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Being situated on the Northern hemisphere, FACT will mostly observe extragalactic objects. Thanks to its technical improvements, the sensitivity of FACT is significantly better than that of the original HEGRA telescope. The scientific aims of the project is the long-term monitoring of bright active galactic nuclei to
BlazarsBlazars are Active Galactic Nuclei (AGNs) characterized by a polarised and highly variable non-thermal continuum emission, extending from radio to γ-rays, produced within a relativistic jet that originates in the central engine and points close to our line of sight. Since the relativistic outflow moves with a bulk Lorentz factor (Γ) and is observed at small angles, the emitted fluxes are affected by a beaming factor δ. The broadband spectral energy distribution (SED) exhibits typically a two bump shape. According to current models, the low energy bump is interpreted as synchrotron emission from highly relativistic electrons, and the high energy bump is related to inverse Compton emission of various underlying radiation fields: the same synchrotron photons in the case of synchrotron self Compton model (SSC), and UV and/or IR thermal photons produced externally to Jet in the case of external radiation Compton (ERC) scenario. Blazars come in two main flavours: BL Lac objects, characterised by featureless optical spectra and interpreted in the framework of pure SSC scenario, and Flat Spectrum Radio Quasars (FSRQ) characterized by prominent emission lines which are likely to have the IC component dominated by the ERC emission. BL Lac objects are often classified according to the peak energy of the synchrotron emission, which reflects the maximum energy the particles can be accelerated in the jet, into: low energy (LBLs), intermediate energy (IBLs), and high energy synchrotron peak objects (HBLs) respectively called LBL, IBL and HBL. HBLs are the largest fraction of Blazars detected at TeV energies, where exhibit often rapid variability, down to the scale of minutes. Acceleration process able to produce particle radiating at TeV energies and the understanding of such a rapid variability, are still open issues, despite the large number of observations, weakened often by the lack of simultaneous multiwavelength data and/or long term sampling. Particle Acceleration
HBLs represent the most extreme accelerator among Blazars. Their synchrotron component, that is typically
peaking at X-ray energies, is often detected up to hard-X-ray energies. The Lorentz factor of the particles
radiating at X-ray and hard-X-ray energies is of the order of γ ≃ 106 Rapid Variability
The Rapid variability observed in TeV emitting Blazar, due to the very short cooling times of the high
energy emitting electrons, gives strong constraints on the size of the accelerating/emitting region, R ≤ ![]() Rapid variability of the blazar PKS 2155-304 observed in the TeV energy range by HESS (top) and as it would have been observed by the much smaller FACT telescope (bottom). Another puzzling feature is represented by TeV orphan flares, as in the case of PKS1959-650 in spring/summer 2002. Such behavior challenges strongly one-zone models and represent further observational constraints to the theoretical models parameters. Also in this case, the FACT monitoring program will be relevant, by increasing the possibility to observe further TeV orphan flares. ![]() ![]() ![]() ![]() ![]() |
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