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

Abstract

The emission of the so-called extreme BL Lacs poses challenges to the particle acceleration models. The hardness of their spectrum, 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 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.

Evidence of a lepto-hadronic two-zone emission in flare states

“…This flux must be attenuated by the photons of the pairplasma, as pointed out by [39] for TBLs. Moreover, it is worth noting that if we take the lowest value allowed by our model to works, i.e., ε p,max ∼ 100 TeV, the observed luminosity would be L ob,pk p,syn ∼ 1 × 10 40 erg s −1 .…”

“…During this campaign, Mrk 501 exhibited low activity at all wavebands. We evoke the same treatment shown in [39] to model the low activity exhibited in Mrk 501. As follows, we estimate the parameter values required by our model.…”

“…In this work, we describe these flares using the recent two-zone lepto-hadronic model proposed by Aguilar-Ruiz et al [39] (hereafter AR2022) introduced to explain the broadband emission of hard TeV BL Lacs. In the AR2022 model, the maxima of the SED's bumps are produced in different dissipation regions.…”