X-ray measurement of electron and magnetic-field energy densities in the west lobe of the giant radio galaxy 3C 236

Abstract

X-ray emission associated with the west lobe of the giant radio galaxy 3C 236 was investigated with the Suzaku observatory to evaluate the energetics in the lobe. After removing contamination from point-like X-ray sources detected with Chandra and subtracting the X-ray and non-X-ray backgrounds, the Suzaku spectrum from the lobe was reproduced by a power-law model with a photon index of Γ = 2.23_{-0.38-0.12}^{+0.44+0.14}, where the first and second errors represent the statistical and systematic ones, respectively. Within the errors, the X-ray index was consistent with the radio synchrotron one, Γ_R = 1.74 ± 0.07, estimated in the 326-2695 MHz range. This agreement supports that the X-ray emission is attributed to the inverse-Compton radiation from the synchrotron electrons filling the lobe, where the cosmic microwave background photons are up-scattered. This result made 3C 236 the largest radio galaxy of which the lobe has ever been probed through the inverse-Compton X-ray photons. When the photon index was fixed at Γ_R, the X-ray flux density at 1 keV was measured as S_X = 12.3 ± 2.0 ± 1.9 nJy. A comparison of the X-ray flux to the radio one (S_R = 1.11 ± 0.02 Jy at 608.5 MHz) yields the energy densities of the electrons and magnetic field in the west lobe as u_e = 3.9_{-0.7 -0.9}^{+0.6 +1.0} × 10^{-14} erg cm^-3 and u_m = 0.92_{-0.15 -0.35}^{+0.21 +0.52}× 10^{-14} erg cm^-3, respectively, indicating a mild electron dominance of u_e/u_m = 4.2_{-1.3 -2.3}^{+1.6 +4.1}. The latter corresponds to the magnetic field strength of B = 0.48_{-0.04 -0.10}^{+0.05 +0.12} μG. These are typical among the lobes of giant radio galaxies. A summary of the u_e-size relation for the inverse-Compton-detected radio galaxies implies that the west lobe of 3C 236 is still actively energized by its jet.