disk can exist closer to its gravitational radius.

The incredible spin of the supermassive black hole in NGC1365 allows the innermost edge of the accretion disk to exist within just 2.5 gravitational radii. Being so close to the black hole’s gravitational radius, the intense gravity warps the fabric of space-time and distorts the X-ray emission from the accretion disk. This distortion is observable and is what allows the spin rate of the supermassive black hole to be measured. From the observed distortion in X-ray emission, the spin of the supermassive black hole in NGC1365 is estimated to be 84 percent as fast as Einstein’s theory of general relativity would allow. Although the spin of a black hole does not translate effectively into units of speed like kilometres per hour, it is safe to say that this supermassive black hole is spinning incredibly rapidly and twisting the fabric of space-time around it.

Figure 2: An illustration showing how the spin and intense gravity of a black hole warps the fabric of space-time and distorts the X-ray emission from the accretion disk. Image credit: NASA/JPL-Caltech

Supermassive black holes are known to acquire most of their rotation as they grow and studying how they spin allows their growth and evolution to be better constrained. The ultra-fast spin of the supermassive black hole in NGC1365 shows that it did not grow from the capture of smaller black holes in randomly-oriented orbits as this is very unlikely to have spun up the black hole in the same direction. Instead, the supermassive black hole is likely to have acquired its ultra-fast spin from a merger event with a comparable mass black hole or the accretion of material from a disk around it. Both scenarios would tend to spin up the black hole in the same direction.

References:

G. Risaliti et al., “A rapidly spinning supermassive black hole at the centre of NGC 1365”, Nature 28 February 2013; doi:10.1038/nature11938

Volonteri, M., Madau, P., Quataert, E. & Rees, M. J., “The distribution and cosmic evolution of massive black hole spins”, Astrophys. J. 620, 69–77 (2005)