Realistic neutron star models in $f(T)$ gravity

TL;DR

This study explores how quadratic torsion modifications in $f(T)$ gravity affect neutron star properties, constraining the model with observational data and revealing limits on the parameter $oldsymbol{eta}$ for realistic stellar models.

Contribution

It provides the first detailed analysis of nonrotating neutron stars in quadratic $f(T)$ gravity using realistic equations of state and observational constraints.

Findings

Positive $oldsymbol{eta}$ decreases maximum neutron star mass.

Negative $oldsymbol{eta}$ allows larger stellar matter content.

Constraints on $oldsymbol{eta}$ depend on the included observational data.

Abstract

We investigate the nonrotating neutron stars in $f(T)$ gravity with $f(T)=T+\alpha{T}^2$, where $T$ is the torsion scalar in the teleparallel formalism of gravity. In particular, we utilize the SLy and BSk family of equations of state for perfect fluid to describe the neutron stellar matter and search for the effects of the $f(T)$ modification on the models of neutron stars. For positive $\alpha$, the modification results in a smaller stellar mass in comparison to general relativity, while the neutron stars will contain larger amount of matter for negative $\alpha$. Moreover, there seems to be an upper limit for the central density of the neutron stars with $\alpha>0$, beyond which the effective $f(T)$ fluid would have a steplike phase transition in density and pressure profiles, collapsing the numerical system. We obtain the mass-radius relations of the realistic models of neutron stars and subject them to the joint constraints from the observed massive pulsars PSR J0030+0451, PSR J0740+6620, and PSR J2215+5135, and gravitational wave events GW170817 and GW190814. For the neutron star model in $f(T)$ gravity to be able to accommodate all the mentioned data, the model parameter $\alpha$ needs to be smaller than $-4.295$, $-6.476$, $-4.4$, and $-2.12$ (in the unit of ${G}^2M_\odot^2/c^4$) for SLy, BSk19, BSk20, and BSk21 equations of state, respectively. If one considers the unknown compact object in the event GW190814 not to be a neutron star and hence excludes this dataset, the constraints can be loosened to $\alpha<-0.594$, $-3.5$, $0.4$ and $1.9$ (in the unit of ${G}^2M_\odot^2/c^4$), respectively.