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Recurrence plot analysis of blazar gamma-ray light curves: Exploiting the time-domain capabilities of Fermi-LAT

Andrea Gokus, Rebecca Phillipson

TL;DR

This paper addresses nonstationary variability in blazar gamma-ray light curves and proposes recurrence plot (RP) analysis as a time-domain method that does not require stationarity. Using Fermi-LAT observations of Mkn 421 and PKS 1424-41, the authors reconstruct phase space with a time-delay embedding and quantify determinism and nonlinearity via $DET$ and $L_{max}$, comparing to $IAAFT$ surrogates. They find that Mkn 421 is more deterministic than PKS 1424-41, and both sources show signs of nonlinearity, though nonstationarity complicates interpretation. The study lays groundwork for extending RP analysis to about $40$-$50$ gamma-ray bright blazars to probe jet dynamics and quasi-periodic behavior across blazar classes.

Abstract

Variability studies of jetted AGN, in particular blazars, have been used to gain a better understanding of the particle acceleration mechanisms in jets. However, statistical methods used for the characterization of variability often rely on stationary time series data, which is not fulfilled for most blazar light curves. We introduce the recurrence plot method for long-term $γ$-ray light curves sampled by Fermi-LAT and present our results for the BL Lac object Mkn 421 and the FSRQ PKS 1424-41. Using surrogates to determine the significance of our findings, we conclude that Mkn 421 exhibits more determinism than PKS 1424-41, and that both sources potentially show nonlinearity. However, the latter has to be tested against more advanced surrogates that are able to replicate the nonstationarity of the original light curves. In future work, we will extend our recurrence analysis to a sample of $\sim50$ $γ$-ray bright sources to probe the jet dynamics of different blazar classes.

Recurrence plot analysis of blazar gamma-ray light curves: Exploiting the time-domain capabilities of Fermi-LAT

TL;DR

This paper addresses nonstationary variability in blazar gamma-ray light curves and proposes recurrence plot (RP) analysis as a time-domain method that does not require stationarity. Using Fermi-LAT observations of Mkn 421 and PKS 1424-41, the authors reconstruct phase space with a time-delay embedding and quantify determinism and nonlinearity via and , comparing to surrogates. They find that Mkn 421 is more deterministic than PKS 1424-41, and both sources show signs of nonlinearity, though nonstationarity complicates interpretation. The study lays groundwork for extending RP analysis to about - gamma-ray bright blazars to probe jet dynamics and quasi-periodic behavior across blazar classes.

Abstract

Variability studies of jetted AGN, in particular blazars, have been used to gain a better understanding of the particle acceleration mechanisms in jets. However, statistical methods used for the characterization of variability often rely on stationary time series data, which is not fulfilled for most blazar light curves. We introduce the recurrence plot method for long-term -ray light curves sampled by Fermi-LAT and present our results for the BL Lac object Mkn 421 and the FSRQ PKS 1424-41. Using surrogates to determine the significance of our findings, we conclude that Mkn 421 exhibits more determinism than PKS 1424-41, and that both sources potentially show nonlinearity. However, the latter has to be tested against more advanced surrogates that are able to replicate the nonstationarity of the original light curves. In future work, we will extend our recurrence analysis to a sample of -ray bright sources to probe the jet dynamics of different blazar classes.

Paper Structure

This paper contains 3 sections, 3 figures.

Table of Contents

  1. Introduction
  2. Method
  3. Results & Discussion

Figures (3)

  • Figure 1: Light curves of the BL Lac object Mkn 421 (left) and the FSRQ PKS 1424$-$41 (right) covering a time range from August 2008 until October 2024. All uncertainties and bins with TS$<1$ are plotted in grey.
  • Figure 2: Recurrence plots of Mkn 421 (left) and PKS 1424$-$41 (right) derived from the lightcurves shown in Fig. \ref{['fig:lcs']}. For both RPs, ten percent of the matrix is filled, meaning our choice of recurrence rate is 0.1.
  • Figure 3: Two RP measures as a function of recurrence rate for Mkn 421 (left) and PKS 1424$-$41 (right). The black line corresponds to the RP measure for the source light curve, while the green vertical bars represent the range of values for the ensemble of surrogates. Top row: The DET measure, which probes the determinism and predictability of the source. Bottom row: The Lmax measure, which is a proxy for nonlinearity in the source.