Deepika Bollimpalli, MPA Garching - July 4th, 2022

Long-term observations have shown that black hole X-ray binaries exhibit strong, aperiodic variability on time scales of a few milliseconds to seconds. In addition, these systems also exhibit rapid variability in their X-ray light curves termed quasi-periodic oscillations (QPOs); also broadly classified into high-frequency and low-frequency QPOs. Understanding the nature of these variabilities and the underlying physical processes helps us probe the nature of the compact object and its surroundings. In this talk, starting with a pedagogical introduction to the accretion discs, I shall give a broad overview of the existing theoretical models for broadband variability and different types of QPOs. I will also focus on the findings from the current state-of-art simulations of accretion discs in reproducing such time variability phenomena. In particular, I will discuss how the mass accretion rate fluctuations generated by the turbulence in the simulations naturally explain the observed broad-band variability in the accreting systems and the viability of the relativistic precession model for the low-frequency QPO.