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.
