General relativistic constraints on the equation of state of dense matter implied by kilohertz quasi-periodic oscillations in neutron-star x-ray binaries

If the observed millisecond variability in the X-ray flux of several neutro n-star low-mass X-ray binaries (LMXBs) is interpreted within a general rela tivistic framework of accretion, in which a cutoff frequency was expected ( Kluzniak, Michelson, & Wagoner), severe constraints can be placed on the eq uation of state (EOS) of matter at supranuclear densities. The reported max imum frequency (1.15 +/- 0.02 kHz) of quasiperiodic oscillations observed i n sources as diverse as Sco X-1 and 4U 1728-34 would imply that the neutron star masses in these LMXBs are M > 1.9 M-circle dot, and hence many equati ons of state would be excluded. Among the very few still viable equations o f state are the EOSs of Phandaripande & Smith and EOS AV14 + UVII of Wiring a, Fiks, & Fabrocini.