ARIES Study Explains High-Frequency QPOs Near Spinning Black Holes

Astronomers at the Aryabhatta Research Institute of Observational Sciences (ARIES), an autonomous institute of the Department of Science and Technology (DST), have identified a single physical mechanism that can explain why matter swirling around a fast-spinning black hole gives off sharply flickering X-rays. The work narrows down the guesswork in theoretical models of these objects.

The science in brief

A black hole is the most compact object known, with gravity so strong that light cannot escape, and it can spin about its axis. Matter dragged towards it forms an accretion flow that falls inward at high speed and gives off non-thermal radiation. The brightness of that radiation varies in an almost regular rhythm, and these variations are called quasi-periodic oscillations (QPOs). In black hole X-ray binaries, QPOs range from less than one cycle a second to several hundred cycles a second. Earlier studies could account for the slow oscillations or had to bring in exotic processes for the fast ones.

How the study was done

The team used a computer simulation built by the Numerical and Theoretical Astrophysics Group at ARIES to model viscous accretion flows, conserving energy, mass and momentum as the matter moves. The work was led by Mr Sanit Debnath and Dr Indranil Chattopadhyay of ARIES, with scientists from the Nicolaus Copernicus Astronomical Center in Poland and MJP Rohilkhand University, Bareilly. The findings appeared in the Journal of High Energy Astrophysics (JHEAP).

What they found

Once the viscosity of the accretion flow rises above a critical value, the normally steady shock in the disc turns unstable and begins to oscillate, and that oscillation shows up as the flickering light observed as QPOs. The spin matters a great deal. Around a slowly spinning black hole, viscosity pushes the average position of the oscillating shock outward, so mostly low-frequency QPOs appear. Around a rapidly spinning one, the shock is pulled closer in and QPOs cover a far wider band; for very high spin the predicted frequencies can exceed 100 Hz for a black hole of about ten solar masses, matching the fast variations seen in X-ray binaries. The study also found a clear link between QPO frequency and the photon index. The researchers call it the first numerical study to simulate an accretion flow with internal friction while using the thermodynamic relations of a relativistic fluid.