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

摘要

X-ray emission associated with the west lobe of the giant radio galaxy, 3C236, was investigated with the Suzaku observatory, to evaluate the energeticsin the lobe. After removing contamination from X-ray sources detected withChandra and subtracting the X-ray and non-X-ray backgrounds, the Suzakuspectrum from the lobe was reproduced by a power-low model with a photon indexof $\Gamma = 2.23_{-0.38-0.12}^{+0.44+0.14}$ where the first and second errorsrepresent 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 agreementsupports that the X-ray emission is attributed to the inverse-Compton (IC)radiation from the synchrotron electrons filling the lobe, where the cosmicmicrowave background photons are up-scattered. This result made 3C 236 thelargest radio galaxy, of which the lobe has ever been probed through the ICX-ray photons. When the photon index was fixed at $\Gamma_{\rm R}$, the X-rayflux density at 1 keV was measured as $S_{\rm X} = 12.3 \pm 2.0 \pm 1.9$ nJy. Acomparison of the X-ray flux to the radio one ($S_{\rm R} = 1.11 \pm 0.02$ Jyat 608.5 MHz) yields the energy densities of the electrons and magnetic fieldin 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 $u_{\rme}/u_{\rm m} = 4.2_{-1.3 -2.3}^{+1.6 +4.1}$. The latter corresponds to themagnetic field strength of $B = 0.48_{-0.04 -0.10}^{+0.05 +0.12}$ $\mu$G.Theseare 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 thewest lobe of 3C 236 is still actively energized by its jet.