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

Abstract

TeV emission from BL Lacertae (BL) objects is commonly modeled as synchrotron self-Compton radiation from relativistically moving homogeneous plasma blobs. In the context of these models, the blob Lorentz factors needed to reproduce the TeV emission corrected for absorption by the diffuse infrared background are large (δ>~50). The main reason for this is that stronger beaming eases the problem of the lack of ~IR-UV synchrotron seed photons needed to produce the deabsorbed ~few TeV peak of the spectral energy distribution. 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 subluminal velocities in the subparsec-scale jets of the best studied TeV blazars, Mrk 421 and Mrk 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 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~15), decelerating down to values compatible with the recent radio interferometric observations, reproduce the ~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 broadband luminosity ratios.