EDP Sciences
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Volume 408, Number 3, September IV 2003
Page(s) 949 - 959
Section Galactic structure and dynamics
DOI http://dx.doi.org/10.1051/0004-6361:20031050

A&A 408, 949-959 (2003)
DOI: 10.1051/0004-6361:20031050

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

M. Gliozzi1, R. M. Sambruna1 and W. N. Brandt2

1  George Mason University, Dept. of Physics & Astronomy & School of Computational Sciences, 4400 University Drive, MS 3F3, Fairfax, VA 22030, USA
2  The Pennsylvania State University, Department of Astronomy & Astrophysics, 525 Davey Lab, University Park, PA 16802, USA

(Received 11 April 2003 / Accepted 7 July 2003)

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 $\Gamma\simeq 1.5$ absorbed by a column density $N_{\rm H}>
5\times 10^{22}{~\rm cm^{-2}}$ . The X-ray luminosity associated with the non-thermal component is ${\sim} 5 \times 10^{41} {~\rm erg~s^{-1}}$. The nuclear source is embedded in a diffuse hot gas ( $kT\sim0.6{-}0.65$ keV), whose density profile implies a Bondi accretion rate of ${\sim} 4.5\times 10^{-2}{~ M_\odot\rm ~ yr^{-1}}$. 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.

Key words: galaxies: active -- galaxies: nuclei -- X-rays: galaxies

Offprint request: M. Gliozzi, mario@physics.gmu.edu

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