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@ARTICLE{Dinesh:644571,
      author       = {Dinesh, A. and Dominguez, A. and Paliya, V. and Contreras,
                      J. L. and Buson, Sara and Ajello, M.},
      title        = {{A} systematic search for spectral hardening in blazar
                      flares with the {F}ermi -{L}arge {A}rea {T}elescope},
      journal      = {Astronomy and astrophysics},
      volume       = {703},
      issn         = {0004-6361},
      address      = {Les Ulis},
      publisher    = {EDP Sciences},
      reportid     = {PUBDB-2026-00410, arXiv:2507.02718},
      pages        = {A162 -},
      year         = {2025},
      note         = {$A\&A$ 703, A162 (2025). 8 pages, 3 figures, 1 table;
                      Accepted by $A\&A$},
      abstract     = {Context. Blazars, a subclass of active galactic nuclei
                      (AGNs), are among the most powerful and variable γ-ray
                      sources in the universe. They emit non-thermal radiation
                      across the electromagnetic spectrum in the form of
                      relativistic jets, characterized by rapid flux and
                      polarization variability. High synchrotron-peaked blazars
                      (HSPs) and extreme high synchrotron-peaked blazars (EHSPs),
                      with synchrotron peaks exceeding 1015 Hz and 1017 Hz,
                      respectively, are crucial for understanding the full range
                      of blazar phenomena and testing models of jet physics. Yet,
                      their understanding remains challenging.Aims. This work aims
                      to systematically identify and characterize the most extreme
                      γ-ray blazars using data from the Large Area Telescope
                      (LAT) on board the Fermi Gamma-ray Space Telescope. The
                      focus is on spectral hardening, where the γ-ray spectrum
                      becomes harder at higher energies, particularly during
                      flaring episodes. This represents the first dedicated
                      analysis of spectral hardening, as previous studies have
                      only explored this phenomenon in a few individual
                      sources.Methods. We analyzed a sample of 138 blazars
                      selected from the 4FGL-DR2 catalog with high synchrotron
                      peak frequencies and well-sampled light curves. Flaring
                      periods were selected using Bayesian Block analysis. Each
                      flare was then analyzed through γ-ray spectral fitting with
                      both power-law and broken power-law models to identify
                      potential spectral hardening. The significance of spectral
                      hardening was assessed using a test statistic, TShardening,
                      based on the likelihood ratio of the two spectral
                      models.Results. We identified two flaring episodes with
                      indications of spectral hardening, one in 4FGL
                      J0238.4−3116 and another in PKS 2155−304, the latter
                      detected independently by both selection methods but
                      referring to the same flaring period. This number of
                      candidate events is consistent with expectations from
                      statistical fluctuations, suggesting that spectral hardening
                      is, at most, a rare occurrence in γ-ray blazars. These
                      results provide the first population-level constraint on the
                      frequency of such events (< $0.1\%).$ The scarcity of events
                      reinforces the notion that the dominant blazar emission
                      mechanism is well described by smoothly varying power-law
                      spectra across the Fermi-LAT range, with sharp spectral
                      hardenings representing rare deviations likely tied to
                      exceptional jet conditions or transient physical processes.
                      Although these flares show notable spectral changes, their
                      statistical significance remains modest and motivates future
                      multi-wavelength studies to assess whether these rare flares
                      reflect genuinely distinct physical processes within blazar
                      jets.Key words: galaxies: active / BL Lacertae objects:
                      general / galaxies: jets / gamma rays: general},
      cin          = {$Z_GA$},
      ddc          = {520},
      cid          = {$I:(DE-H253)Z_GA-20210408$},
      pnm          = {613 - Matter and Radiation from the Universe (POF4-613)},
      pid          = {G:(DE-HGF)POF4-613},
      experiment   = {EXP:(DE-MLZ)NOSPEC-20140101},
      typ          = {PUB:(DE-HGF)16},
      eprint       = {2507.02718},
      howpublished = {arXiv:2507.02718},
      archivePrefix = {arXiv},
      SLACcitation = {$\%\%CITATION$ = $arXiv:2507.02718;\%\%$},
      doi          = {10.1051/0004-6361/202556241},
      url          = {https://bib-pubdb1.desy.de/record/644571},
}