Mass and spin measurements for the neutron star 4U1608−52 through the relativistic precession model

Abstract

We present a systematic analysis of the complete set of observations of the neutron star low-mass X-ray binary 4U1608-52 obtained by the Rossi X-ray Timing Explorer's Proportional Counter Array. We study the spectral and fast-time variability properties of the source in order to determine the mass and spin of the neutron star via the relativistic precession model, and find 24 observations containing usable sets of the necessary three quasi-periodic oscillations (triplets) with which to accomplish this task, along with a further 7 observations containing two of the three quasi-periodic oscillations each. We calculate the spin and mass of the source for each of the triplets, and find that they give physically realistic estimates clustering in the spin range $0.19 < a < 0.35$ and mass range $2.15 < M/\textrm{M}_{\odot} < 2.6$. Neutron stars present environments for studying matter under the most extreme conditions of pressure and density; as their equation of state is not yet known, accurate measurements of their mass and spin will eventually allow for the discrimination between various models. We discuss the implications of our findings in the context of equation of state predictions, physically allowed spin ranges, emission proximity to the innermost stable circular orbit and possible model inaccuracies.