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research
Corotational Instability, Magnetic Resonances and Global Inertial-Acoustic Oscillations in Magnetized Black-Hole Accretion Discs
Authors
Arras
Balbus
+67Â more
Baruteau
Blokland
Bondeson
Brandenburg
Davis
Done
Dong Lai
Feng
Ferreira
Frieman
Fromang
Fromang
Fu
Fu
Gierlinski
Goldreich
Goossens
Guan
Hameiri
Hawley
Heinemann
Hirose
Kato
Kato
Kato
Kato
Keppens
Lai
Latter
Li
Longaretti
Lovelace
Lovelace
Middleton
Middleton
Mikhailovskii
Miller
Muto
Narayan
Nowak
O 'Neill
Okazaki
Paczynski
Pessah
Pessah
Press
Remillard
Reynolds
Sakurai
Silbergleit
Simon
Sorathia
Stone
Strohmayer
Swank
Tagger
Tagger
Tassev
Terquem
Tsang
Tsang
Tsang
Varniere
Wagoner
Wen Fu
Yu
Zhang
Publication date
1 January 2010
Publisher
'Wiley'
Doi
Cite
View
on
arXiv
Abstract
Low-order, non-axisymmetric p-modes (also referred as inertial-acoustic modes) trapped in the inner-most region of hydrodynamic accretion discs around black holes, are plausible candidates for high-frequency quasi-periodic oscillations (QPOs) observed in a number of accreting black-hole systems. These modes are subject to global instabilities due to wave absorption at the corotation resonance (where the wave pattern frequency
ω
/
m
\omega/m
ω
/
m
equals the disc rotation rate
Ω
\Omega
Ω
), when the fluid vortensity,
ζ
=
κ
2
/
(
2
Ω
Σ
)
\zeta=\kappa^2/(2\Omega\Sigma)
ζ
=
κ
2
/
(
2ΩΣ
)
(where
κ
\kappa
κ
and
Σ
\Sigma
Σ
are the radial epicyclic frequency and disc surface density, respectively), has a positive gradient. We investigate the effects of disc magnetic fields on the wave absorption at corotation and the related wave super-reflection of the corotation barrier, and on the overstability of disc p-modes. For discs with a pure toroidal field, the corotation resonance is split into two magnetic resonances, 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 magnetic resonances, but with opposite signs. The combined effect of the two magnetic resonances is to reduce the super-reflection and the growth rate of the overstable p-modes. We show that even a subthermal toroidal field may suppress the overstability of hydrodynamic (B=0) p-modes. For accretion discs with mixed (toroidal and vertical) magnetic fields, two additional Alfven resonances appear, where
\tomega
matches the local Alfven wave frequency. They further reduce the growth rate of p-modes. Our results suggest that in order for the non-axisymmetric p-modes to be a viable candidate for the observed high-frequency QPOs, the disc magnetic field must be appreciably subthermal, or other mode excitation mechanisms are at work.Comment: 21 pages, 11 figures, MNRAS accepte
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