The Origin of Hard/Soft Lags in Galactic BHBs and AGN: Implications for Accretion Disk Dynamics

Time lag properties are important because it gives a model independent view of accretion dynamics. The time lag properties are inclination dependent and the explanation of the origin of time lag is difficult due to the interplay of multiple non-linear physical mechanisms that are contributing to the time lag. We attempted various Galactic black hole transients (GRS 1915+105, XTEJ 1550-564, GX 339-4, H 1743-322, XTE J1650-500 etc.) and a bare-type AGN (Ark 120) that exhibit similar spectro-temporal properties. We have calculated the X-ray time delay/lag between the different energy band light curve for both AGN and GBHs. We found that the evolution of Low frequency QPOs in GBHs is inclination-independent. However, the time lag/delay changes sign with variations in the accretion rate and disk geometry in both AGN and BH transient sources (at QPO frequency ~ 2-3 Hz). These variations in the accretion geometry result in alterations in the dominant physical mechanisms of the governing accretion dynamics. We attribute the origin of time lag/delays to a combination of physical mechanisms, including Comptonization, reflection, variations in accretion geometry, gravitational light bending, and disk-jet connections in Galactic black hole binaries.