Indian scientists decode flickering X-ray light around spinning black holes

A team of Indian astrophysicists has identified the physical mechanism behind the flickering X-ray radiation emitted by matter falling into spinning black holes, resolving a long-standing puzzle in high-energy astrophysics.

Researchers at the Aryabhatta Research Institute of Observational Sciences (ARIES) in Nainital, an autonomous institute under the Department of Science and Technology, led the study in collaboration with scientists from Poland's Nicolaus Copernicus Astronomical Center and MJP Rohilkhand University in Bareilly. The team demonstrated that fluid friction and rapid rotation work together to destabilise matter flowing towards black holes, causing shock waves to oscillate and emit fluctuating X-ray light.

The findings, published in the Journal of High Energy Astrophysics, were led by Sanit Debnath and Dr Indranil Chattopadhyay using numerical simulations developed by ARIES's Numerical and Theoretical Astrophysics Group.

How viscosity and spin trigger X-ray flickering

As black holes draw surrounding celestial gas into rapid accretion flows, the material heats up and radiates intense X-rays. Astronomers have long observed that this X-ray brightness varies in nearly regular cycles known as quasi-periodic oscillations (QPOs). In black hole X-ray binary systems, these fluctuations range from under one cycle per second to several hundred hertz. Existing theoretical models had struggled to explain both high and low frequencies through a single physical process.

Using simulations built for relativistic fluids, the ARIES team discovered that when internal friction — or viscosity — in the accretion flow exceeds a critical threshold, previously stable shock waves in the gas become unstable. These oscillating shocks cause periodic pulses in the emitted radiation.

The spin rate of the black hole determines where these shock waves form and how rapidly they flicker.

In slowly spinning black holes, viscosity pushes the oscillating shock front farther away, generating low-frequency variations. In rapidly spinning black holes, the shock moves closer to the event horizon, driving much faster oscillations.

For a black hole weighing approximately ten times the mass of the Sun, the model shows that high spin rates can produce QPO frequencies above 100 hertz. This matches high-frequency variations observed in real-world X-ray binary systems.

According to the researchers, the study is the first numerical investigation to combine viscous accretion flows with thermodynamic relations for a relativistic fluid within this framework, highlighting India's growing leadership in computational astrophysics and deep-space research.