The coherence of kHz quasi-periodic oscillations in the X-rays from accreting neutron stars

Our examination of archival Rossi X-ray Timing Explorer data on neutron star low-mass X-ray binaries has revealed a number of trends in the quality factors of their lower and upper kHz QPOs. We report our analysis of the sources 4U 1636--536, 4U 1608--522, 4U 1735--44, 4U 1728--34, 4U 1820--303 and 4U 0614+09. All the sources except 4U 0614+09 show a precipitous drop in the quality factor of the lower kHz QPO at the highest observed frequencies. In contrast, in all sources except 4U 1728--34, the upper kHz QPO has a quality factor that increases monotonically with increasing frequency. We demonstrate that both observed trends are consistent with generic expectations based on the idea that the region determining the frequencies approaches the innermost stable circular orbit, a feature unique to strong gravity. If this interpretation is correct, it suggests that the neutron stars in several of these systems have masses ∼ 2 M_⊙, which has important implications for the equilibrium state of nonrelativistic matter at several times nuclear saturation density.