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This paper asks whether some compact objects found by gravitational-wave and X‑ray observatories — objects with masses in the so‑called mass gap (2.5–5 times the mass of the Sun) — could be neutron stars if gravity is slightly different from Einstein’s theory. The authors work in a simple version of f(R,T) gravity, a modification of general relativity that adds a small coupling between matter and geometry. They combine this gravity model with a realistic microscopic description of dense neutron matter to compute neutron star structure.
To build their models the team used an equation of state (EoS) derived from microscopic nuclear interactions, the AV18 potential, computed with the lowest‑order constrained variational (LOCV) method. They included the effects of strong magnetic fields, which make the pressure inside the star different in the radial and tangential directions (pressure anisotropy). The magnetic field was modeled as varying with density using a Gaussian profile, with representative surface values of 5×10^16 and 1×10^17 gauss and a central value up to 2×10^18 gauss. Anisotropy in the fluid was introduced using a standard Bowers–Liang prescription, and the equilibrium stars were found by solving the modified Tolman–Oppenheimer–Volkoff equations appropriate to f(R,T)=R+2λT gravity (λ is the matter–geometry coupling).
Their results show clear trends. Increasing the surface magnetic field tends to soften the EoS (the matter resists compression less) and so reduces the maximum mass and radius of the models. At fixed gravity coupling λ, increasing the anisotropy parameter β raises both the maximum mass and the radius. Conversely, at fixed β, decreasing the coupling λ produces larger masses and radii. With suitable choices of λ and anisotropy, some models reach masses in the 2.5–3 Msun range. The authors report that these configurations can reproduce the measured properties of several observed systems, including GW170817, the pulsars PSR J0952−0607 and PSR J0740+6620, and the secondary (less massive) components reported for GW190814 (m2 = 2.59+0.08−0.09 Msun) and GW200210‑092254 (m2 = 2.83+0.47−0.42 Msun). They also check simple diagnostics (Schwarzschild radius and surface gravitational redshift) and find values consistent with visible neutron stars rather than black holes (for example the surface redshift remains below about 0.4 in their models).
Why this matters: if confirmed, the result would mean that at least some of the compact objects sitting in the mass gap might be massive neutron stars described by a modified theory of gravity, rather than unusually small black holes. The study uses a realistic, microscopic nuclear interaction (AV18) rather than a simple toy EoS. That makes the conclusions about how gravity, anisotropy and magnetic fields interact to set maximum masses more physically grounded than many earlier parametric studies.
Important caveats and uncertainties remain. The results depend on several specific modeling choices: the particular f(R,T) form (R+2λT), the AV18 equation of state, the Bowers–Liang model for anisotropy, and a two‑body truncation in the LOCV calculation. Internal magnetic fields in neutron star cores are not directly measured, so the chosen magnetic profiles and strengths are only plausible scenarios. The authors note that a more robust assessment would marginalize over a wide family of EoS models and perform a joint statistical inference of gravity and matter parameters. In short, the paper shows a viable possibility, but the claim that specific mass‑gap objects are neutron stars depends on the model choices and parameter values used here.