Viscous Transonic Accretion Flows in Kerr Black Hole Geometry

TL;DR

This paper investigates how viscosity influences transonic accretion flows around Kerr black holes, affecting shock formation, flow structure, and potential observational signatures like QPOs, using a pseudo-Kerr formalism for better modeling.

Contribution

It introduces a pseudo-Kerr formalism to analyze viscous transonic accretion flows, detailing how viscosity impacts shock formation, flow topology, and observational features around spinning black holes.

Findings

Shocks become weaker and are located farther from the black hole with increased viscosity.

Beyond a critical viscosity, shocks do not form, and flow remains subsonic and Keplerian.

QPO frequencies depend on black hole spin and shock locations.

Abstract

We study viscous transonic accretion flows in vertical equilibrium in Kerr geometry. We employ the pseudo-Kerr formalism which accurately describes transonic flows around Kerr black holes and is applicable for modelling observational data. We study the effects of viscosity on the nature of sonic points and the parameter space that allows an accretion flow to possess multiple sonic points. We concentrate on the accretion solutions that can have centrifugal pressure supported shock waves and find that the shocks are weaker and are located farther from the black hole as the viscosity is enhanced. Moreover, if the viscosity is greater than a critical value, shocks do not form and the accretion flow can pass only through the inner sonic point close to the black hole and remains subsonic and Keplerian throughout the accretion disk. Since the resonance oscillation frequencies of the shock waves provide a measure of the observed Quasi Periodic Oscillation (QPO) frequencies, and since the location of shock waves depend on the spin of a black hole, it is clear that the QPO frequencies must depend on the spin of black hole as well. Our pseudo-Kerr approach makes it easier to compute spectra from an accretion flow with viscous dissipation and radiative cooling around a spinning black hole.