A time sensitive modelling of the two extreme BL Lacertae objects 1ES 1218+304 and 1ES 0229+200

In this thesis, the broadband spectral energy distribution (SED) of the sources 1ES 1218+304 and 1ES 0229+200 is studied. 1ES 1218+304 and 1ES 0229+200 are two well-known extreme high-frequency-peaked BL Lac objects (EHBLs), a type of blazar. Blazars are in turn a type of active galactic nuclei (AGN) with the presence of a jet of relativistic charged particles where, by random chance, the jet points towards Earth. Data taken by the ultraviolet/optical telescope UVOT, and the X-ray telescope XRT, on board the Neils Gehrels Swift Observatory satellite, are used. The ultraviolet/optical and X-ray data are analysed together with gamma-ray data in the 30 MeV to 300 GeV range taken by the Fermi-LAT detector and in the 50 GeV to 30 TeV range, provided by the VERITAS Collaboration. The light curves of each energy band were built and analysed with a Bayesian blocks algorithm. The intrinsic spectral indices in the VHE range were found to be Γ𝑉 𝐻 𝐸 ≳ 2, except during the quietest period of 1ES 0229+200, from 57419 MJD to 58017 MJD, where the intrinsic spectral index was found to be Γ𝑉 𝐻 𝐸 = 1.2 ± 0.4. Evidence of non-stationary extreme states was found, after analysis of the frequencies and luminosities of the synchrotron peak relative to the TeV slope 𝑆 = 2 − Γ𝑉 𝐻 𝐸. The Bayesian blocks were used to select four periods of data from each source, and SEDs were constructed from the combined data from these periods. A one-zone leptonic synchrotron self-Compton (SSC) emission scenario is used to describe the broadband SEDs. We used a Monte Carlo Markov chain (MCMC) to sample the parameter space of the SSC model and obtain the median and asymmetric 1-sigma standard deviations. The values of the minimum Lorentz factors of electrons are found to be compatible with 1, while the values of the Doppler factor 𝛿𝐷 vary between 10 and 90. The estimated values of the equipartition parameter 𝑘𝑒𝑞 (the ratio between the electrons energy density and the magnetic field energy density) have a maximum of 104 and a minimum of 2, where the latter is not observed in the literature.