A Two-Zone Model as origin of Hard TeV Spectrum in Extreme BL Lacs

TL;DR

This paper introduces a two-zone lepto-hadronic model to explain the complex multi-wavelength spectra of extreme BL Lac objects, addressing challenges faced by traditional one-zone models in reproducing their hard gamma-ray spectra.

Contribution

The work presents a novel two-zone lepto-hadronic model that successfully explains the multi-wavelength observations of extreme BL Lacs, incorporating photo-hadronic processes near the AGN core.

Findings

Model reproduces VHE gamma-ray spectra via photo-hadronic interactions.

Parameter values are consistent with typical BL Lac ranges.

Outer blob explains X-ray synchrotron emission.

Abstract

The emission of the so-called extreme BL Lacs poses challenges to the particle acceleration models. The hardness of their spectrum, $\lesssim 2$, in the high-energy band demands unusual parameters using the standard one-zone synchrotron self-Compton (SSC) model with a deficient magnetized plasma. Some authors use either two-zone or hadronic/lepto-hadronic models to relax these atypical values. In this work, we present a lepto-hadronic two-zone model to explain the multi-wavelength observations of the six best-known \textit{extreme} BL Lacs. The very-high-energy gamma-ray observations are described by the photo-hadronic processes in a blob close to the AGN core and by SSC and external inverse Compton-processes in an outer blob. The photo-hadronic interactions occur when accelerated protons in the inner blob interact with annihilation line photons from a sub-relativistic pair plasma. The X-ray observations are described by synchrotron radiation from the outer blob. The parameter values found from the description of the spectral energy distribution for each object with our phenomenological model are similar to each other, and lie in the typical range reported in BL Lacs.