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Magnetohydrodynamics of turbulent accretion discs around black holes Pudritz, Ralph Egon

Abstract

The Cyg X-1 X-ray source is believed to be comprised of an accretion disc around a central black hole. We apply the methods of Mean Field Electrodynamics to the study of magnetic processes in such an accretion disc. By decomposing the magnetic field in the disc into mean and fluctuating components, the observed X-ray properties of this system may be accounted for. It is found that intense, short lived magnetic fluctuations may occur which give rise to solar-like flares on the surfaces of the accretion disc. The energy releases and time scales of such flares is found to provide a physical basis for the observed shot-noise like character of the X-ray emission from the system. It is demonstrated that a rather strong, large scale magnetic field can be generated by turbulent dynamo action in the accretion disc. This result is the reason why magnetic fields may play a vital role in these systems. The long time averaged structure of the accretion disc is determined by the Maxwell-stress due to the mean field, and is in agreement with the "standard" cool accretion disc models. We prove that on intermediate time and length scales, the Maxwell stresses due to the magnetic fluctuations remove the known instability of "standard" accretion disc models to ring- like "clumping" and subsequent heating of the gas. This result shows that the hard X-ray emission of the Cyg X-1 source must arise from either a hot corona, or intense solar-type flares above the disc surfaces. If the hard X-ray emission arises from non-thermal electron populations accelerated in the flares, it is found that this emission must occur in a rapid "flash-phase" on submillisecond time scales. These flares occur well away from the inner disc boundaries so that we believe that submillisecond variations of the Cyg X-1 source need not be a test of the rotation of the central black hole.

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