Characterising the emission mechanisms of Extreme High Synchrotron Peak blazars: a CTAO pathfinder project

Blazars are jetted AGNs with spectral energy distribution (SED) characterised by two broad peaks often attributed respectively to synchrotron and inverse Compton emission from the jets. The precise frequency of the peaks is variable and is used for classification: High Synchrotron Peak (HSP) and Extreme High Synchrotron Peak (EHSP) blazars are those displaying the synchrotron peak in the UV to hard x-ray band (∼0.1-500 keV), and they are particularly interesting for studies in the Very High Energy band (VHE, i.e., > 100 GeV). In fact, in BL Lacs the known correlation between the x-ray and the VHE emission suggests that a significant number of x-ray bright HSP/EHSP blazars could be detected in the VHE band by next-generation Cherenkov telescopes. For this reason, in this work I studied x-ray and γ-ray data of a sample of HSP/EHSP blazars that were detected in the x-rays, but lacked a counterpart in the 4FGL-DR4 catalog from Fermi-LAT, to test their viability as extreme emitters and evaluate their detectability in the TeV regime with the Cherenkov Telescope Array Observatory (CTAO). I performed an x-ray analysis, by retrieving data from multiple telescopes and fitting their spectra using both a powerlaw and a log parabola model to select, by comparing the C-stat of both fits, the sources best described by a logparabola model, a result that supports a scenario where the synchrotron peak falls in the x-rays. I then modelled the broadband SED of 9 blazars selected among the sources with x-ray spectrum best fitted with a log parabola, using two models, one assuming a log parabola electron distribution and the other one with a broken power law electron distribution, using parameters consistent with those derived by Hota et al. (2024), and I constrained the models with Fermi-LAT upper limits I personally derived. The models resulted to be generally consistent with the available data and confirmed that a subsample of sources have a significant chance of detection by CTAO.