Detection of Simultaneous Quasiperiodic Oscillation Triplets in 4U 1728-34 and Constraining the Neutron Star Mass and Moment of Inertia

Abstract

We report simultaneous detection of twin kHz and ∼40 Hz quasiperiodic oscillations (QPOs) in the time-resolved analysis of the AstroSat/LAXPC observation of the neutron star low-mass X-ray binary, 4U 1728-34. The frequencies of the multiple sets of triplets are correlated with each other and are consistent with their identification as the orbital, periastron precession, and twice the nodal precession frequencies. The observed relations, along with the known spin of the neutron star, put constraints on the mass and the ratio of the moment of inertia to the mass of the neutron star to be ${M}_{\odot }^{* }=1.92\pm 0.01$ and ${I}_{45}/{M}_{\odot }^{* }=1.07\pm 0.01$ under the simplistic assumption that the metric is a Kerr one. We crudely estimate that the mass and moment of inertia values obtained may differ by about 1% and 5%, respectively, if a self-consistent metric is invoked. Using the Tolman–Oppenheimer–Volkoff equations for computing the moment of inertia of a neutron star in slow rotation approximation, having different equations of state, we find that the predicted values of neutron star parameters favor stiffer equations of state. We expect more stringent constraints would be obtained using a more detailed treatment, where the equation of state–dependent metric is used to compute the expected frequencies rather than the Kerr metric used here. The results provide insight into both the nature of these QPOs and the neutron star interior.

Keywords

• X-ray binary stars
• Low-mass x-ray binary stars
• Neutron stars
• Accretion
• General relativity
• Astrophysical black holes