X-Ray and TeV Emissions from High-frequency-peaked BL Lac Objects

Themajority of the extragalactic sources yet detected at TeV photon energies belong to the class of “high-frequencypeaked BL Lac objects” (HBLs) that exhibit a spectral energy distribution with a lower peak in the X-ray band. Such spectra are well described in terms of a log-parabolic shape with considerable curvature, and widely interpreted as synchrotron emission from ultrarelativistic electrons outflowing in a relativistic jet; these are expected to radiate also in γ -rays through the inverse Compton process. Recently, we have compared the X-ray spectral parameter distributions of TeV detected HBLs (TBLs) with those undetected (UBLs), and found that the distributions of the peak energies Ep are similarly symmetric around a value of a few keVs for both subclasses, while the X-ray spectra are broader for TBLs than for UBLs. Here we propose an acceleration scenario to interpret both the Ep and the spectral curvature distributions in terms of a coherent and a stochastic acceleration mechanisms, respectively. We show how the curvature parameter b~0.3–0.7 of the synchrotron X-rays, which depends only on the latter acceleration component, can be related to the inverse Compton luminosity in γ -rays, thus introducing a link between the X-ray and the TeV observations of HBLs.