A Comprehensive Power Spectral Density Analysis of Astronomical Time Series. II. The Swift/BAT Long Gamma-Ray Bursts

Abstract: We investigated the prompt light curves (LCs) of long gamma-ray bursts (GRBs) from the Swift/BAT catalog. We aimed to characterize their power spectral densities (PSDs), search for quasiperiodic oscillations (QPOs), and conduct novel analyses directly in the time domain. We analyzed the PSDs using Lomb-Scargle periodograms, and searched for QPOs using wavelet scalograms. We also attempted to classify the GRBs using the Hurst exponent, $H$, and the $\mathcal{A}-\mathcal{T}$ plane. The PSDs fall into three categories: power law (PL; $P(f)\propto 1/f^\beta$) with index $\beta\in(0,2)$, PL with a non-negligible Poisson noise level (PLC) with $\beta\in(1,3)$, and a smoothly broken PL (SBPL; with Poisson noise level) yielding high-frequency index $\beta_2\in(2,6)$. The latter yields break time scales on the order of 1--100\,seconds. The PL and PLC models are broadly consistent with a fully developed turbulence, $\beta=5/3$. For an overwhelming majority of GRBs (93\%), $H>0.5$, implying ubiquity of the long-term memory. We find no convincing substructure in the $\mathcal{A}-\mathcal{T}$ plane. Finally, we report on 34 new QPOs: with one or more constant leading periods, as well as several chirping signals. The presence of breaks and QPOs suggests the existence of characteristic time scales that in at least some GRBs might be related to the dynamical properties of plasma trajectories in the accretion disks powering the relativistic jets.