Variability of black hole accretion discs: effects of local and global inertial-acoustic oscillations

• Published in: Monthly notices of the Royal Astronomical Society 1996-12, Vol.283 (3), p.919-929
• Main Authors: Milsom, John A., Taam, Ronald E.
• Format: Article
• Language: English
• Subjects: accretion; accretion discs; black hole physics; hydrodynamics; instabilities; waves
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/283.3.919

The non-linear time-dependent evolution of inertial-acoustic oscillations in black hole accretion discs has been investigated as a function of mass accretion rate, Ṁ/ṀEdd, black hole mass, M, and viscosity, v. The results indicate qualitatively distinct behaviour in three regimes. In particular, for Ṁ ≳ 0.25ṀEdd the disc is stable to inertial-acoustic oscillations. For 0.01MEdd ≲ Ṁ ≲ 0.25 ṀEdd, a global oscillation occurs at the maximum epicyclic frequency, and local oscillations at the local epicyclic frequencies are also present. At accretion rates lower than ∼0.01 ≲ṀEdd, the disc exhibits local oscillations at all radii with no global oscillation. The disc luminosity power spectra are characterized by power-law slopes of between 0.80 and 1.95, with the flattest spectra corresponding to the lowest accretion rates. In each regime the luminosity oscillation amplitudes are largest for large viscosities (rms variations always ≲ 1 per cent), and the oscillations vanish if the viscosity is sufficiently small. Constant-viscosity models and α-viscosity models were used, and these yield qualitatively similar results. The locations of the transitions between these three regimes are approximately independent of M. Unless disc coronae or multidimensional effects reduce the amplitude of these oscillations, observations of these oscillations should be possible with the X-ray Timing Explorer (XTE).