Joint PDF(t_s, τ_s) in Eq. (7) from merged (H1,L1)-spectrograms by χ-image analysis following time-slide analysis (top-left panels). Clustering is seen in the (t_s, τ_s)-plane about a global maximum in the extended foreground over small time-slides Eq. (11). (The peak in the cluster in extended foreground (S₁) is slightly larger than what is seen in conventional foreground (S₀).) This cluster is extremal in the sense of Eq. (13). Tracing produces PDF’s of τ_s and the delay time t_s − t_m, where t_m is the merger time of GW170817. There is notable consistency in τ_s with the duration s of GRB170817A (Pozanenko et al. 2018), expected from the timescale of black hole spin-down in interaction with surrounding high-density matter following gravitational collapse of the hyper-massive neutron star in the immediate aftermath of this merger. Clustering in τ_s and t_s as a function of time-slide Δt in the post-merger emission to GW170817 extracted from (H1,L1)-spectrograms merged by frequency coincidences |f_H1 − f_L1| < 10 Hz (top-right panels). Contour plots of (maximal over τ_s, f_s, f₀) over all time-slides Δt ϵ [−32, 32] s (S_0 − 2) in the bottom panels. The right panel zooms in on Δt ϵ [−1, 1] s, highlighting clustering within Δt ϵ [−τ, τ] set by the intermediate time scale τ ≃ 0.5 s of our template bank. Crucially, this cluster is extremal in the sense of Eq. (13) in extended foreground (S₁).