We study the properties of viscous accretion flow having low angular momentum in presence of various cooling processes such as bremsstrahlung and synchrotron. We find that a significant fraction of the accretion flow could pass through a standing, oscillating or a propagating shock. At the shock, the flow kinetic energy is converted into the thermal energy forming a hot electron bath. These hot electrons can inverse-Comptonize the soft photons from cooled Keplerian disk and hard X-rays are produced. When shock oscillates, periodic breathing of shock starts to occur and therefore, the number of intercepted soft photons at the inner part of the disk varies accordingly. This causes the periodic changes in hard X-rays which are called as quasi-periodic oscillations (QPOs). We discuss the role of shock waves while explaining the observational features. |