Rotating neutron stars with chaotic magnetic fields in general relativity and Rastall gravity

TL;DR

This work investigates rotating neutron stars with chaotic magnetic fields in GR and Rastall gravity using a GM1-based QHD equation of state. The authors extend the Hartle–Thorne formalism to Rastall gravity and adopt a chaotic-field ansatz that couples magnetic energy density to matter, enabling analysis of mass–radius relations, deformation, and moment of inertia. They find that magnetic fields can raise the maximum mass and increase compactness, while larger Rastall parameters mainly shrink radii and suppress deformation; rotation increases deformation, particularly at higher spins. The magnetic-field–driven increase in the moment of inertia occurs notably in the 1.5–1.99 solar-mass range, with Rastall parameter effects being modest, and all results remain consistent with pulsar-based I constraints, supporting the viability of RG as a testbed for strong-field gravity in neutron stars.

Abstract

Observations indicate that the magnetic fields on neutron stars (NSs) lie in the range of $10^{8}$-$10^{15}$ G. We investigate rotating NSs with chaotic magnetic fields in both general relativity (GR) and Rastall gravity (RG). The equation of state (EOS) of NS matter is formulated within the framework of quantum hadrodynamics (QHD). The Hartle-Thorne formalism, extended to RG, is employed as an approximation for describing rotating NSs, while the magnetic field is modeled through an ansatz in which it is coupled to the energy density. We find that at high masses, neither rotation nor the Rastall parameter significantly affects the total mass, whereas the magnetic field strength can increase the maximum allowed mass. At lower masses, both the magnetic field and an increasing Rastall parameter reduce the stellar radius in the static configuration. Although higher angular velocities enhance stellar deformation, both magnetic field and larger Rastall parameter tend to suppress it. Regarding the moment of inertia, the Rastall parameter has little impact, whereas the magnetic field strength can increase it within the mass range $1.50$-$1.99 M_\odot$. All parameters considered in this study are consistent with observational constraints on the moment of inertia obtained from radio observations of massive pulsars.

Figures (6)

• Figure 1: Relation between total energy density $\varepsilon$ and total pressure $p$.
• Figure 2: Mass-radius relation of NSs with and without chaotic magnetic field within static configuration.
• Figure 3: Mass-radius relation of NSs for (a) $\Omega = 1000$ s$^{-1}$ and $B=0$, (b) $\Omega = 2000$ s$^{-1}$ and $B=0$, (c) $\Omega = 1000$ s$^{-1}$ and $\gamma=3$, (d) $\Omega = 2000$ s$^{-1}$ and $\gamma=3$, (e) $\Omega = 1000$ s$^{-1}$ and $\gamma=5$, and (f) $\Omega = 2000$ s$^{-1}$ and $\gamma=5$.
• Figure 4: Illustrations of 3- and 2-dimensional deformation of NSs in each particular mass at $\lambda=-1\times10^{-5}$, $\gamma=5$, and $\Omega=2000$ s$^{-1}$.
• Figure 5: Eccentricity of NSs at (a) $\Omega =$ 1000 s$^{-1}$, (b) $\Omega =$ 2000 s$^{-1}$ as a function of mass