Testing linear non-minimal coupling in f(Q,T) gravity using X-ray binary pulsars constrained by NICER observations

Errehymy A. Maurya S.K. Donmez O. Umbetova Z. Rayimbaev J. Khashan M.M. Eid M.R.
December 2025 Elsevier B.V.

Physics of the Dark Universe
2025 #50

X-ray binary pulsars are valuable for testing alternative gravity theories like f(Q,T) gravity because they host neutron stars (NSs) with strong gravitational fields and measurable matter interactions. The linear non-minimal coupling between Q and T can affect the stars structure and emission, which Neutron Star Interior Composition Explorer (NICER) can constrain through high-precision X-ray timing and spectral data. In this work, we explore the physical implications of such couplings within the f(Q,T)=χ₁Q+χ₂T gravity framework, focusing on realistic, anisotropic stellar configurations. Using analytical methods, we model spherically symmetric NSs and calibrate our approach against well-observed pulsars including Her X-1, 4U 1538-52, and SMC X-1. The resulting configurations satisfy essential physical requirements: positive, finite energy density and pressure profiles; regularity at the center; vanishing radial pressure at the surface; and stable, causal behavior throughout. Pressure anisotropy remains positive, contributing to hydrostatic balance via a repulsive force that opposes gravity. The radial and tangential sound speeds remain subluminal, and the adiabatic index Γ exceeds 4/3 across the star, increasing outward—indicating resistance to collapse under perturbations. The M−R relations predicted by the model align well with observations. For example, with χ₁=0.8, we find M≈0.95,M_⊙ and R≈9.18 km; increasing χ₁ to 1.0 gives M≈1.19,M_⊙ and R≈11.39 km. Similarly, for χ₂=0.0, we obtain M≈1.21,M_⊙ and R≈11.61 km, while increasing χ₂ to 0.8 yields M≈1.19,M_⊙ and R≈11.38 km. These trends highlight the role of χ₁ and χ₂ in controlling stellar compactness and stiffness. The model naturally captures a broad range of NS configurations, without relying on a fixed equation of state (EOS), and remains consistent with nuclear saturation density constraints. Altogether, these results establish f(Q,T) gravity as a physically robust and observationally viable extension of general relativity (GR)—one capable of capturing the internal dynamics of compact stars with precision.

Exact solutions , f(Q, T) gravity theory , X-ray binary pulsars

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Astrophysics Research Centre, School of Mathematics, Statistics and Computer Science, University of KwaZulu-Natal, Private Bag X54001, Durban, 4000, South Africa
Department of Mathematical and Physical Sciences, College of Arts and Sciences, University of Nizwa, Nizwa 616, Oman
Research Center of Astrophysics and Cosmology, Khazar University, 41 Mehseti Street, AZ, Baku, 1096, Azerbaijan
College of Engineering and Technology, American University of the Middle East, Egaila, 54200, Kuwait
Department of General and Theoretical Physics, L.N. Gumilyov Eurasian National University, Astana, 010008, Kazakhstan
Institute of Theoretical Physics, National University of Uzbekistan, Tashkent, 100174, Uzbekistan
Department of Information Technologies, Tashkent International University of Education, Imom Bukhoriy 6, Tashkent, 100207, Uzbekistan
University of Tashkent for Applied Sciences, Str. Gavhar 1, Tashkent, 100149, Uzbekistan
Institute of Fundamental and Applied Research, National Research University TIIAME, Kori Niyoziy 39, Tashkent, 100000, Uzbekistan
Tashkent State Technical University, Tashkent, 100095, Uzbekistan
Department of Basic Sciences, Common First Year, King Saud University, Riyadh, 11451, Saudi Arabia
Center for Scientific Research and Entrepreneurship, Northern Border University, Arar, 73213, Saudi Arabia

Astrophysics Research Centre
Department of Mathematical and Physical Sciences
Research Center of Astrophysics and Cosmology
College of Engineering and Technology
Department of General and Theoretical Physics
Institute of Theoretical Physics
Department of Information Technologies
University of Tashkent for Applied Sciences
Institute of Fundamental and Applied Research
Tashkent State Technical University
Department of Basic Sciences
Center for Scientific Research and Entrepreneurship

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