On the development of QPOs in Bondi-Hoyle accretion flows

TL;DR

This paper demonstrates that quasi-periodic oscillations naturally occur in the shock cone of Bondi-Hoyle accretion flows around rotating black holes, with frequencies influenced by black hole spin and flow properties, potentially explaining observed QPOs.

Contribution

It introduces the first relativistic simulations showing QPOs in the shock cone of Bondi-Hoyle accretion, highlighting their dependence on black hole spin and flow parameters.

Findings

QPOs are produced in the shock cone of accretion flows.

QPO frequencies scale linearly with inverse black-hole mass.

Development of flip-flop instability in relativistic regime.

Abstract

The numerical investigation of Bondi-Hoyle accretion onto a moving black hole has a long history, both in Newtonian and in general-relativistic physics. By performing new two-dimensional and general-relativistic simulations onto a rotating black hole, we point out a novel feature, namely, that quasi-periodic oscillations (QPOs) are naturally produced in the shock cone that develops in the downstream part of the flow. Because the shock cone in the downstream part of the flow acts as a cavity trapping pressure perturbations, modes with frequencies in the integer ratios 2:1 and 3:1 are easily produced. The frequencies of these modes depend on the black-hole spin and on the properties of the flow, and scale linearly with the inverse of the black-hole mass. Our results may be relevant for explaining the detection of QPOs in Sagittarius A*, once such detection is confirmed by further observations. Finally, we report on the development of the flip-flop instability, which can affect the shock cone under suitable conditions; such an instability has been discussed before in Newtonian simulations but was never found in a relativistic regime.