Constraining $$f({\mathcal {R}})$$ gravity by Pulsar SAX J1748.9-2021 observations

Abstract

AbstractWe discuss spherically symmetric dynamical systems in the framework of a general model of $$f(\mathcal{R})$$ f ( R ) gravity, i.e. $$f(\mathcal{R})=\mathcal{R}e^{\zeta \mathcal{R}}$$ f ( R ) = R e ζ R , where $$\zeta $$ ζ is a dimensional quantity in squared length units [L$$^2$$ 2 ]. We initially assume that the internal structure of such systems is governed by the Krori–Barua ansatz, alongside the presence of fluid anisotropy. By employing astrophysical observations obtained from the pulsar SAX J1748.9-2021, derived from bursting X-ray binaries located within globular clusters, we determine that $$\zeta $$ ζ is approximately equal to $$\pm 5$$ ± 5 km$$^2$$ 2 . In particular, the model is capable of producing stable configurations for SAX J1748.9-2021, encompassing both its geometric and physical characteristics. We show that, within the framework of $$f(\mathcal{R})$$ f ( R ) gravity, the Krori–Barua ansatz establishes semi-analytical connections between the radial ($$p_r$$ p r ) and tangential ($$p_t$$ p t ) pressures, and the density ($$\rho $$ ρ ). These relations are described as $$p_r\approx v_r^2 (\rho -\rho _{I})$$ p r ≈ v r 2 ( ρ - ρ I ) and $$p_t\approx v_t^2 (\rho -\rho _{II})$$ p t ≈ v t 2 ( ρ - ρ II ) . In this context, $$v_r$$ v r and $$v_t$$ v t denote the sound speeds in the radial and tangential directions, respectively. Meanwhile, $$\rho _I$$ ρ I pertains to the surface density, and $$\rho _{II}$$ ρ II is derived from the model parameters. These connections are consistent with the equations of state derived from the best-fit solutions identified in the ongoing investigation. Notably, within the framework of $$f(\mathcal{R})$$ f ( R ) gravity where $$\zeta $$ ζ is negative, the maximum compactness, denoted as C, is inherently limited to values that do not exceed the Buchdahl limit. This contrasts with general relativity or $$f(\mathcal{R})$$ f ( R ) gravity with $$\zeta $$ ζ positive, where the compactness has the potential to asymptotically reach the black hole threshold ($$C\rightarrow 1$$ C → 1 ). The model predictions suggest a central density that largely exceeds the saturation nuclear density, which is $$\rho _{\text {nuc}} = 3\times 10^{14}$$ ρ nuc = 3 × 10 14 g/cm$$^3$$ 3 . Also the surface density $$\rho _I$$ ρ I surpasses $$\rho _{\text {nuc}}$$ ρ nuc . We obtain a mass-radius diagram, corresponding to the boundary density, which is consistent with other observational data.