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Register a new userQuasi-periodic oscillations from Rossby Wave Instability
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Added by
Frederic H. Vincent
Publication date
2013
fields
Physics
and research's language is
English
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We study the Rossby wave instability model of high-frequency quasi-periodic oscillations (QPO) of microquasars. We show ray-traced light curves of QPO within this model and discuss perspectives of distinguishing alternative QPO models with the future Large Observatory For X-ray Timing (LOFT) observations.
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The rather elusive High-Frequency Quasi-Periodic Oscillations(HFQPO) observed in the X-ray lightcurve of black holes have been seen in a wide range of frequencies, even within one source. It is also notable to have been detected in pairs of HFQPOs with a close to integer ratio between the frequencies. The aim of this paper is to investigate some of the possible observable that we could obtain from having the Rossby Wave Instability (RWI) active in the accretion disc surrounding the compact object. Using the newly developed GR-AMRVAC code able to follow the evolution of the RWI in a full general relativistic framework, we explore how RWI can reproduce observed HFQPO frequencies ratios and if it is compatible with the observations. In order to model the emission coming from the disc we have linked our general relativistic simulations to the general relativistic ray-tracing GYOTO code and delivered synthetic observables that can be confronted to actual data from binary systems hosting HFQPOs.}{We have demonstrated in our study that some changes in the physical conditions prevailing in the part of the disc where RWI can be triggered leads to various dominant RWI modes whose ratio recovers frequency ratios observed in various X-ray binary systems. In addition, we have also highlighted that when RWI is triggered near the last stable orbit of a spinning black hole, the amplitude of the X-ray modulation increases with the spin of the black hole. Revisiting published data on X-ray binary systems, we show that this type of relationship actually exists in the five systems where an indirect measure of the spin of the black hole is available.
We simulate an oscillating purely hydrodynamical torus with constant specific angular mo- mentum around a Schwarzschild black hole. The goal is to search for quasi-periodic oscil- lations (QPOs) in the light curve of the torus. The initial torus setup is subjected to radial, vertical and diagonal (combination of radial and vertical) velocity perturbations. The hydro- dynamical simulations are performed using the general relativistic magnetohydrodynamics code Cosmos++ and ray-traced using the GYOTO code. We found that a horizontal velocity perturbation triggers the radial and plus modes, while a vertical velocity perturbation trig- gers the vertical and X modes. The diagonal perturbation gives a combination of the modes triggered in the radial and vertical perturbations.
We report the results of a systematic timing analysis of all archival Rossi X-Ray Timing Explorer (RXTE) observations of the bright black-hole binary GRS 1915+105 in order to detect high-frequency quasi-periodic oscillations (HFQPO). We produced power-density spectra in two energy bands and limited the analysis to the frequency range 30-1000 Hz. We found 51 peaks with a single trial significance larger than 3 sigma. As all but three have centroid frequencies that are distributed between 63 and 71 Hz, we consider most of them significant regardless of the number of trials involved. The average centroid frequency and FWHM are 67.3 +/- 2.0 Hz and 4.4 +/- 2.4 Hz respectively. Their fractional rms varies between 0.4% and 2% (total band detections) and between 0.5% and 3% (hard ban detections). As GRS 1915+105 shows large variability on time scales longer than 1s, we analysed the data in 16s intervals and found that the detections are limited to a specific region in the colour-colour diagram, corresponding to state B of the source, when the energy spectrum is dominated by a bright accretion disk component. However, the rms spectrum of the HFQPO is very hard and does not show a flattening up to 40 keV, where the fractional rms reaches 11%. We discuss our findings in terms of current proposed models and compare them with the results on other black-hole binaries and neutron-star binaries.
When the accretion disc around a weakly magnetised neutron star (NS) meets the stellar surface, it should brake down to match the rotation of the NS, forming a boundary layer. As the mechanisms potentially responsible for this braking are apparently inefficient, it is reasonable to consider this layer as a spreading layer (SL) with negligible radial extent and structure. We perform hydrodynamical 2D spectral simulations of an SL, considering the disc as a source of matter and angular momentum. Interaction of new, rapidly rotating matter with the pre-existing, relatively slow material co-rotating with the star leads to instabilities capable of transferring angular momentum and creating variability on dynamical timescales. For small accretion rates, we find that the SL is unstable for heating instability that disrupts the initial latitudinal symmetry and produces large deviations between the two hemispheres. This instability also results in breaking of the axial symmetry as coherent flow structures are formed and escape from the SL intermittently. At enhanced accretion rates, the SL is prone to shearing instability and acts as a source of oblique waves that propagate towards the poles, leading to patterns that again break the axial symmetry. We compute artificial light curves of an SL viewed at different inclination angles. Most of the simulated light curves show oscillations at frequencies close to 1kHz. We interpret these oscillations as inertial modes excited by shear instabilities near the boundary of the SL. Their frequencies, dependence on flux, and amplitude variations can explain the high-frequency pair quasi-periodic oscillations observed in many low-mass X-ray binaries.
We performed axisymmetric hydrodynamical simulations of oscillating tori orbiting a non-rotating black hole. The tori in equilibrium were constructed with a constant distribution of angular momentum in a pseudo-Newtonian potential (Klu{z}niak-Lee). Motions of the torus were triggered by adding sub-sonic velocity fields: radial, vertical and diagonal to the tori in equilibrium. As the perturbed tori evolved in time, we measured $L_{2}$ norm of density and obtained the power spectrum of $L_{2}$ norm which manifested eigenfrequencies of tori modes. The most prominent modes of oscillation excited in the torus by a quasi-random perturbation are the breathing mode and the radial and vertical epicyclic modes. The radial and the plus modes, as well as the vertical and the breathing modes will have frequencies in an approximate 3:2 ratio if the torus is several Schwarzschild radii away from the innermost stable circular orbit. Results of our simulations may be of interest in the context of high-frequency quasi-periodic oscillations (HF QPOs) observed in stellar-mass black hole binaries, as well as in supermassive black holes.
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