The beat frequency model for QPOs

The quasi-periodic oscillation (QPO) phenomenon has come to pose increasingly complex problems as sources have been discovered; these currently number 12, of which at least seven are X-ray binaries. The beat-frequency model (BFM), originally devised to characterize the first and simplest of the QPOs, GX5-1, is presently discussed with a view to its assumptions and applicability to other QPOs. The most important problem with the BFM is identified as the assumption that the neutron star involved rotates at an angular frequency of the order, in the case of GX5-1, of 10 msec.