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The presence of a surface at the inner boundary, such as in a neutron star ora white dwarf, allows the existence of a standing shock in steady sphericalaccretion.The standing shock can become unstable in 2D or 3D; this is calledthe {\em standing accretion shock instability} (SASI).Two mechanisms --advective-acoustic and purely acoustic -- have been proposed to explain SASI.Using axisymmetric hydrodynamic (HD) and magnetohydrodynamic (MHD) simulations,we find that the advective-acoustic mechanism better matches the observedoscillation timescales in our simulations. The global shock oscillationspresent in the accretion flow can explain many observed high frequency($\gtrsim 100$ Hz) quasi-periodic oscillations (QPOs) in X-ray binaries (XRBs).The presence of a moderately strong magnetic field adds more features to theshock oscillation pattern, giving rise to low frequency modulation in thecomputed light curve. This low frequency modulation can be responsible for$\sim 100$ Hz QPOs (known as hHz QPOs). We propose that the appearance of hHzQPO determines the separation of twin peak QPOs of higher frequencies.

Magnetized SASI: its mechanism and possible connection to some QPOs in XRBs