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 ( \delta \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 ( \Gamma \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 .