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

TL;DR

This study links a 1.7-year gamma-ray QPO in PKS 2155-304 to spectral hardening events, revealing jet geometry effects and supporting a two-component jet model through advanced time-series analysis.

Contribution

It introduces a geometric masking scenario explaining spectral hardening during low-flux states in blazars, supported by 17.4 years of Fermi-LAT data analysis.

Findings

Spectral hardening is phase-locked to the QPO trough.

A softer-when-brighter chromatic trend supports a geometric origin.

Jet geometry influences the visibility of acceleration processes.

Abstract

Blazar gamma-ray variability is predominantly stochastic and well described by red-noise processes. However, a subset of sources shows 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 17.4 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.