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

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 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-low model with a photon index of $\Gamma = 2.23_{-0.38-0.12}^{+0.44+0.14}$ 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, $\Gamma_{\rm R} = 1.74 \pm 0.07$, estimated in the 326 -- 2695 MHz range. This agreement supports that the X-ray emission is attributed to the inverse-Compton (IC) 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 IC X-ray photons. When the photon index was fixed at $\Gamma_{\rm R}$, the X-ray flux density at 1 keV was measured as $S_{\rm X} = 12.3 \pm 2.0 \pm 1.9$ nJy. A comparison of the X-ray flux to the radio one ($S_{\rm R} = 1.11 \pm 0.02$ Jy at 608.5 MHz) yields the energy densities of the electrons and magnetic field in the west lobe as $u_{\rm e} = 3.9_{-0.7 -0.9}^{+0.6 +1.0} \times 10^{-14} $ ergs cm$^{-3}$ and $u_{\rm m} = 0.92_{-0.15 -0.35}^{+0.21 +0.52}\times 10^{-14} $ ergs cm$^{-3}$, respectively, indicating a mild electron dominance of $u_{\rm e}/u_{\rm m} = 4.2_{-1.3 -2.3}^{+1.6 +4.1}$. The latter corresponds to the magnetic field strength of $B = 0.48_{-0.04 -0.10}^{+0.05 +0.12}$ $\mu$G.These are typical among the lobes of giant radio galaxies. A compilation of the $u_{\rm e}$-size relation for the IC-detected radio galaxies implies that the west lobe of 3C 236 is still actively energized by its jet.