Time-dependent photoionization simulations for the case of the light curve of the step function. At t = 0, the radiation flux is suddenly increased from 1 to 2, corresponding to a change amplitude of f = 1. This leads to a positive response in the low-ionization state (log₁₀U = −3) and a negative response in the high-ionization state (log₁₀U = 0). The dashed red lines represent the response timescales at the initial time predicted by Equation (11). The response timescales for log₁₀U = −3 and log₁₀U = 0 are 48.3 days and 5.5 days, respectively, indicating that the C IV concentration evolves more rapidly at higher ionization (log₁₀U = 0).