A Multi-Wavelength Study of the High Surface Brightness Hotspot in PKS1421-490

TL;DR

This study presents detailed multi-wavelength observations and modeling of a high surface brightness hotspot in PKS1421-490, revealing insights into jet dynamics, magnetic fields, and electron energy distributions in active galactic nuclei.

Contribution

It provides the first detailed spectral energy distribution modeling of this hotspot, suggesting Doppler beaming and a specific electron energy cutoff, advancing understanding of jet physics.

Findings

Hotspot X-ray luminosity comparable to entire jet luminosity.

Evidence of Doppler beaming in hotspot emission.

Electron energy distribution cutoff at gamma ~ 650.

Abstract

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.