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@ARTICLE{Adhikari:642083,
      author       = {Adhikari, Sagar and Peñil, Pablo and Domínguez, Alberto
                      and Ajello, Marco and Buson, Sara and Rico, Alba},
      title        = {{D}ecade-long periodicity study of 2{FHL} blazars with
                      historical optical data},
      reportid     = {PUBDB-2025-05345, arXiv:2409.18334},
      year         = {2025},
      note         = {Funding for APPLAUSE has been provided by DFG (German
                      Research Foun-dation, Grant)},
      abstract     = {In our recent investigation, we utilized a century's worth
                      of archival optical data to search for a decade-long
                      periodicity from the blazar PG 1553+113, finding a hint of a
                      22-yr period. Building on this foundation, the current study
                      extends our analysis to include 10 blazars from the
                      Fermi-Large Area Telescope Second Catalog of Hard Sources
                      (2FHL) catalogue to uncover similar long-term periodic
                      behaviour. To ensure the reliability of our findings, we
                      consider the impact of observational limitations, such as
                      temporal gaps and uneven sampling, which could potentially
                      introduce artefacts or false periodic signals. Our analysis
                      initially identifies decade-scale periodicity in four of
                      these blazars (AP Librae, MKN 421, MKN 501, PG 1246+586).
                      However, further investigation reveals that three of these
                      are likely influenced by noise and poor sampling. The most
                      promising candidate, approximately 51 $\pm$ 9 yr signal in
                      MKN 421, corresponds to fewer than three full cycles and
                      cannot be considered significant. Furthermore, global
                      significance suggests none of the candidate periodicities
                      meet the threshold for statistical significance. These
                      results underscore the importance of accounting for sampling
                      artefacts and highlight the need for robust methodologies in
                      long-term periodicity searches.},
      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)25},
      eprint       = {2409.18334},
      howpublished = {arXiv:2409.18334},
      archivePrefix = {arXiv},
      SLACcitation = {$\%\%CITATION$ = $arXiv:2409.18334;\%\%$},
      doi          = {10.3204/PUBDB-2025-05345},
      url          = {https://bib-pubdb1.desy.de/record/642083},
}