Evidences of an innermost stable bound orbit predicted by general relativity from the amplitude of the twin-peak quasi-periodic oscillations

TL;DR

This paper proposes a model linking the energy of twin-peak high-frequency QPOs in X-ray binaries to plasma clumps near a compact object, providing evidence for the innermost stable orbit predicted by general relativity.

Contribution

It introduces an intuitive model connecting QPO energy to plasma clumps and constrains their properties, supporting the existence of the innermost stable orbit in strong gravity.

Findings

Energy variation correlates with the lower HF QPO frequency.

Constraints on plasma clump size and mechanical properties.

Evidence supporting the innermost stable orbit predicted by GR.

Abstract

The twin-peak high-frequency quasi-periodic oscillations (HF QPOs), observed in the power spectra of low-mass X-ray binaries, might carry relevant clues about the physics laws reigning close to a compact object. Their frequencies are typical of the orbital motion time-scales a few gravitational radii away from the compact object. The aim of the manuscript is to propose an intuitive model explaining that the energy carried by the lower HF QPO can be related to differences of potential energy released by clumps of plasma spiraling in a curved space-time. Our model provides estimates on both the size of clumps of matter that can survive to the strong tidal force and energy loaded by tides on the clump. We also have obtained some constraints on the mechanical properties of the plasma orbiting into the accretion disk. We note that the systematic behavior of the emitted energy as function of the central frequency of the lower HF QPO, observed in several sources with a neutron star, might give clues related to an innermost stable bound orbit predicted by the General Relativity theory in strong field regime.