Can the viscosity in astrophysical black hole accretion disks be close to its string theory bound?

Mukhopadhyay, Banibrata (2013) Can the viscosity in astrophysical black hole accretion disks be close to its string theory bound? In: Physics Letters B, 721 (1-3). pp. 151-158.

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Abstract

String theory and gauge/gravity duality suggest the lower bound of shear viscosity (eta) to entropy density (s) for any matter to be mu h/4 pi k(B), when h and k(B) are reduced Planck and Boltzmann constants respectively and mu <= 1. Motivated by this, we explore eta/s in black hole accretion flows, in order to understand if such exotic flows could be a natural site for the lowest eta/s. Accretion flow plays an important role in black hole physics in identifying the existence of the underlying black hole. This is a rotating shear flow with insignificant molecular viscosity, which could however have a significant turbulent viscosity, generating transport, heat and hence entropy in the flow. However, in presence of strong magnetic field, magnetic stresses can help in transporting matter independent of viscosity, via celebrated Blandford-Payne mechanism. In such cases, energy and then entropy produces via Ohmic dissipation. In,addition, certain optically thin, hot, accretion flows, of temperature greater than or similar to 10(9) K, may be favourable for nuclear burning which could generate/absorb huge energy, much higher than that in a star. We find that eta/s in accretion flows appears to be close to the lower bound suggested by theory, if they are embedded by strong magnetic field or producing nuclear energy, when the source of energy is not viscous effects. A lower bound on eta/s also leads to an upper bound on the Reynolds number of the flow.

Item Type:	Journal Article
Publication:	Physics Letters B
Publisher:	Elsevier Science
Additional Information:	Copyright of this article belongs to Elsevier Science.
Keywords:	Infall; Accretion; and Accretion Disks; Gauge/String Duality; Shear Rate Dependent Viscosity; Magnetic Field; Black Holes; Relativistic Plasmas
Department/Centre:	Division of Physical & Mathematical Sciences > Physics
Date Deposited:	10 Jun 2013 08:10
Last Modified:	10 Jun 2013 08:10
URI:	http://eprints.iisc.ac.in/id/eprint/46651

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