A Multiwavelength Study of the High Surface Brightness Hot Spot in PKS 1421–490

Abstract

Long Baseline Array imaging of the z = 0.663 broadline radio galaxy PKS 1421−490 reveals a 400 pc diameter high surface brightness hot spot at a projected distance of ∼40 kpc from the active galactic nucleus. The isotropic X-ray luminosity of the hot spot, L 2-10 keV = 3 × 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 hot spot 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, which we model by incorporating a cutoff in the electron energy distribution at γ 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 Γ jet 5.

RAiSE III: 3C radio AGN energetics and composition

“…Specifically, the minimum Lorentz factor of this electron energy distribution is related to the luminosity as L rad ∝ γ min s−2 (Equation 8 of . The minimum Lorentz factor in the hotspot has been estimated by other authors for selected radio sources including: Cygnus A (McKean et al 2016), PKS 1421-490 (Godfrey et al 2009), 3C123, 3C196 and 3C295 (Hardcastle 2001). These hotspot Lorentz factor estimates are mostly in the order of a few hundred although these estimates are quite uncertain due to the confounding effects of free-free and synchrotron selfabsorption.…”

“…The minimum Lorentz factor of the electron energy distribution at the hotspot reduces accordingly as the electrons propagate throughout the lobe; the Lorentz factor in the lobe does not need to be explicitly measured. Following Godfrey et al (2009) we assume a typical hotspot minimum Lorentz factor of γ min = 500 for the sources in our sample. Systematic errors in AGN intrinsic parameter estimates are derived for realistic Lorentz factors in the range γ min = 100-1000, yielding a 0.19 dex uncertainty in jet power estimates, 0.06 dex in source ages, and 0.01 dex in equipartition factors.…”

Interpreting the radio/X-ray correlation of black hole X-ray binaries based on the accretion–jet model

“…The existence of protons in the jet is supported by several observations of the AGN jets (see Sikora 2011 for a review): (1) the detection of circular polarization and Faraday rotation of the radio core. This is because the electron-positron plasma cannot generate any polarization (Park & Blackman 2010); (2) the low-energy cutoff in the radio spectra (and also the electron energy distribution) of hotspots in radio-lobes, which is likely a consequence of dissipation of bulk kinetic energy in an electron-proton jet (Godfrey et al 2009). We mainly follow Spada et al (2001) to calculate the radiation from the jet 1 .…”

Multiwavelength study of Cygnus A

“…For the same hotspot, the authors derived a low magnetic field strength, 21.5 μG. Godfrey et al (2009) studied the radio and X-ray emission from the hotspots in PKS 1421−490. Fitting the radio and optical fluxes with a one-zone synchrotron self-Compton emission, they concluded that the minimum Lorentz factor is about 650, or higher if beaming is important.…”