On the origin of the X-rays and the nature of accretion in NGC 4261

Abstract

We report on the X-ray properties of the radio galaxy NGC 4261, combining information from the XMM-Newton, Chandra, and BeppoSAX satellites. Goals of this study are to investigate the origin of the X-rays from this low-power radio galaxy and the nature of the accretion process onto the central black hole. The X-ray spectrum of the nuclear source extending up to 100-150 keV is well described by a partially covered (covering factor >0.8) power law with a photon index Gamma =~ 1.5 absorbed by a column density N_H> 5x 10²² cm^-2. The X-ray luminosity associated with the non-thermal component is ~ 5 x 10⁴¹ erg s^-1. The nuclear source is embedded in a diffuse hot gas (kT ~ 0.6-0.65 keV), whose density profile implies a Bondi accretion rate of ~4.5x 10^-2 M_sunyr^-1. For the first time rapid X-ray variability is detected in a low-power radio galaxy at more than 99% confidence level. The observed X-ray spectral and variability properties indicate the accretion flow as the most likely origin of the bulk X-ray continuum. This conclusion is strengthened by energetic considerations based on a comparison between the X-ray luminosity and the kinetic power of the jet, which also suggest that the Bondi accretion rate overestimates the actual accretion rate onto the black hole.