Time-dependent photoionization solutions: starting from the photoionization equilibrium for the 2015b epoch (shown in the bottom panel of Fig. 3), we assume a step-function flux increase of 13% (see text), and track the changes in the relative fraction of N V for a range of n_e. The log(n_e) of each solution labels each curve. The shortest time-scale where we measured definitive changes in the N V trough between two epochs was 700 000 s. In red we mark the N V fraction curve that shows a 50% change between the initial and final values after 700 000 s.