Resonance Oscillation of Radiative Shock Waves in Accretion Disks around Compact Objects

We extend our previous numerical simulation of accretion disks with shock waves when cooling effects are also included. We consider bremsstrahlung and other power-law processes: A p²Tα to mimic cooling in our simulation. We employ the smoothed particle hydrodynamics technique as in the past. We observe that for a given angular momentum of the flow, the shock wave undergoes a steady, radial oscillation with the period roughly equal to the cooling time. Oscillations seem to take place when the disk and cooling parameters (i.e., accretion rate, cooling process) are such that the infall time from shock is of the same order as the postshock cooling time. The amplitude of oscillation could be up to 10% of the distance of the shock wave from the black hole when the black hole is accreting. When the accretion is impossible due to the centrifugal barrier, the amplitude variation could be much larger. As a result of the oscillation, the energy output from the disk is also seen to vary quasi-periodically. We believe that these oscillations might be responsible for the quasi-periodic oscillation (QPO) behaviors seen in several black hole candidates, in neutron star systems as well as dwarf novae candidates such as SS Cyg and VW Hyi.