2018MNRAS.479.1210G

We study several Galactic black hole candidates using long-time Rossi X-ray Timing Explorer/All Sky Monitor X-ray data to search for telltale signatures of differences in viscous timescales in the two components used in the two-component advective flow paradigm. In high-mass X-ray binaries (HMXBs) mainly winds are accreted. This nearly inviscid and dominant sub-Keplerian flow falls almost freely towards the black hole. A standard Keplerian disc can form out of this sub-Keplerian matter in presence of a significant viscosity and could be small in size. However, in low-mass X-ray binaries (LMXBs), highly viscous and larger Keplerian accretion disc is expected to form inside the sub-Keplerian disc due to the Roche lobe overflow. Due to two viscous timescales in these two components, it is expected to have a larger lag between the times of arrival of these components in LMXBs than that in HMXBs. Direct cross-correlation between the photon fluxes will not reveal this lag since they lack linear dependence, however, they are coupled through the viscous processes which bring in both matter. To quantify the aforesaid time lag, we introduce an index (Θ), which is a proxy of the usual photon index (Γ). Thus, when Θ, being dynamically responsive to both fluxes, is considered as a reference, the arrival time lag between the two fluxes in LMXBs is found to be much larger than that in HMXBs. Our result establishes the presence of two dynamical components in accretion and shows that the Keplerian disc size indeed is smaller in HMXBs as compared to that in LMXBs.