X-ray and TeV emissions from High Frequency Peaked BL Lacs

TL;DR

This paper investigates the X-ray and TeV gamma-ray emissions from high frequency peaked BL Lacs, proposing an acceleration model that links spectral curvature in X-rays to gamma-ray luminosity, enhancing understanding of their emission mechanisms.

Contribution

It introduces a coherent and stochastic acceleration model explaining spectral features and their relation to gamma-ray emission in HBLs, based on spectral parameter analysis.

Findings

Spectral peak energies are similar for TeV detected and undetected HBLs.

Broader X-ray spectra are observed in TeV detected HBLs.

Spectral curvature correlates with gamma-ray luminosity, linking X-ray and TeV emissions.

Abstract

The majority of the extragalactic sources yet detected at TeV photon energies belong to the class of "high frequency peaked BL Lacs" (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 a considerable curvature, and widely interpreted as synchrotron emission from ultrarelativistic electrons outflowing in a relativistic jet; these are expected to radiate also in gamma-rays by 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 E_p 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 E_p 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, that depends only on the latter acceleration component, can be related to the inverse Compton luminosity in gamma-rays, so introducing a link between the X-ray and the TeV observations of HBLs.