Quadratic Rastall Gravity: from low-mass HESS J1731-347 to high-mass PSR J0952-0607 pulsars

Abstract

Similar to Rastall gravity we introduce matter-geometry nonminimal coupling which is proportional to the gradient of quadratic curvature invariants. Those are mimicking the conformal trace anomaly when backreaction of the quantum fields to a curved spacetime geometry is considered. We consider a static spherically symmetric stellar structure with anisotropic fluid and Krori-Barua metric potentials model to examine the theory. Confronting the model with NICER+XMM-Newton observational constraints on the pulsar PSR J0740$+$6620 quantifies the amount of the nonminimal coupling via a dimensionless parameter $\epsilon\simeq -0.01$. We verify that the conformal symmetry is broken everywhere inside the pulsar as the trace anomaly $\Delta>0$, or equivalently the trace of the stress-energy tensor $\mathfrak{T}<0$, whereas the adiabatic sound speed does not violate the conjecture conformal upper limit $v_r^2/c^2 = 1/3$. The maximum compactness accordingly is $C_\text{max}=0.752$ which is $4\%$ higher than GR. Notably, if the conformal sound speed constraint is hold, observational data excludes $\epsilon \geq 0$ up to $\geq 1.6\sigma$. The stellar model is consistent with the self-bound structure with soft linear equation of state. Investigating possible connection with MIT bag model of strange quarks sets physical bounds from microscopic physics which confirm the negative value of the parameter $\epsilon$. We estimate a radius $R=13.21 \pm 0.96$ km of the most massive observed compact star PSR J0952$-$0607 with $M=2.35\pm0.17 M_\odot$. Finally, we show that the corresponding mass-radius diagram fits well lowest-mass pulsar HESS J1731$-$347 and highest-mass pulsar PSR J0952$-$0607 ever observed as well as the intermediate mass range as obtained by NICER and LIGO/Virgo observations.