Axion-angle dependence of the rotational and tidal properties of PNJL quark stars

Abstract

We investigate how a fixed, spatially uniform axion angle modifies the rotational and static tidal properties of self-bound quark stars in general relativity, using an axion-extended Polyakov-loop Nambu--Jona-Lasinio equation of state. We vary $\theta$ from $0$ to $\pi$ while holding the remaining model parameters fixed, and compute the leading-order moment of inertia and the second-order spin-induced quadrupole moment and mass shift. The calculation includes finite-surface-density contributions to the rotational mass shift and static tidal response. Across this range, the static maximum mass increases from $1.68$ to $1.95M_{\odot }$, remaining below the lower edge of the quoted $1\sigma$ timing interval for PSR~J0740+6620. At $M=1.338M_{\odot }$, the moment of inertia increases by $13\%$ and the dimensionless tidal deformability rises from $342$ to $547$. Over the common mass range examined, the differences in the moment of inertia between adjacent sampled angles exceed the tested interpolation and table-resolution sensitivities. Despite these changes, joint fits to the five sequences describe the $\bar{I}$--$C$ and I-Love-Q relations with maximum departures of $2.7\%$ and $0.4\%$, respectively, over the fitted domains. The predicted binary tidal-deformability curves lie within the estimated 90\% credible region of the adopted GW170817 posterior over the mass-ratio ranges examined. Within this EOS family, stellar properties vary appreciably with $\theta$, while the normalized quantities admit accurate joint fits across the five sequences.

arXiv comment14 pages, 9 figures, 2 tables