Synchrotron and inverse-Compton emission from blazar jets - III.
  Compton-dominant blazars
  In this paper we develop the extended jet model of Potter & Cotter to model
the simultaneous multi-wavelength spectra of six Compton-dominant blazars. We
include an accelerating parabolic base transitioning to a slowly decelerating
conical jet with a geometry set by observations of M87 and consistent with
simulations and theory. We investigate several jet models and find that the
optically thick to thin synchrotron break in the radio spectrum requires the
jet to first come into equipartition at large distances along the jet (10^5
Schwarzschild radii), consistent with the observed transition from parabolic to
conical in the jet of M87. We confirm this result analytically and calculate
the expected frequency core-shift relations for the models under consideration.
We find that a parabolic jet transitioning to a ballistic conical jet, which
starts in equipartition and becomes more particle dominated at larger
distances, fits the multiwavelength data of the six blazars well, whilst an
adiabatic equipartition conical section requires very large bulk Lorentz
factors to reproduce the Compton-dominance of the blazars.
  We find that all these blazars require high power, high bulk Lorentz factor
jets observed close to the line of sight as we expect from the blazar sequence
and consistent with the results from Paper II. The inverse-Compton emission in
our fits is due to inverse-Compton scattering of high-redshift CMB photons at
large distances along the jet due to the high bulk Lorentz factors of the jets.
We postulate a new interpretation of the blazar sequence based on the radius of
the transition region of the jet (where the jet is brightest in synchrotron
emission) scaling linearly with black hole mass.
