Global oscillations of a fluid torus as a modulation mechanism for black‐hole high‐frequency QPOs

Abstract

Abstract. We study strong-gravity effects on modulation of radiation emerging from accreting compact objects as a possible mechanism for flux modulation in QPOs. We construct a toy model of an oscillating torus in the slender approximation assuming thermal bremsstrahlung for the intrinsic emissivity of the medium and we compute observed (predicted) radiation signal including contribution of indirect (higher-order) images and caustics in the Schwarzschild spacetime. We show that the simplest oscillation mode in an accretion flow, axisymmetric up-and-down motion at the meridional epicyclic frequency, may be directly observable when it occurs in the inner parts of accretion flow around black holes. Together with the second oscillation mode, an in-and-out motion at the radial epicyclic frequency, it may then be responsible for the high-frequency modulations of the X-ray flux observed at two distinct frequencies (twin HF-QPOs) in micro-quasars.

QPO as the Rosetta Stone for understanding black hole accretion

“…However, it was shown by a large scale ray tracing method that in this case, the X-ray luminosity is modulated by strong gravity effects (gravitational lensing and Doppler beaming) in photon propagation from oscillating disk to observer. It was found that the "observed" power in PDS Fourier spectra very strongly depends on lensing and beaming effects -for example, power in the vertical mode increases with inclination (Bursa 2005).…”

“…Of course, QPO appearance may also be intrinsically influenced by different physical conditions of the flow in different spectral states. This we study by a direct ray tracing from Mami Machida's accretion flow MHD 3D simulations in Schwarzschild spacetime (Bursa 2005).…”

Investigating the instability of the torus around the blackhole in the presence of the radial and angular velocity perturbations

“…Since the oscillation epicyclic frequencies are strongly related with the black hole spin and mass, the instabilities and QPOs could be used to constrain the black hole properties [1,3,4]. On the other hand, we might also deduce the properties of the strong general relativistic region and it will give an opportunity to compare the various theories of the gravity [5][6][7][8].…”

Deciphering signatures of Bardeen black holes from the observed quasi-periodic oscillations

“…These models include non-geodesic effects in the accretion physics originating from the pressure forces by modelling the accretion flow in terms of a slightly non-slender pressure-supported perfect fluid torus[50,87,88]. There exist two main variants of non-axisymmetric disk-oscillation models: the first (denoted by NADO1 or Vertical Precession Resonance Model[84][85][86]) considers resonance between the vertical epicyclic frequency (f 1 = ν θ ) with m = −1 non-axisymmetric radial epicyclic frequency (f 2 = ν φ − ν r ) while the second (denoted by NADO2) assumes coupling between the m = −2 non-axisymmetric vertical epicyclic frequency (f 1 = 2ν φ − ν θ )[48][49][50] with the m = −1 nonaxisymmetric radial epicyclic frequency (f 2 = ν φ − ν r ). Here m refers to the azimuthal wave number of the non-axisymmetric perturbation.…”