Millisecond oscillators in accreting neutron stars and black holes

Millisecond variability detected in low-mass X-ray binaries (LMXBs) probably reflects motions of accreting fluid in the strong-field gravity of neutron stars or neutron stars. Parametric resonance between two oscillators with unequal eigenfrequencies provides a natural explanation for the 2:3 frequency ratio observed in some black-hole systems, and probably also in the brightest neutron stars. The two oscillators likely correspond to the meridional and radial epicyclic motions.