Estimation of neutron star mass and radius of FRB 20240114A by identification of crustal oscillations

TL;DR

This work treats FRB 20240114A QPOs as signatures of neutron-star crustal torsional oscillations to extract core EOS information. By mapping observed rest-frame frequencies to fundamental and first-overtone crustal modes within non-magnetic crust models (anchored to $L$ and $K_0$ via OI-EOS parametrizations and the crustal shear properties), the authors derive a consistent NS mass–radius region centered around $R\approx 13$ km and constrain the symmetry-energy slope $L$ to $59.5$–$96.8$ MeV, with a derived overtone-linked combination $\varsigma=(K_0^4L^5)^{1/9}$ informing $K_0$ limits. The results yield two mass–radius possibilities depending on the overtone identification, roughly $M\sim1.0$–$1.55\,M_\odot$ or $M\sim1.17$–$1.76\,M_\odot$, which are broadly compatible with current NICER and GW170817 constraints but rely on the crust-only, non-magnetic assumption. The analysis highlights the potential of FRB QPOs as a new asteroseismology probe of cold dense matter and underscores the need for broader FRB QPO samples to perform population-wide EOS inferences, while cautioning about possible shifts if magneto-elastic effects are significant.

Abstract

By identifying quasi-periodic oscillations (QPOs) reported in FRB 20240114A (from the Five-hundred-meter Aperture Spherical Telescope) with neutron star crustal torsional oscillations, together with experimental constraints on the incompressibility $K_0$ of symmetric nuclear matter at saturation density, we constrain the mass and radius of an extragalactic neutron star at redshift $z\approx0.13$. Identifying the low-order QPO frequencies as fundamental oscillations, and frequencies of $567.7\,\mathrm{Hz}$ or $655.5\,\mathrm{Hz}$ (rest frame) as first overtone candidates, implies neutron star mass ranges of $1.00$--$1.55\,M_\odot$ or $1.17$--$1.76\,M_\odot$, respectively. The radius is also constrained, with a self-consistent value around $13$~km, further supported by the calculation of the NS structure within the low-mass/low-central density regime. Simultaneously, we also constrain another nuclear saturation parameter, namely the density dependence of the nuclear symmetry energy at saturation density (i.e., the slope parameter), $L$, and determine it to be $L=59.5-96.8$ MeV with $\sim 10\%$ systematic uncertainty, which is broadly consistent with previous constraints on $L$ obtained from experiments and astronomical observations. Thus, a mapping of FRB QPOs to crustal torsional modes seems reasonable. This can be confirmed with upcoming FRB surveys over a broad range of redshifts and more elaborate data analyses.

Figures (7)

• Figure 1: The rest-frame frequencies, $\nu_0$, listed in Table \ref{['tab:QPO1']} (except for 205.6 Hz), identified with the fundamental frequencies of crustal torsional oscillations with various values of $\ell$, for the NS model with $1.4M_\odot$ and 12 km. The top and bottom panels correspond to the results with $N_s/N_d=0$ and 1, respectively. The horizontal solid lines with the QPO frequencies written in bold font are frequencies with statistical significance greater than or equal to 3$\sigma$, while the dashed lines with the QPO frequencies written in thin font are those with significance less than 3$\sigma$. The vertical lines denote the optimal value of $L$ to identify the QPO frequencies with the fundamental frequencies of the crustal torsional oscillations, while the shaded regions denote $L\pm 1\sigma$.
• Figure 2: The optimal ranges of $L \pm 1\sigma$ to identify the observed QPO frequencies with the crustal torsional oscillations, shown for various NS models. The gray shaded regions denote the range of fiducial value of $L$, i.e., $L=60\pm 20$ MeV in light gray PhysRevC.86.015803refId0Li:2019xxz, and the range of $L$ constrained from the identification of magnetar QPOs observed in SGR 1806-20 with crustal torsional oscillations, i.e., $L=58-73$ MeV in dark gray 10.1093/mnras/sty1755universe10060231.
• Figure 3: The $\sim 600$ Hz candidate QPO frequencies observed in FRB 20240114A, compared with the 1st overtone excited in a NS model with $1.4M_\odot$, 12 km, and $N_s/N_d=1$.
• Figure 4: Constraints on the parameter $\varsigma$, obtained by identifying the QPO frequencies of $\sim 600$ Hz with the 1st overtone of crustal torsional oscillations. The left, middle, and right panels correspond to stellar models whose radii are 10, 12, and 14 km, respectively, while the top and bottom panels correspond to results with $N_s/N_d=0$ and 1. In each panel, the lower and upper bounds come from the upper and lower bounds of the uncertainties in the QPO frequency (see Fig. \ref{['fig:1st-M14']}).
• Figure 5: Constraint on $K_0$ obtained from the combination of the constraints on $L$ shown in Fig. \ref{['fig:LL_Ns']} and $\varsigma$ shown in Fig \ref{['fig:ss_RNs10']} by identifying the QPO frequencies with the crustal torsional oscillations. For reference, the fiducial value of $K_0$ obtained from the terrestrial experiments, i.e., $K_0=240\pm20$ MeV 2006EPJA...30...23S, is also shown.