Decoding accretion mysteries Spectral-timing insights into black hole and neutron star X-ray binaries

This thesis investigates the dynamics of black hole and neutron star systems, focusing on the hard states of black holes MAXI J1820+070, Cyg X-1, and the equivalent hard state for the neutron star Aql X-1 to understand the interactions between accretion disks and the inner corona (boundary layer). Divided into three science chapters, Chapter 2 probes the spectral-timing behaviour of two black hole X-ray binaries - MAXI J1820+070 (low mass) and Cyg X-1 (high mass) in their hard states, utilising data from XMM-Newton, NICER, and INTEGRAL. The time lags between the disk and the power-law energies at long timescales highlight the propagation of mass accretion fluctuations through the disk and are observed for both sources. The differences in the observed time-lags between the two sources at shorter timescales are likely due to variations in their accretion rates and/or spectral states but not explained by the difference in masses of the black holes alone. Chapter 3 is dedicated to an in-depth analysis of MAXI J1820+070 during its 2018 outburst in the hard state. The results support models where mass accretion fluctuations propagate through the accretion disk before reaching a compact corona. This work also underscores the changes in coronal geometry and the movement of the disk inward during the hard state evolution. Chapter 4 focuses on the neutron star system Aql X-1. The solid surface and a compact boundary layer in neutron stars reduce geometrical uncertainties. Using NICER data, this chapter applies a propagating fluctuations model to Aql X-1, fitting spectral-timing data products, reinforcing the commonalities between black hole and neutron star systems, in terms of a variable outer accretion disk, while simultaneously highlighting the differences i.e. corona vs a compact boundary layer region.