Time-frequency domain illustrating filtering signals with frequencies slowly wandering in time, that appear as trajectories with finite time-derivative (top panel). Schematically, these trajectories passing through “butterflies" (green) defined by a finite slew rate (time rate-of-change) in frequency |df(t)/dt| ≥ δ > 0 (insert). It is realized by matched filtering over a dense bank of chirp-like time-symmetric templates of intermediate duration. (Reprinted from van Putten (2016).) Identification of broadband Kolmogorov spectrum in an ensemble averaged spectrum to the Nyquest frequency of 1 kHz of 42 bright long GRBs in the BeppoSAX catalog by butterfly filtering over a dense bank of 8.64 million templates, shown as a purple line (bottom panel). This result demonstrates sensitivity to turbulence better than an order of magnitude compared to conventional Fourier analysis (blue). Note: reprinted from van Putten et al. (2014a).