Eight response curves of the two-level pipeline by χ-image analysis of merged (H1,L1)-spectrograms produced by butterfly matched filtering using a total of 56 signal injection experiments on LIGO O2 data covering GW170817 (top-left panel). These model injections are defined by Eq. (7). The dashed line is the mean of -peaks averaged over (tiny) time-sides Δt within the light-travel time of 10 ms between H1 and L1 identified with extended emission to GW170817EE in a post-merger descending branch (lower panels). Precision realized is shown by parameter recovery (top-right panel). The parameter “field-of-view” covered for descending chirps covers energies 0 < ℰ_gw < 8% M_⊙c² (7 steps) and timescales of descent τ_s (eight steps, in units of seconds) with start frequency f_s = 680 Hz for a model source distance D = 40 Mpc given by GW170817. These injection signals are in the shot-noise dominated regime of the LIGO detectors, where noise increases with frequency and hence sensitivity decreases with τ_s. Inserted is a plot of observed τ_s of post-merger emission to GW170817 (black square), overlaid to the calibration curve (blue curve) to infer uncertainty (along abscissa) from observed scatter (along the ordinate). Sample of a single injection of a combined merger and post-merger signal (1810–1825 s) in the snippet of H1L1-data containing GW170817 (t_m = 1842.43 s) (reprinted from Paper II), shown in the lower panel. The present 56 injections Eq. (7) vary over energy ℰ_GW and time-scale of descent τ_s, keeping merger signal injection fixed at the parameters of GW170817.