Yesterday a study was published showing that a black hole in the center of NGC1365 is spinning at near the speed limit of the universe – but how did this black hole get to spin that fast in only one direction?
The supermassive black hole at the center of NGC1365 has a mass about 2 million times that of our own Sun and has been the subject of observation by both XMM Newton (dealing with relatively low energy X-Rays) and NuStar (looking at high energy X-rays) that look for the X-rays produced by the accretion disk of material spinning around the black hole. As material enters the proximity of the black hole any coherent structure like a rock is effectively broken down and its constituent particles get very,very hot due to friction with the billions of other particles.
Where the friction is highest (IE around the event horizon of the black hole) higher energy X-Rays are produced by these hot particles, and more friction is produced by a faster black hole spin rate. Normally when measuring spin rates in this way, astrophysicists look for the signature of Iron ions that are reflecting or emitting X-rays, as they produce a distinctive, easily-separable peak on an energy trace.
The study clears up an important uncertainty when measuring the rate of spin in this way. The uncertainty lies with measuring the X-rays from the region close to the event horizon, where the particle energies are highest but where a combination of the distorting effect of the black holes gravity and obscuring effect of the accretion disk itself becomes a bone of contention between scientists. One school of thought held that the gravitational distortion was the dominant source of inaccuracy, and the other that the cloud of material around the accretion disk was the principle reason for errors.
Data from the XMM Newton observatory showed that low energy X-rays from the black hole were being distorted (for an unknown reason), after which NuSTAR was turned onto the same black hole. Using NuSTAR to detect the source of higher energy iron x-ray signatures revealed that the radiation sources were in such close proximity to the black hole that it must be the gravity affecting the readings instead of the obscuring effect of the gas cloud, discounting the cloud-interference model entirely.
The study subsequently calculated the black hole to be spinning at an astonishing 84% of the speed of the light – although there’s a reasonably large margin for error in this due to the distortion effects of the black hole’s massive gravity affecting the iron signature, causing the “peak” on the trace to be spread out instead of a nice sharp point. A side effect of this study is that this distortion itself is an indicator of the speed of rotation, with faster spinning black holes causing a greater error margin (as I understand it- I am not a particle physicist).
So this black hole is spinning really, really, really fast. Why is this significant?
Analysing the movement of bodies is a subject called dynamics. Taking a cube on a table top as an example, it can move in at least six different ways – it can slide forward and back along x,y and z axes, and it can also rotate about the x,y and z axes. Lets say our cube is spinning only around the Z axis like a spinning top, in. It has a property called angular momentum around that Z axis, denoted in the image below by Theta ( O with a bar through it) Z.
Supermassive black holes start small and get larger (gain more mass) by sucking in all sorts of material- asteroids, planets, suns, star clusters, galaxies, even other black holes; all with their own angular momentums oriented in different directions.
For this supermassive black hole to be spinning at 84% of its maximum possible rate (about 85% of the speed of light) in *one* direction reveals that A) Not only is it very old, B) it must have sucked in a large amount of mass moving in roughly the same consistent direction, like multiple star clusters, or is itself a conglomeration of several black holes that have sucked in other star clusters, making it a very rare black hole indeed.
The study by Risaliti et al is published in the February 28th edition of Nature: A rapidly spinning supermassive black hole at the centre of NGC 1365.
EDIT: Thanks to comments received a clarification has to be made that the majority of sources used when writing this article were inaccurate – the black hole is rotating at 84% of the maximum speed theoretically possible for the rotation of a black hole, which is 85% of the speed of light. Therefore, this black hole is spinning at around 71.4% of the speed of light.
Image: Artists impression of a supermassive black hole with accretion disk. Image Credit: NASA/JPL.