Neutron Stars in Energy--Momentum Squared Gravity: Structure, Stability, and Multimessenger Constraints

Abstract

We study nonrotating neutron stars in energy--momentum squared gravity (EMSG). The stellar models are obtained from the modified Tolman--Oppenheimer--Volkoff equations and four original tabulated hadronic equations of state: WFF1, SLy4, APR4, and MPA1. We use the common coupling set $α\in\{-20,-10,0,+5,+7.5\}\,\mathrm{km}^{2}$ and construct mass--radius and mass--central-density sequences. We compare these sequences with the $2\,M_\odot$ mass requirement, the NICER measurements of PSR J0030+0451 and PSR J0740+6620, and the region inferred from GW170817. Within the sampled coupling range, the computed pre-turning-point branches remain compatible with these four benchmarks. Negative values of $α$ generally shift the high-density branches toward larger masses. Positive values produce smaller shifts in the opposite direction. The microscopic sound speed becomes superluminal at high density in the WFF1 and SLy4 tables, whereas APR4 and MPA1 remain causal over their tabulated ranges. For WFF1, the causal boundary falls within a coarse table segment. Its compatibility with the $2\,M_\odot$ requirement therefore cannot be decided at the available density resolution. At $α=-20\,\mathrm{km}^{2}$, the effective-fluid sound-speed diagnostic becomes singular for WFF1, SLy4, and APR4. This singularity is absent from the explicit modified TOV system. All computed sequences remain below the standard Buchdahl reference values for compactness and surface redshift.