Thermodynamic Irreversibility in Underdamped Brownian Motion with Spatial Temperature Gradients

We analyze underdamped Brownian motion in non-isothermal media with quadratic, linear, and piecewise-constant temperature profiles. Exact identities for entropy production and entropy extraction are derived, addressing whether a vanishing rate implies equilibrium. For a free particle (no external forces or ratchets), the instantaneous entropy production and extraction rates decay to zero at long times despite a spatial temperature gradient. However, their time-integrated values remain finite, demonstrating intrinsic irreversibility arising from kinetic-energy-mediated heat flow from hot to cold regions. Hence, zero entropy-production rate does not certify equilibrium in non-isothermal underdamped systems. Without loads/potentials, most thermodynamic rates vanish asymptotically while irreversibility persists via cumulative heat transfer.