Obscuration and Origin of Nuclear X-Ray Emission in FR I Radio Galaxies

We present X-ray observations of the nuclear region of 25 Fanaroff-Riley type I (FR I) radio galaxies from the 3CRR and B2 catalogs, using data from the Chandra and XMM-Newton archives. We find the presence of a X-ray central compact core (CCCX) in 13/25 sources; in 3/25 sources the detection of a CCCX is uncertain, while in the remaining 9/25 sources no CCCX is found. All the sources are embedded in a diffuse soft X-ray component, generally on kiloparsec scales, which is in agreement with the halo of the host galaxy and/or with the intracluster medium. The X-ray spectra of the cores are described by a power law with photon indices Γ=1.1-2.6. In eight sources excess absorption over the Galactic value is detected, with rest-frame column densities N^z_H~10²⁰-10²¹ cm^-2 thus, we confirm the previous claim, based on optical data, that most FR I radio galaxies lack a standard optically thick torus. We find significant correlations between the X-ray core luminosity and the radio and optical luminosities, suggesting that at least a fraction of the X-ray emission originates in a jet; however, the origin of the X-rays remains ambiguous. If the X-ray emission is entirely attributed to an isotropic, accretion-related component, we find very small Eddington ratios, L_bol/L_Edd~10^-3to10^-8, and we calculate the radiative efficiency to be η~10^-2to10^-6 on the basis of the Bondi accretion rates from the spatial analysis. This suggests that radiatively inefficient accretion flows are present in the cores of low-power radio galaxies.