A New, Simple Model for Black Hole High Frequency QPOs

Abstract. Observations of X-ray emissions from binary systems have always been considered important tools to test the validity of General Relativity in strong-field regimes. The pairs and triplets of high frequency quasi-periodic oscillations observed in binaries containing a black hole candidate, in particular, have been proposed as a means to measure more directly the black hole properties such as its mass and spin. Numerous models have been suggested over the years to explain the QPOs and the rich phenomenology accompanying them. Many of these models rest on a number of assumptions and are at times in conflict with the most recent observations. We here propose a new, simple model in which the QPOs result from basic p-mode oscillations of a non-Keplerian disc of finite size. We show that within this new model all of the key properties of the QPOs: a) harmonic ratios of frequencies even as the frequencies change; b) variations in the relative strength of the frequencies with spectral energy distribution and with photon energy; c) small and systematic changes in the frequencies, can all be explained simply given a single reasonable assumption. 1 A number of models have been proposed to explain the high frequency quasi-periodic oscillations (HFQPOs) seen in accreting black hole systems [1, 2] and two of these seem particularly promising in our view. The first model proposed, the discoseismic model [3], asserts that g-modes should become trapped in the potential well of a Keplerian (i.e. geometrically thin) disc in a Kerr potential. The size of the region where the modes are trapped depends on both the mass and the spin of the accreting black hole. Additional frequencies of oscillation should be expected from pressure modes and corrugation modes. The predictions of the model are well summarised by Kato [4]. Given pairs of high frequency QPOs and a proper identification of the frequencies with the particular modes, one can measure both the black hole mass and spin to relatively high accuracy [5]. The discoveries of three systems where the QPOs show a harmonic structure with relatively strong peaks seen in integer ratios

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