An international team led by the University of Southampton have been observing a rapidly spinning black hole rotating around its axis at its near maximum rate in our galaxy. It is hoped that the study will shed more light on the characteristics of black holes and their surrounding environment.
One of the most exciting predictions of Einstein’s theory of General Relativity is that a spinning black hole–known as a Kerr black hole– should drag space around with it–a phenomenon known as frame dragging. This behaviour is not seen in non-rotation black holes.
These effects should be visible in the shape of the radiation from the material rotating very close to the black hole before disappearing. Therefore, if the change in shape of the emitting spectra can be determined somehow, then the equations of general relativity can be used to measure the black hole spin.
The study, published in the Astrophysical Journal, is significant because by using state-of-the-art technology, the team of researchers found evidence that a stellar-mass black hole in our galaxy (known as 4U 1630-472) is rotating rapidly (at a speed of 92-95 per cent of the theoretically-allowed rotational speed) around its axis while sucking in-falling material. It is subject to gravitational stresses and temperatures so high that it begins to shine brightly in X-rays, which were seen by astronomers using telescopes.
Mayukh Pahari, from the University of Southampton and lead author, said: “Detecting signatures that allow us to measure spin is extremely difficult. The signature is embedded in the spectral information which is very specific to the rate at which matter falls into the black hole. The spectra, however, are often very complex mostly due to the radiation from the environment around the black hole.
“During our observations, we were lucky enough to obtain a spectrum directly from the radiation of the matter falling into the black hole and simple enough to measure the distortion caused by the rotating black hole.”
Black holes are created when the fusion of heavy elements ceases in massive stars–the end of star’s life’– reach the collapse under their own gravitational force. The mass is so great and concentrated that it creates a point known as the event horizon–where even light does not have enough speed to achieve escape velocity.
Black holes are characterised by very few qualities, mass, charge and angular momentum–known is astronomy as the ‘no hair theorem’. Measurement of these characteristics is vital in understanding the physics of black holes and the surrounding regions of space.
This isn’t the first time that 4U 1630-472 has been used to observe black hole behaviour. In 2014, a team of astronomers from Cornell University observed disk wind passing through the material accreting onto the black hole.
Original research: http://dx.doi.org/10.3847/1538-4357/aae53b
Disk wind study: https://arxiv.org/abs/1401.3646