Transient X-Ray Sources, Luminosity Gaps, and Neutron Star Densities

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Abstract
A simple model of accretion onto magnetized neutron stars predicts a large luminosity change during a transition from magnetospheric accretion to neutron star accretion. It is pointed out that, for this model, the ratio of the minimum neutron star accretion luminosity and maximum magnetospheric accretion luminosity depends on just the neutron star density and rotation period. Hence, the detection of such a luminosity gap would, at least in principle, give direct insight into the mass-radius relationship of neutron stars. Some potential problems that may affect this measurement are also discussed and some observational results are briefly reviewed.