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New insights on the nature of blazars from a decade of multi-wavelength observations : Discovery of a very large shift of the synchrotron peak frequency, long-term optical-gamma-ray flux correlations, and rising flux trend in the BL Lac 1ES 1215+303

Abstract : Blazars are known for their variability on a wide range of timescales at all wavelengths; and their classification (into flat spectrum radio quasars, low-, intermediate- or high-frequency-peaked BL Lac; FSRQ, LBL, IBL, HBL) is based on broadband spectral characteristics that do not consider the source being at, possibly, different states of activity. Recently, it was proposed that blazars could be classified according to the kinematics of their radio features. Most studies of TeV gamma-ray blazars focus on short timescales, especially during flares, due to the scarcity of observational campaigns or due to the relatively young existence of specialized, sensitive enough detectors.With a decade of observations from the Fermi-LAT and VERITAS, I present an extensive study of the long-term multi-wavelength variability of the blazar 1ES 1215+303 from gamma-rays to radio. This unprecedented data set reveals multiple strong gamma-ray flares and a long-term increase in the gamma-ray and optical flux baseline of the source over the ten-year period, which results in a linear correlation between these two energy bands over a decade. Typical HBL behaviors are identified in the radio morphology of the source. However, analyses of the broadband spectral energy distribution at different flux states of the source, unveil an extreme shift in energy of the synchrotron peak frequency from IR to soft X-rays; indicating that the source exhibits IBL characteristics during quiescent states and HBL behavior during high states. A two-component synchrotron self-Compton model is used to describe this dramatic change.A detailed framework of the analysis of the data from the Fermi-LAT instrument is provided, and could serve as a guideline for researchers interested in this field. I present the thorough efforts that were employed in validating the methods used and the sanity checks that were performed on the results obtained. A description of the higher-level analyses are provided, including the flare-selection algorithms, the search for harder-when-brighter behavior in the Fermi-LAT data, the multi-wavelength cross-correlation and variability analysis, the search for trends, log-normality and variability, the characterization of flares and of the spectral energy distributions, and the search for simultaneousFermi-LAT - VERITAS observations. These are the heart of this PhD work.The different methods applied and presented in this work provide a complete and detailed panorama of the intricate nature of this blazar, and possibly even challenge our current classification scheme. Moreover, this work provides an illustration of the type of long-term analyses that future imaging atmospheric instruments, such as the Cherenkov Telescope Array, will not only allow but potentially improve.
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Submitted on : Tuesday, October 20, 2020 - 4:30:11 PM
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Janeth Veronica Valverde Quispe. New insights on the nature of blazars from a decade of multi-wavelength observations : Discovery of a very large shift of the synchrotron peak frequency, long-term optical-gamma-ray flux correlations, and rising flux trend in the BL Lac 1ES 1215+303. Cosmology and Extra-Galactic Astrophysics [astro-ph.CO]. Institut Polytechnique de Paris, 2020. English. ⟨NNT : 2020IPPAX013⟩. ⟨tel-02972820⟩

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