  We study several Galactic black hole candidates using long-time RXTE/ASM
X-ray data to search for telltale signatures of differences in viscous
timescales in the two components used in the Two-Component Advective Flow
(TCAF) 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/delay 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
({\Theta}), which is a proxy of the usual photon index ({\Gamma}). Thus, when
{\Theta}, 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.
