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MIPT Calculated How Black Hole Brakes its Rotation by Emitting Jets
February 15, 2018 19:15


 The Laboratory for Research of Relativistic Objects of the Universe at the Moscow Institute of Physics and Technology has developed a model to test one of the hypotheses describing the interaction of supermassive black holes in the centers of galaxies with the jets emitted by them. From the measured magnetic field of the jet, experts can now predict the rotational energy loss of the black hole emitting it. The study was published in the journal Frontiers in Astronomy and Space Science.

Today astrophysics can observe hundreds of jets - powerful streams going out from the nuclei of active galaxies, supermassive black holes, with velocities close to the light speed (relativistic). Their dimensions are huge even against the background of other astronomical objects - the length of the jet can reach several percent of the radius of the galaxy, and be around 300,000 times larger than the size of the black hole emitting it. These objects give experts a chance to look into the bygone past of the universe. However, scientists still have a lot of questions as per their design. It is not yet even known for sure what the jets consist of, since they do not record any spectral lines. It is commonly believed today that they consist of electrons and positrons or protons. Rotating black holes with an accretion disk (the matter moving around the central body) that produce jets, are considered to be the most efficient engines. The efficiency of jets, which are ejected by the active nuclei of galaxies, sometimes exceeds 100 per cent. It turns out that the process of slowing down the rotation of a black hole contributes to the energy of the jet. That is, by emitting the jet, the black hole slightly slows down its rotation.

Thus, the jet enables the black hole to get rid of the excess torque that it receives from the accreted matter rotating at high speed.

More recently, astrophysicists have developed a method by which it is possible to establish the magnetic field in jets of active galactic nuclei. In her work, astrophysicist Elena Nokhrina showed that by using this method one can assess the presence of the energy contribution from the slowing down of the rotation of the black hole into the total power of the jet. Until now, the formula, which gives an answer to the question of whether the flow of a black hole’s rotation energy into the jet energy has occurred, has not been verified on the data obtained from observations. At the same time, an important parameter that determines the rate of loss of the rotational energy of a black hole - its rotation speed - cannot yet be reliably assessed with observations. A black hole cannot have its own magnetic field. But a vertical magnetic field is created around it; it is associated with the magnetic field of the substance of the accretion disk. To estimate the loss of a black hole’s rotational energy, it is necessary to determine the magnitude of the flux of the magnetic field passing through the black hole horizon.

The calculations performed indicate a correlation between the total thickness of the jet emitted by the black hole and the losses of the rotational energy of the black hole. The amount of energy transferred by the jet, according to the hypothesis considered, depends on the flux of the magnetic field and the rotation speed of the black hole. Thus, it is possible to estimate the contribution of a black hole’s rotational energy losses to the power of the jet.

A remarkable result of this theoretical work is the possibility to estimate a black hole’s loss of rotational energy by measuring the magnetic field in the jet, without information on the rotation speed of the black hole.

The research work was supported by Russian Science Foundation.




Author: Vera Ivanova

Tags: Astrophysics     

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