Corotational Instability, Magnetic Resonances and Global Inertial-Acoustic Oscillations in Magnetized Black-Hole Accretion Discs - Astrophysics > High Energy Astrophysical PhenomenaReportar como inadecuado




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Abstract: Low-order, non-axisymmetric p-modes also referred as inertial-acousticmodes trapped in the inner-most region of hydrodynamic accretion discs aroundblack holes, are plausible candidates for high-frequency quasi-periodicoscillations QPOs observed in a number of accreting black-hole systems. Thesemodes are subject to global instabilities due to wave absorption at thecorotation resonance where the wave pattern frequency $\omega-m$ equals thedisc rotation rate $\Omega$, when the fluid vortensity,$\zeta=\kappa^2-2\Omega\Sigma$ where $\kappa$ and $\Sigma$ are the radialepicyclic frequency and disc surface density, respectively, has a positivegradient. We investigate the effects of disc magnetic fields on the waveabsorption at corotation and the related wave super-reflection of thecorotation barrier, and on the overstability of disc p-modes. For discs with apure toroidal field, the corotation resonance is split into two magneticresonances, where the wave frequency in the corotating frame of the fluid,$\tomega=\omega-m\Omega$, matches the slow magnetosonic wave frequency.Significant wave energy-angular momentum absorption occurs at both magneticresonances, but with opposite signs. The combined effect of the two magneticresonances is to reduce the super-reflection and the growth rate of theoverstable p-modes. We show that even a subthermal toroidal field may suppressthe overstability of hydrodynamic B=0 p-modes. For accretion discs with mixedtoroidal and vertical magnetic fields, two additional Alfven resonancesappear, where $\tomega$ matches the local Alfven wave frequency. They furtherreduce the growth rate of p-modes. Our results suggest that in order for thenon-axisymmetric p-modes to be a viable candidate for the observedhigh-frequency QPOs, the disc magnetic field must be appreciably subthermal, orother mode excitation mechanisms are at work.



Autor: Wen Fu, Dong Lai

Fuente: https://arxiv.org/







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