Does Cyg X-1 have a Tiny Accretion Disk?

Results of analysis of several outbursts in a few transient sources are studied using the RXTE/PCA (2.5-25 keV) data as well as RXTE/ASM (1.5-12 keV) data spanning over a long time. A typical time lag is found between the arrival times of the Keplerian disk component and the halo component of the Two-Component Advective Flow (TCAF) when the spectral data is fitted with TCAF solution. This is due to the fact that Keplerian disks have higher viscosity and matter takes a longer time to fall, while the halo has a lower viscosity and falls almost in the free-fall time scale. The wind-fed high-mass X-ray binaries have low angular momentum dominated flow and the lag is expected to be smaller due to the difficulty in making a large Keplerian disk and the arrival time would not differ significantly in the two components of TCAF. On the contrary, a bigger Keplerian disk is likely to form when the accretion is through the Roche-lobe overflow in LMXBs and thus the lag is expected to be very large. Using earlier results in several outburst LMXBs and fitting with TCAF during state transitions of a well known HMXB Cyg X-1, we find that this is indeed the case. Several days of time lags were found in LMXBs while for Cyg X-1 the lag is negligible. We also computed the cross-correlation between the disk and the halo accretion rates and found that the lag is roughly what is observed directly from TCAF extracted parameters. This confirms that the size of the Keplerian disk in Cyg X-1 must be relatively smaller than that in LMXBs studied here.