Abstract
We present the results of our Power Spectral Density (PSD) analysis for the BL Lac object PKS 2155-304, utilizing the nightly-binned long-term light curve from the decade-long monitoring, as well as the minute-binned intra-night light curve from the High Energy Stereoscopic Survey (H.E.S.S.; >200 GeV). The source is unique for exhibiting the shortest flux-doubling timescale at Very High Energy (VHE) among its class and thus provides a rare opportunity to study the particle acceleration on the smallest spatial scales in blazar jets. The light curves are modeled in terms of the Continuous-Time Auto-Regressive Moving Average (CARMA) process. The combined long-term and intra-night PSD extends up to ∼6 decades in the temporal frequency range; unprecedented at the TeV energies for a blazar source. Our systematic approach reveals that PKS 2155-304 shows, on average, a complex shape of variability power spectrum, with more variability power on longer timescales. The long-term variability is best modeled by the CARMA(2,1) process, while the intra-night variability is modeled by a CARMA(1,0) process. We note that the CARMA(1,0) process refers to an Ornstein–Uhlenbeck process where the power-law PSD slope (PSD varies as a function of variability frequency to the power of the negative slope) changes from two to zero, above a certain “characteristic/relaxation” timescale. Even though the derived power spectrum of the intra-night light curve did not reveal a flattening, we speculate such relaxation must occur on timescales longer than a few hours for the source.
Highlights
Blazars, including Flat-Spectrum Radio Quasars (FSRQs) and BL Lac objects, are the most prolific class of GeV emitters in terms of apparent luminosity, constituting ∼62% of point sources in the eight-year Fermi-Large Area Telescope (LAT) catalog of Fermi-associated sources (4FGL; [1])
Power Spectral Density (PSD) characterized by β ∼ 1–2 and the log-normal shapes of fluxes have been a common feature in galactic black binary systems where they are linked to underlying accretion processes [32]
For blazar sources, the radio and γ-fluxes seem to follow a log-normal distribution [33,34], meaning that the variability is driven by a multiplicative process, and not the additive ones such as superposition of many shocks or magnetic reconnection regions
Summary
Blazars, including Flat-Spectrum Radio Quasars (FSRQs) and BL Lac objects, are the most prolific class of GeV emitters in terms of apparent luminosity, constituting ∼62% of point sources in the eight-year Fermi-Large Area Telescope (LAT) catalog of Fermi-associated sources (4FGL; [1]) Their characteristic two-peak Spectral Energy Distribution (SED) is believed to result from the nonthermal processes in a highly-magnetized relativistic nuclear jet. Large total observed luminosities (∼ 1047−48 ergs s−1 ; [7]), coupled with a factor of few intensity changes on timescales as short as minutes at the highest energies, pose several challenges to the current understanding of blazar emission scenarios [8,9]. This is due to: (1) extreme photon deficiencies at the VHE energies due to power-law shapes of HE SEDs [16] and (2) the HE (>100 MeV) spectra of blazars showing absorption features that arise due to the interaction of the NHE (>100 GeV) photons with low energy photons from the Extragalactic
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