2026/06/30 by M. H. Naddaf, M. Ghasemnezhad, H. Ghanbarnejad +2
Physics and Astronomy · #astro-ph.HE #astro-ph.IM
The standard α-disk formalism parametrizes turbulent angular momentum transport through a dimensionless coefficient α, assumed to be spatially and thermodynamically invariant. While analytically convenient, this assumption leads to the well-known thermal and viscous instabilities in radiation-pressure dominated (RPD) regions. We show that this instability is not the consequence of radiation pressure, but is due to enforcing a constant α across distinct thermodynamic regimes. Requiring the steady thin-disk (TD) to remain thermally stable and single-valued in the M--Σ plane yields a necessary condition on the stress response, expressed as η\rm x ≡ dlnα\rm x / dln X > 4/7, where X ≡ P\rm gas/P\rm rad. The resulting viscosity law α\rm x ≡ α(X) emerges directly from the internal consistency of TD equations, without modifying the stress law or invoking any additional physics. α\rm x removes the RPD unstable branch. The disk structure becomes smooth and globally single-valued, with higher Σ and τ in the inner RPD disk, while preserving the standard effective-temperature profile. This increases thermal and inflow timescales, offering a natural route to accretion-state dependent variability without large-amplitude radiation-pressure limit cycles. It also motivates revisiting AGN disk tensions, including microlensing sizes and continuum reverberation lags with improved radiative-transfer modeling. The results show that the RPD instability, and possibly some associated AGN disk tensions, reflect an inconsistent viscosity closure.