Long Baseline Array imaging of the z=0.663 broad line radio galaxy PKS 1421–490 reveals a 400 pc diameter high surface brightness hotspot at a projected distance of approximately 40 kpc from the active galactic nucleus .
The isotropic X-ray luminosity of the hotspot , L _ { 2 - 10 \ > keV } = 3 \times 10 ^ { 44 } \ > { ergs\ > s ^ { -1 } } , is comparable to the isotropic X-ray luminosity of the entire X-ray jet of PKS 0637–752 , 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 \approx 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 \sim 0.5 to \alpha \lesssim 0.2 , which we model by incorporating a cut-off in the electron energy distribution at \gamma _ { min } \approx 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 } \gtrsim 5 .