Statistical properties of flux variations in blazar light curves at GeV and TeV energies

TL;DR

This study analyzes gamma-ray light curves of blazars at GeV and TeV energies, revealing that their flux variability can be modeled by an Ornstein-Uhlenbeck process and is likely driven by magnetic reconnection rather than shock acceleration.

Contribution

It introduces a novel application of Bayesian blocks and the HOP algorithm to characterize blazar flux variations and compares variability properties across different energy bands.

Findings

Flux variations are symmetric with no systematic asymmetry.

Blazar light curves follow an Ornstein-Uhlenbeck stochastic process.

Variability suggests magnetic reconnection as a plausible mechanism.

Abstract

Despite numerous detections of individual flares, the physical origin of the rapid variability observed from blazars remains uncertain. Using Bayesian blocks and the Eisenstein-Hut HOP algorithm, we characterize flux variations of high significance in the $\gamma$-ray light curves of two samples of blazars. Daily binned long-term light curves of TeV-bright blazars observed with the First G-APD Cherenkov Telescope (FACT) are compared to those of GeV-bright blazars observed with the Large Area Telescope on board the $Fermi$ Gamma-ray Space Telescope ($Fermi$-LAT). We find no evidence for systematic asymmetry of the flux variations based on the derived rise and decay time scales. Additionally, we show that the daily-binned blazar light curves can be described by an exponential stochastic Ornstein-Uhlenbeck (OU) process with parameters depending on energy. Our analysis suggests that the flux variability in both samples is a superposition of faster fluctuations. This is, for instance, challenging to explain by shock-acceleration but expected for magnetic reconnection.