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Spectral Hardening Revealed by Geometric De-boosting in the Masked Jet of PKS 2155$-$304

Alberto Domínguez, Adithiya Dinesh, Elena Madero

Abstract

Blazar gamma-ray variability is predominantly stochastic and well described by red-noise processes. However, a subset of sources exhibits quasi-periodic oscillations (QPOs) on year-long timescales, whose physical origin remains debated. In high-synchrotron-peaked (HSP) blazars, departures from a single power-law gamma-ray spectrum, manifested as high-energy upturns in the GeV band, may probe emission mechanisms and the intrinsic duty cycle. We investigate the link between the 1.7 yr gamma-ray QPO in PKS 2155-304 and an exceptional spectral hardening event identified in the Fermi-LAT HSP blazar population. We analyze 18 years of Fermi-LAT data using 30-day binning, applying Singular Spectrum Analysis to mitigate red-noise effects and a Moving Block Bootstrap approach to quantify the correlation between photon flux and photon index. We find a statistically significant softer-when-brighter chromatic trend, supporting a geometric origin of the flux modulation. The spectral hardening event is phase-locked to the QPO trough, implying that the hardening signature is detectable only when geometrically boosted soft emission is suppressed at the flux minimum. We propose a Geometric Masking scenario in which jet geometry regulates the visibility of acceleration processes. These results favor a two-component jet structure and suggest that spectral hardening during low-flux states, even in non-periodic sources, may reveal jet physics otherwise obscured by relativistic amplification.

Spectral Hardening Revealed by Geometric De-boosting in the Masked Jet of PKS 2155$-$304

Abstract

Blazar gamma-ray variability is predominantly stochastic and well described by red-noise processes. However, a subset of sources exhibits quasi-periodic oscillations (QPOs) on year-long timescales, whose physical origin remains debated. In high-synchrotron-peaked (HSP) blazars, departures from a single power-law gamma-ray spectrum, manifested as high-energy upturns in the GeV band, may probe emission mechanisms and the intrinsic duty cycle. We investigate the link between the 1.7 yr gamma-ray QPO in PKS 2155-304 and an exceptional spectral hardening event identified in the Fermi-LAT HSP blazar population. We analyze 18 years of Fermi-LAT data using 30-day binning, applying Singular Spectrum Analysis to mitigate red-noise effects and a Moving Block Bootstrap approach to quantify the correlation between photon flux and photon index. We find a statistically significant softer-when-brighter chromatic trend, supporting a geometric origin of the flux modulation. The spectral hardening event is phase-locked to the QPO trough, implying that the hardening signature is detectable only when geometrically boosted soft emission is suppressed at the flux minimum. We propose a Geometric Masking scenario in which jet geometry regulates the visibility of acceleration processes. These results favor a two-component jet structure and suggest that spectral hardening during low-flux states, even in non-periodic sources, may reveal jet physics otherwise obscured by relativistic amplification.
Paper Structure (10 sections, 2 figures)

This paper contains 10 sections, 2 figures.

Table of Contents

  1. Introduction
  2. Data and Methodology
  3. Results
  4. Global Spectral Behavior
  5. Phase-Locked Spectral Hardening
  6. Discussion
  7. The Geometric Masking Scenario
  8. Constraints on Jet Structure and Extreme States
  9. Comparison with PG 1553+113 and Population Context
  10. Summary and Conclusions

Figures (2)

  • Figure 1: Gamma-ray light curve of PKS 2155$-$304 (30-day bins, 2008--2026): Fermi-LAT flux (blue circles, $1\sigma$ errors), the SSA baseline (red line) used for detrending, and the reconstructed QPO component (green dashed line).
  • Figure 2: Spectral variability of PKS 2155$-$304. Left: Photon index vs. photon flux, showing a positive correlation ($\rho \approx 0.3$), weak but robust, indicative of an atypical softer-when-brighter trend. Right: Phase-resolved spectral evolution folded at the 1.7 yr period, with the 18-year baseline variability (green dots) consistent with an achromatic trend given the lack of significant correlation between photon index and phase ($\rho = 0.036$, $p = 0.60$), and the two consecutive 30-day bins of the extreme spectral hardening event reported by Dinesh2025.