Scientists from the Aryabhatta Research Institute of Observational Sciences (ARIES), an autonomous institute of the Department of Science and Technology, have uncovered a pivotal mechanism explaining how black hole spin influences the flickering of X-rays emitted from matter around black holes. The study, conducted in collaboration with researchers from Nicolaus Copernicus Astronomical Center, Poland and MJP Rohilkhand University, Bareilly, was led by Mr. Sanit Debnath and Dr. Indranil Chattopadhyay from ARIES and published in the Journal of High Energy Astrophysics (JHEAP).
The research focused on quasi-periodic oscillations (QPOs) in black hole X-ray binaries, which show nearly regular variations in brightness with frequencies ranging from less than one cycle per second to several hundred cycles per second. Using computer simulations developed by the Numerical and Theoretical Astrophysics Group at ARIES, the team modeled the behavior of viscous accretion flows around black holes while conserving energy, mass, and momentum.
The key finding reveals that when the viscosity of the accretion flow exceeds a critical value, normally stable shocks in the disc become unstable and begin to oscillate, producing the observed QPOs. The study demonstrates that black hole spin strongly influences shock movement and QPO frequencies: slowly spinning black holes push the oscillating shock farther away, producing mainly low-frequency QPOs, while rapidly spinning black holes pull the shock closer, generating QPOs across a much wider frequency range. For black holes with very high spin, the predicted QPO frequencies can exceed 100 Hz for a black hole approximately ten times the mass of the Sun, matching observed high-frequency variations in black hole X-ray binary systems.
The simulations also show that as black hole spin increases, the shock moves closer to the black hole and its oscillations become more pronounced. The research identified a clear relationship between QPO frequency and the gradient of photon flux with frequency (photon index), providing additional insight into the energetic environment around spinning black holes. This study represents the first numerical simulation to incorporate internal friction/viscosity in accretion flows while implementing thermodynamic relations of relativistic fluid, demonstrating how black hole spin influences QPO frequencies in the most realistic scenario to date.
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