A Multi - Wavelength Study of the High Surface Brightness Hotspot in Pks 1421 – 490

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, L2−10 keV = 3× 10 ergs s, 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 (α ≈ 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 α ∼ 0.5 to α . 0.2, which we model by incorporating a cut-off in the electron energy distribution at γmin ≈ 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 Γjet & 5. Subject headings: galaxies:active – galaxies:jets – quasars:individual (PKS 1421–490)