Super-Keplerian Frequencies in Accretion Disks. Implications for Mass and Spin Measurements of Compact Objects from X-ray Variability Studies

TL;DR

This paper investigates the limitations of current methods for measuring the mass and spin of black holes and neutron stars using X-ray variability, through numerical simulations showing the assumptions often do not hold.

Contribution

The study uses numerical simulations to demonstrate cases where traditional assumptions in variability-based measurements of compact objects are invalid, questioning their robustness.

Findings

Assumptions of sub-Keplerian modes are often violated in simulations.

Resonant mode excitation at the same disk radius may not occur as assumed.

Mass and spin estimates from variability can be unreliable under certain conditions.

Abstract

The detection of fast quasi-periodic variability from accreting black holes and neutron stars has been used to constrain their masses, radii, and spins. If the observed oscillations are linear modes in the accretion disks, then bounds can be placed on the properties of the central objects by assuming that these modes are locally sub-Keplerian. If, on the other hand, the observed oscillations correspond to non-linear resonances between disk modes, then the properties of the central objects can be measured by assuming that the resonant modes are excited at the same radial annulus in the disk. In this paper, we use numerical simulations of vertically integrated, axisymmetric hydrodynamic accretion disks to provide examples of situations in which the assumptions implicit in both methods are not satisfied. We then discuss our results for the robustness of the mass and spin measurements of compact objects from variability studies.