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 {\it Swift} and the {\it Rossi} X-ray Timing Explorer, high and very high energy $\gamma -$rays from the {\it 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 $\sim$1.8, indicative of random-walk type noise processes; (2) the power-law form of the variability power spectra at higher frequencies, from X-rays to very high energy \,$\gamma$-rays, however, have slopes $\sim$1.2, suggesting a flicker noise type process; (3) there is significantly more variability power at X-rays, high and very high energy $\gamma$-rays on timescales $\lesssim$ 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.

MNRAS accepted

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• X-ray intraday variability of the TeV blazar Mrk 421 with {\it XMM-Newton}
• Understanding the X-ray spectral curvature of Mkn 421 using broadband AstroSat observations
• Optical variability power spectrum analysis of blazar sources on intranight timescales
• Study of correlation between optical flux and polarization variations in BL Lac objects
• Broadband study of OQ 334 during its flaring state
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• Probing the disc-jet coupling in S4 0954+65, PKS 0903-57, & 4C +01.02 with $\gamma$-rays
• Multi-frequency Variability Study of Flat-Spectrum Radio Quasar PKS 0346-27
• Study of Temporal and Spectral variability for Blazar PKS 1830-211 with Multi-Wavelength Data
• Estimating longterm power spectral densities in AGN from simulations
• Optical variability of eight FRII-type quasars with 13-yr photometric light curves
• X-ray Flux and Spectral Variability of the TeV Blazars Mrk 421 and PKS 2155-304