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

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 hbar/4pi k_B, when hbar 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 >~ 10^9K, 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.

arXiv comment16 pages including 4 figures; version modified substantially including new figures (removing some old figures) and effects of magnetic field; to appear in Physics Letters B

References in corpus (28)

• Viscosity in Strongly Interacting Quantum Field Theories from Black Hole Physics
• X-ray Properties of Black-Hole Binaries
• Advection-Dominated Accretion: A Self-Similar Solution
• The order of the quantum chromodynamics transition predicted by the standard model of particle physics
• The QCD equation of state with dynamical quarks
• The QCD transition temperature: results with physical masses in the continuum limit
• Is there still any Tc mystery in lattice QCD? Results with physical masses in the continuum limit III
• The QCD transition temperature: results with physical masses in the continuum limit II.
• QCD Equation of State and Hadron Resonance Gas
• Nearly Perfect Fluidity: From Cold Atomic Gases to Hot Quark Gluon Plasmas
• Beyond eta/s = 1/4pi
• The Shear Viscosity to Entropy Ratio: A Status Report
• Description of Pseudo-Newtonian Potential for the Relativistic Accretion Disk around Kerr Black Holes
• The viscosity bound in string theory
• Properties of the propagating shock wave in the accretion flow around GX 339-4 in the 2010 outburst
• Two temperature accretion around rotating black holes: Description of general advective flow paradigm in presence of various cooling processes to explain low to high luminous sources
• The bound on viscosity and the generalized second law of thermodynamics
• Hysteresis effects and diagnostics of the shock formation in low angular momentum axisymmetric accretion in the Kerr metric
• The bulk kinetic power of the jets of GRS 1915+105
• Global solution of viscous accretion disk around rotating compact objects: a pseudo-general-relativistic study
• Stability of accretion disk around rotating black holes: a pseudo-general-relativistic fluid dynamical study
• Hydrodynamic simulations of viscous accretion flows around black holes
• Stability of Accretion Disks in Presence of Nucleosynthesis
• Two temperature viscous accretion flows around rotating black holes: Description of under-fed systems to ultra-luminous X-ray sources
• On the efficiency of the Blandford-Znajek mechanism for low angular momentum relativistic accretion
• Implications of a viscosity bound on black hole accretion
• Pion production in the inner disk around Cygnus X-1
• Is the energy generation rate of nuclear reactions in hot accretion flows important?

Cited by in corpus (7)

• Origin of nonlinearity and plausible turbulence by hydromagnetic transient growth in accretion disks: Faster growth rate than magnetorotational instability
• A pure hydrodynamic instability in shear flows and its application to astrophysical accretion disks
• Fate of an Accretion Disc around a Black Hole when both the Viscosity and Dark Energy is Effecting
• Violation of Universal Lower Bound for the Shear Viscosity to Entropy Density Ratio in Dark Energy Dominated Accretion
• Origin of hydrodynamic instability from noise: from laboratory flow to accretion disk
• Hydrodynamical instability with noise in the Keplerian accretion discs: Modified Landau equation
• Forced linear shear flows with rotation: rotating Couette-Poiseuille flow, its stability and astrophysical implications