Scientists Explain Flickering Light Around Spinning Black Holes

Indian scientists have identified a physical mechanism that could explain why matter surrounding rapidly spinning black holes produces X-rays that flicker at different frequencies. The findings offer new insight into the behaviour of matter under extreme gravitational conditions and could help explain observations that existing models have struggled to account for.

The study was led by researchers at the Aryabhatta Research Institute of Observational Sciences (ARIES), an autonomous institute under the Department of Science and Technology (DST), in collaboration with scientists from Poland’s Nicolaus Copernicus Astronomical Center and MJP Rohilkhand University in Bareilly.

Black Hole Spin and X-ray Oscillations

Black holes draw surrounding matter into rapidly moving flows known as accretion flows. As this matter moves towards the black hole, it emits radiation, including X-rays, whose brightness can vary in nearly regular patterns called quasi-periodic oscillations (QPOs).

In black hole X-ray binary systems, these oscillations range from less than one cycle per second to several hundred cycles per second. Previous studies have struggled to explain both low- and high-frequency oscillations through a single physical mechanism.

The ARIES team investigated whether oscillations in shocks within the accretion flow could account for this broad range of frequencies.

Simulations Reveal the Role of Viscosity

The researchers used a computer simulation developed by ARIES’s Numerical and Theoretical Astrophysics Group to model viscous matter flows around black holes. The simulations tracked the movement of matter while conserving key physical quantities, including mass, energy and momentum.

The team found that when viscosity in the accretion flow exceeds a critical threshold, a normally stable shock can become unstable and begin oscillating. These movements can produce the periodic variations in emitted radiation observed as QPOs.

The research was led by Sanit Debnath and Dr Indranil Chattopadhyay of ARIES. The findings were published in the Journal of High Energy Astrophysics.

Faster-Spinning Black Holes Produce Higher Frequencies

The simulations showed that black hole spin strongly influences the shock’s position and the frequency of its oscillations.

In slowly spinning black holes, viscosity tends to push the oscillating shock farther away, producing predominantly low-frequency QPOs. In rapidly spinning black holes, the shock moves closer to the black hole, allowing oscillations across a wider range of frequencies.

For a black hole with approximately ten times the Sun’s mass, the model predicts that very high spin rates can produce QPO frequencies exceeding 100 hertz, comparable to high-frequency variations observed in X-ray binary systems.

The researchers also found that faster spin makes shock oscillations more pronounced and identified a relationship between QPO frequency and the photon index, which describes the spectral distribution of emitted radiation.

A More Realistic Model of Black Hole Behaviour

According to the researchers, the study is the first numerical investigation to combine viscous accretion flows with thermodynamic relations for a relativistic fluid in this modelling framework.

The results provide a physical explanation for how black hole spin and internal friction can work together to produce rapid X-ray variations. Further observations and modelling will be important for testing the predictions and improving scientific understanding of the energetic environments surrounding black holes.