Blazar variability power spectra from radio up to TeV photon energies: Mrk 421 and PKS 2155−304

Abstract

We present the results of the power spectral density (PSD) analysis for the blazars Mrk 421 and PKS 2155−304, using good-quality, densely sampled light curves at multiple frequencies, covering 17 decades of the electromagnetic spectrum, and variability timescales from weeks up to a decade. The data were collected from publicly available archives of observatories at radio from OVRO, optical and infrared (B, V, R, I, J, H, and K-bands), X-rays from the Swift and the Rossi X-ray Timing Explorer, high and very high energy γ−rays from the Fermi and Very Energetic Radiation Imaging Telescope Array System as well as the High Energy Stereoscopic System. Our results are: (1) the power-law form of the variability power spectra at radio, infra-red and optical frequencies have slopes ∼1.8, indicative of random-walk type noise processes;(2) the power-law form of the variability power spectra at higher frequencies, from Xrays to very high energy γ-rays, however, have slopes ∼1.2, suggesting a flicker noise type process; (3) there is significantly more variability power at X-rays, high and very high energy γ-rays on timescales 100 days, as compared to lower energies. Our results do not easily fit into a simple model, in which a single compact emission zone is dominating the radiative output of the blazars across all the timescales probed in our analysis. Instead, we argue that the frequency-dependent shape of the variability power spectra points out a more complex picture, with highly inhomogeneous outflow producing non-thermal emission over an extended, stratified volume.

A comprehensive power spectral density analysis of astronomical time series I: the Fermi-LAT gamma-ray light curves of selected blazars

“…Our results are consistent with Bhatta & Dhital (2020), except for 3C 454.3, for which the authors reported β = 1.3 ± 0.17, while we obtained β ∼ 2. Finally, regarding Mrk 421 and PKS 2155−304, we obtained values broadly consistent with those of Goyal (2020).…”

X-ray Flux and Spectral Variability of the TeV Blazars Mrk 421 and PKS 2155-304

“…PSDs were well fitted with power-law, and their slopes were found in the range of 2.1-2.3 [109]. PSD analysis of long term Swift X-ray observations of PKS 2155-304 fitted well with power-law, and its slope was found to be 1.3 ± 2.1 [65].…”

“…PSD analysis of long term RXTE and Swift X-ray observations of Mrk 421 were well fitted with power-law. For RXTE and Swift X-ray light curves, the PSD slopes were found to be 1.1 ± 1.6 and 1.3 ± 0.7, respectively [65].…”

Simulations of Stochastic Long-Term Variability in Leptonic Models for External-Compton and Synchrotron Self-Compton Dominated Blazars

“…No significant distinctive differences are present among the different test cases. This indicates that the one-zone emission model might be incapable of producing light curve variability that posses different PSD indices for different wavebands (Goyal 2020).…”