Decelerating Flows in TeV Blazars

The peak of the de-absorbed energy distribution of the TeV emitting blazars, all of the BL Lacertae (BL) class, can reach values up to ∼ 10 TeV. In the context of synchrotron-self Compton (SSC) models of relativistic uniformly moving blobs of plasma, such high energy peak emission can be reproduced only by assuming Doppler factors of δ ∼ 50. However, such high values strongly disagree with the unification of FR I radio galaxies and BLs. Additionally, the recent detections of slow, possibly sub-luminal velocities in the sub-pc scale jets of the TeV BLs MKN 421 and MKN 501 suggest that the jets in these sources decelerate very early to mildly relativistic velocities (Γ ∼ a few). In this work we examine the possibility that the relativistic flows in the TeV BLs are longitudinally decelerating. In this case, modest Lorentz factors (Γ ∼ 15), decelerating down to values compatible with the recent radio interferometric observations, can reproduce the ∼ few TeV peak energy of these sources. Furthermore, such decelerating flows are shown to reproduce the observed broadband BL FR I luminosity ratios.