  Long Baseline Array imaging of the z=0.663 broad line radio galaxy
PKS1421-490 reveals a 400 pc diameter high surface brightness hotspot at a
projected distance of approximately 40kpc from the active galactic nucleus. The
isotropic X-ray luminosity of the hotspot, L_{2-10 keV} = 3 10^{44} ergs/s, is
comparable to the isotropic X-ray luminosity of the entire X-ray jet of
PKS0637-752, and the peak radio surface brightness is hundreds of times greater
than that of the brightest hotspot in Cygnus A. We model the radio to X-ray
spectral energy distribution using a one-zone synchrotron self Compton model
with a near equipartition magnetic field strength of 3 mG. There is a strong
brightness asymmetry between the approaching and receding hotspots and the hot
spot spectrum remains flat (alpha ~ 0.5) well beyond the predicted cooling
break for a 3 mG magnetic field, indicating that the hotspot emission may be
Doppler beamed. A high plasma velocity beyond the terminal jet shock could be
the result of a dynamically important magnetic field in the jet. There is a
change in the slope of the hotspot radio spectrum at GHz frequencies from
alpha~0.5 to alpha<0.2, which we model by incorporating a cut-off in the
electron energy distribution at gamma_{min} ~ 650, with higher values implied
if the hotspot emission is Doppler beamed. We show that a sharp decrease in the
electron number density below a Lorentz factor of 650 would arise from the
dissipation of bulk kinetic energy in an electron/proton jet with a Lorentz
factor Gamma_{jet} ~ 5.
