Suzaku Diagnostics of the Energetics in the Lobes of the Giant Radio Galaxy 3C 35

Abstract

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, Γ = 1.35^+0.56 _-0.86 ^+0.11 _-0.10, where the first and second errors represent the statistical and systematic ones, was found to agree with the synchrotron radio index from the lobes, Γ_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 ± 5.4^+4.0 _-3.6 nJy with the photon index fixed at the radio value. The X-ray surface brightness from these lobes (~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 ± 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 _e = (5.8 ± 2.3^+1.9 _-1.7) × 10^-14 erg cm^-3 over the electron Lorentz factor of 10³-10⁵. The magnetic energy density was calculated as u _m = (3.1^+2.5 _-1.0 ^+1.4 _-0.9) × 10^-14 erg cm^-3, corresponding to the magnetic field strength of 0.88^+0.31 _-0.16 ^+0.19 _-0.14 μG. These results suggest that the energetics in the 3C 35 lobes are nearly consistent with equipartition between the electrons and magnetic fields.