Twelve and a Half Years of Observations of Centaurus A with RXTE

TL;DR

This study presents 12.5 years of RXTE observations of Centaurus A, analyzing its X-ray spectra, variability, and spectral energy distribution, suggesting the hard X-ray emission may originate from the jet base behind absorbing material.

Contribution

First long-term X-ray observational analysis of Centaurus A, revealing variability patterns and spectral features, and proposing a jet-based origin for the hard X-ray emission.

Findings

Spectra fit with absorbed power law and iron line; no better fit with cut-off or reflection.

Detected a variability break at ~18 days, inconsistent with standard relations for radio-quiet AGN.

Hard X-ray emission likely originates from the jet base behind absorbing material.

Abstract

The Rossi X-ray Timing Explorer has observed the nearest radio galaxy, Centaurus A, in 13 intervals from 1966 August to 2009 February over the 3--200 keV band. Spectra accumulated over the 13 intervals were well described with an absorbed power law and iron line. Cut-off power laws and Compton reflection from cold matter did not provide a better description. For the 2009 January observation, we set a lower limit on the cut-off energy at over 2 MeV. The power spectral density function was generated from RXTE/ASM and PCA data, as well as an XMM-Newton long look, and clear evidence for a break at 18+10-7 days (68% conf.) was seen. Given Cen A's high black hole mass and very low value of Lx/LEdd, the break was a factor of 17+/-9 times higher than the break frequency predicted by the McHardy and co-workers' relation, which was empirically derived for a sample of objects, which are radio-quiet and accreting at relatively high values of Lbol/LEdd. We have interpreted our observations in the context of a clumpy molecular torus. The variability characteristics and the broadband spectral energy distribution, when compared to Seyferts, imply that the bright hard X-ray continuum emission may originate at the base of the jet, yet from behind the absorbing line of sight material, in contrast to what is commonly observed from blazars.