Analysis of Neutron Star $f-$mode Oscillations in General Relativity with Spectral Representation of Nuclear Equations of State

TL;DR

This paper investigates neutron star f-mode oscillations using a large set of nuclear equations of state, revealing correlations with star radii and implications for constraining nuclear matter models through astrophysical observations.

Contribution

It introduces a spectral representation of nearly 9000 nuclear EOSs to analyze f-mode oscillations and explores their correlations with neutron star properties, providing new constraints from NICER data.

Findings

Weak correlation between f-mode frequencies and nuclear saturation properties.

Strong linear relationship between neutron star radii and f-mode frequencies.

NICER observations partially constrain the nuclear EOS models.

Abstract

We study quasinormal $f-$mode oscillations in neutron star(NS) interiors within the linearized General Relativistic formalism. We utilize approximately 9000 nuclear Equations of State (EOS) using spectral representation techniques, incorporating constraints on nuclear saturation properties, chiral Effective Field Theory ($\chi$EFT) for pure neutron matter, and perturbative Quantum Chromodynamics (pQCD) for densities pertinent to NS cores. The median values of f-mode frequency, $\nu_f$ (damping time, $\tau_f$) for NS with masses ranging from 1.4 - 2.0 $M_\odot$ lie between 1.80 - 2.20 kHz (0.13 - 0.22 s) for our entire EOS set. Our study reveals a weak correlation between $f-$mode frequencies and individual nuclear saturation properties, prompting the necessity for more intricate methodologies to unveil multi-parameter relationships. We observe a robust linear relationship between the radii and $f-$mode frequencies for different NS masses. Leveraging this correlation alongside NICER observations of PSR J0740+6620 and PSR J0030+0451, we establish constraints that exhibit partial and minimal overlap for observational data from Riley et al. and Miller et al. respectively with our nucleonic EOS dataset. Moreover, NICER data aligns closely with radius and frequency values for a few hadron-quark hybrid EOS models. This indicates the need to consider additional exotic particles such as deconfined quarks at suprasaturation densities. We conclude that future observations of the radius or $f-$mode frequency for more than one NS mass, particularly at the extremes of viable NS mass scale, would either rule out nucleon-only EOS or provide definitive evidence in its favour.