Thermal timescale of an accretion disk
= Thermal timescale of an accretion disk
{title2=$t_{\rm th}$}
The thermal timescale compares the internal energy stored in a disc annulus with its local heating or cooling. In a gas-pressure-supported <alpha disk>, $t_{\rm th}\sim(\alpha\Omega_K)^{-1}$, whereas the <viscous timescale> is longer by approximately $(R/H)^2$. A local thermal perturbation is therefore studied at fixed <surface density of a disk>, rather than fixed steady-state <mass accretion rate>.