Decelerating Flows in TeV Blazars: A Resolution to the BL Lac -- FR I Unification Problem

Abstract

TeV emission from BL Lacertae (BL) objects is commonly modeled as Synchrotron-Self Compton (SSC) radiation from relativistically moving homogeneous plasma blobs. In the context of these models, the blob Lorentz factors needed to reproduce the corrected for absorption by the diffuse IR background (DIRB) TeV emission are large ($δ\gtrsim 50$). The main reason for this is that stronger beaming eases the problem of the lack of $\sim$ IR-UV synchrotron seed photons needed to produce the de-absorbed $\sim $ few TeV peak of the spectral energy distribution (SED). However, such high Doppler factors are in strong disagreement with the unified scheme, according to which BLs are FR I radio galaxies with their jets closely aligned to the line of sight. Here, motivated by the detection of sub-luminal velocities in the sub-pc scale jets of the best studied TeV blazars, MKN 421 and MKN 501, we examine the possibility that the relativistic flows in the TeV BLs decelerate. In this case, the problem of the missing seed photons is solved because of Upstream Compton (UC) scattering, a process in which the upstream energetic electrons from the fast base of the flow `see' the synchrotron seed photons produced in the slow part of the flow relativistically beamed. Modest Lorentz factors ($Γ\sim 15$), decelerating down to values compatible with the recent radio interferometric observations, reproduce the $\sim $ few TeV peak energy of these sources. Furthermore, such decelerating flows are shown to be in agreement with the BL - FR I unification, naturally reproducing the observed BL/FR I broad band luminosity ratios.