Suzaku diagnostics of the energetics in the lobes of the giant radio galaxy 3C 35
The Suzaku observation of a giant radio galaxy 3C 35 revealed faint extended X-ray emission, associated with its radio lobes and/or host galaxy. After careful subtraction of the X-ray and non-X-ray background and contaminating X-ray sources, the X-ray spectrum of the faint emission was reproduced by a sum of the power-law (PL) and soft thermal components. The soft component was attributed to the thermal plasma emission from the host galaxy. The photon index of the PL component, $\Gamma = 1.35_{-0.86}^{+0.56}$$_{-0.10}^{+0.11}$ where the first and second errors represent the statistical and systematic ones, was found to agree with the synchrotron radio index from the lobes, $\Gamma_{\rm R} = 1.7$. Thus, the PL component was attributed to the inverse Compton (IC) X-rays from the synchrotron electrons in the lobes. The X-ray flux density at 1 keV was derived as $13.6\pm 5.4_{-3.6}^{+4.0}$ nJy with the photon index fixed at the radio value. The X-ray surface brightness from these lobes ($\sim 0.2$ nJy arcmin$^{-2}$) is lowest among the lobes studied through the IC X-ray emission. In combination with the synchrotron radio flux density, $7.5 \pm 0.2$ Jy at 327.4 MHz, the electron energy density spatially averaged over the lobes was evaluated to be the lowest among those radio galaxies, as $u_{\rm e} = (5.8 \pm 2.3 _{-1.7}^{+1.9}) \times 10^{-14}$ ergs cm$^{-3}$ over the electron Lorentz factor of $10^{3}$ -- $10^{5}$. The magnetic energy density was calculated as $u_{\rm m}=(3.1_{-1.0}^{+2.5}$$_{-0.9}^{+1.4}) \times 10^{-14}$ ergs cm$^{-3}$, corresponding to the magnetic field strength of $0.88_{-0.16}^{+0.31}$$_{-0.14}^{+0.19}$ $\mu$G. These results suggest that the energetics in the 3C 35 lobes are nearly consistent with equipartition between the electrons and magnetic fields.