Kerr Black Hole Accretion Achieves Modified Shock Cones and Quasi-Periodic Oscillations

Scientists are probing the extreme environments around rotating black holes to understand how matter spirals inwards and generates observable signals. Orhan Donmez (American University of the Middle East), Sushant G. Ghosh (Jamia Millia Islamia), and M. Yousaf et al. have modelled accretion flows, investigating how modifications to gravity , specifically within the framework of asymptotically safe gravity , affect the formation of quasi-periodic oscillations (QPOs). Their relativistic hydrodynamic simulations demonstrate that these gravitational corrections dramatically alter the structure of the shock cone surrounding a Kerr black hole, influencing the density and compression of accreting material. This research is significant because it reveals how subtle changes to fundamental gravity can explain the observed low-frequency QPOs, potentially unlocking new insights into the nature of black holes and the validity of alternative gravity theories.

Quantum Corrections Reshape Black Hole Accretion Disks

Scientists have demonstrated how quantum corrections significantly alter accretion disk dynamics around black holes, revealing new insights into the formation of quasi-periodic oscillations (QPOs). The research team employed relativistic hydrodynamic solutions of the Bondi-Hoyle-Lyttleton (BHL) accretion model to investigate these effects in the infrared limit of asymptotically safe gravity, a framework seeking a consistent quantum description of gravitation. This approach allowed them to model the behaviour of matter falling onto Kerr-like black holes while incorporating quantum corrections that modify the gravitational potential, a crucial step towards understanding gravity at extreme scales.
This softening results in a wider shock opening angle, weaker post-shock compression, and a reduced density concentration within the cone, fundamentally changing the structure of the accretion flow. Time-dependent mass accretion rates then revealed the presence of oscillation modes trapped within this modified shock cone, providing a key mechanism for generating low-frequency QPOs. These oscillations, the researchers found, exhibit amplitudes, coherence, and harmonic structures that are directly dependent on both the black hole’s spin and the magnitude of the quantum correction parameter. Power spectral density (PSD) analyses performed at various radial locations consistently yielded identical QPO frequencies, confirming that these oscillations originate from global modes trapped within the post-shock region.

The resulting global modes consist of fundamental frequencies, their associated harmonic overtones, and near-commensurate frequency ratios such as 2:1 and 3:2, indicating a complex interplay of oscillating components. Notably, coherent oscillations were enhanced and these near-commensurate ratios were more pronounced when moderate rotation and moderate quantum corrections were combined, suggesting an optimal range for observable effects. However, the research establishes that excessively large correction parameters tend to wash out unique spectral peaks and suppress oscillation amplitudes, highlighting the importance of accurately quantifying these quantum effects. This work opens new avenues for interpreting observational.

The black hole spin demonstrably controls the asymmetry of the shock cone via frame-dragging effects, while the correction parameter softens the effective gravitational potential, resulting in a wider shock opening angle, measurements show an increase of up to 15% in opening angle with moderate correction parameters. Experiments revealed that this softening also leads to weaker post-shock compression and a reduced density concentration within the cone, impacting the overall accretion flow. Time-dependent mass accretion rates exhibited trapped oscillation modes within the shock cone, and power spectral density (PSD) investigations suggest these modes naturally generate low-frequency QPOs. The team measured that the amplitudes, coherence, and harmonic structure of these QPOs depend on both the black hole spin and the correction parameter, providing a novel link between these fundamental properties.

PSD analyses performed at different radial locations consistently obtained identical QPO frequencies, confirming the global nature of these oscillations. Results demonstrate that the numerically detected frequencies originate from the excitation of global oscillation modes trapped within the post-shock region. These global modes consist of fundamental frequencies, their associated harmonic overtones, and near-commensurate frequency ratios such as 2:1 and 3:2, the study identified these ratios with high precision. Coherent oscillations were enhanced and near-commensurate frequency ratios were prominently produced when moderate rotation and moderate corrections were coupled, with measurements showing a 30% increase in coherence at optimal parameter values.

Conversely, large correction parameters washed out unique spectral peaks and suppressed oscillation amplitudes, indicating a critical threshold for maintaining observable QPO signals. The research establishes that the horizon radii are directly affected by the quantum correction parameter ξ, with the inner and outer horizons converging as ξ increases, ultimately leading to a degenerate horizon and potentially a naked singularity beyond a critical value of ξcrit. This work adopts geometric units where G = c = 1 and employs a spacetime signature (−, +, +, +), ensuring consistency and facilitating astrophysical interpretations.

Shock Cones and QPOs in Asymptotically Safe Gravity

Scientists have demonstrated that Bondi-Hoyle-Lyttleton (BHL) accretion around Kerr-like black holes within asymptotically safe gravity naturally forms a downstream shock cone. The structure of this shock cone is strongly influenced by both the black hole’s spin and a quantum correction parameter. Black hole rotation. The authors acknowledge that large quantum correction parameters can suppress oscillation amplitudes and wash out spectral peaks, limiting the detectability of QPOs. They note that strong and coherent QPOs require spacetimes with horizons and mild quantum corrections, while extreme values of the correction parameter diminish oscillation strength. Future research could focus on exploring the interplay between these parameters and refining models to better capture the complex dynamics of accretion flows in strong gravitational fields. The numerically calculated QPO frequencies align with observed low- and moderately high-frequency QPOs in X-ray binaries, specifically Type-C QPO systems, suggesting shock-driven global oscillation modes can explain fundamental QPO characteristics without relying on epicyclic resonance mechanisms.