Universal I-Q relations for rapidly rotating neutron and strange stars in scalar-tensor theories

TL;DR

This paper investigates how rapid rotation affects the universal relation between normalized moment of inertia and quadrupole moment in scalarized neutron stars, finding near EOS independence but notable deviations from general relativity that are below observational detection thresholds.

Contribution

It demonstrates that the normalized I-Q relation remains nearly EOS independent in scalar-tensor theories for rapid rotation, highlighting limitations in using these relations to distinguish gravity theories.

Findings

Normalized I-Q relation is nearly EOS independent for scalarized stars.

Deviations from GR increase with negative scalar coupling parameters but remain below observational thresholds.

Unnormalized moments differ significantly between scalar-tensor theories and GR.

Abstract

We study how rapid rotation influences the relation between the normalized moment of inertia $\bar{I}$ and quadrupole moment $\bar{Q}$ for scalarized neutron stars. The questions one has to answer are whether the EOS universality is preserved in this regime and what are the deviations from general relativity. Our results show that the $\bar{I}-\bar{Q}$ relation is nearly EOS independent for scalarized rapidly rotating stars, but the differences with pure Einstein's theory increase compared to the slowly rotating case. In general, smaller negative values of the scalar field coupling parameters $\beta$ lead to larger deviations, but these deviations are below the expected accuracy of the future astrophysical observations if one considers values of $\beta$ in agreement with the current observational constraint. An important remark is that although the normalized $\bar{I}-\bar{Q}$ relation is quite similar for scalar-tensor theories and general relativity, the unnormalized moment of inertia and quadrupole moment can be very different in the two theories. This demonstrates that although the $\bar{I}-\bar{Q}$ relations are potentially very useful for some purposes, they might not serve us well when trying to distinguish between different theories of gravity.