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| Natura: | Preprint |
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2026
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| Accesso online: | https://arxiv.org/abs/2603.04529 |
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| _version_ | 1866917412661624832 |
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| author | Domínguez, Alberto Dinesh, Adithiya Madero, Elena |
| author_facet | Domínguez, Alberto Dinesh, Adithiya Madero, Elena |
| contents | 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. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2603_04529 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | Spectral Hardening Revealed by Geometric De-boosting in the Masked Jet of PKS 2155$-$304 Domínguez, Alberto Dinesh, Adithiya Madero, Elena High Energy Astrophysical Phenomena Astrophysics of Galaxies Blazar gamma-ray variability is predominantly stochastic and well described by red-noise processes. However, a subset of sources shows quasi-periodic oscillations (QPOs) on year-long timescales, whose physical origin remains debated. In high-synchrotron-peaked (HSP) blazars, departures from a single power-law gamma-ray spectrum, manifested as high-energy upturns in the GeV band, may probe emission mechanisms and the intrinsic duty cycle. We investigate the link between the 1.7 yr gamma-ray QPO in PKS 2155-304 and an exceptional spectral hardening event identified in the Fermi-LAT HSP blazar population. We analyze 17.4 years of Fermi-LAT data using 30-day binning, applying Singular Spectrum Analysis to mitigate red-noise effects and a Moving Block Bootstrap approach to quantify the correlation between photon flux and photon index. We find a statistically significant softer-when-brighter chromatic trend, supporting a geometric origin of the flux modulation. The spectral hardening event is phase-locked to the QPO trough, implying that the hardening signature is detectable only when geometrically boosted soft emission is suppressed at the flux minimum. We propose a Geometric Masking scenario in which jet geometry regulates the visibility of acceleration processes. These results favor a two-component jet structure and suggest that spectral hardening during low-flux states, even in non-periodic sources, may reveal jet physics otherwise obscured by relativistic amplification. |
| title | Spectral Hardening Revealed by Geometric De-boosting in the Masked Jet of PKS 2155$-$304 |
| topic | High Energy Astrophysical Phenomena Astrophysics of Galaxies |
| url | https://arxiv.org/abs/2603.04529 |