Synchrotron and inverse-Compton emission from blazar jets – IV. BL Lac type blazars and the physical basis for the blazar sequence

In this paper we investigate the properties of a sample of six BL Lacs by fitting their spectra using our inhomogeneous jet model with an accelerating, magnetically dominated, parabolic base, which transitions to a slowly decelerating conical jet with a geometry based on observations of M87. Our model is able to fit very well to the simultaneous multiwavelength spectra of all the BL Lacs including radio observations. We find that the BL Lacs have lower jet powers and bulk Lorentz factors than the sample of Compton-dominant blazars investigated in Paper III, consistent with the blazar sequence. Excitingly, we find a correlation between the radius at which the jet first comes into equipartition and the jet power, in agreement with our prediction from Paper II. We interpret this result as one of two physical scenarios: a universal jet geometry which scales linearly with black hole mass or a dichotomy in Eddington accretion rates between flat spectrum radio quasars (FSRQs) and BL Lacs. If we assume that the jet geometry of all blazars scales linearly with black hole mass then we find a plausible range of masses (10^7-10^10 solar masses). We find that the quiescent gamma-ray spectrum of Markarian 421 is best fitted by scattering of external CMB photons. We are unable to fit the spectrum using synchrotron self-Compton emission due to strong gamma-ray absorption via pair production even using a compact, rapidly decelerating, jet with a very large bulk Lorentz factor (50), as has been suggested recently. Finally, we fit to the SEDs of the four high power HSP BL Lacs recently found by Padovani et al. 2012. We find that their high peak frequency emission is caused by high maximum electron energies whilst the rest of their jet properties are typical of relatively high power BL Lacs and consistent with our predictions.