Synthetic spectra for the quiescent phase in IRAS 15398, scaled as indicated. The different colors correspond to the same post-burst time steps as in Fig. 1: 10 yr (black), 150 yr (red), 800 yr (gray), 2600 yr (purple), 6200 yr (green), 24 000 yr (orange), and 1 × 10⁵ yr (blue). The arrows indicate how the intensity of each line changes as a function of time; the colored flags mark the peak intensities at the listed times. The N₂H⁺ 1–0 panel shows the isolated F₁ = 0−1 hyperfine component, whereas the 6–5 panel shows the sum over all unresolved components.