On the Origin of X‐Ray Emission in Some FR I Galaxies: ADAF or Jet?

Abstract

We investigate the origin of X-ray emission in FR I galaxies using radio, submillimeter, optical, and Chandra X-ray data for a small sample of eight FR I sources. These sources are very dim, with X-ray luminosities LX/LEdd ~ 10-4 to 10-8 (with LX the X-ray luminosity between 2 and 10 keV). We try to fit the multi-wave-band spectra using a coupled accretion-jet model. In this model, the accretion is described by an advection-dominated accretion flow (ADAF); in the innermost region of the ADAF, a fraction of the flow is transferred into the vertical direction and forms a jet. We find that X-ray emission in the source with the highest LX (~1.8 × 10-4LEdd) is from the ADAF. The results for the four sources with moderate LX (several times 10-6LEdd) are complicated. Two are dominated by the ADAF, one by the jet, and the other by the sum of the jet and ADAF. The X-ray emission in the three least luminous sources (LX ≲ 1.0 × 10-6LEdd) is mainly from the jet, although for one source it can also be interpreted as the ADAF, since the quality of the X-ray data is low. We conclude that these results roughly support the prediction of Yuan & Cui that when the X-ray luminosity of a system is below some critical value, the X-radiation will not be dominated by the emission from the ADAF any longer, but by the jet. We also investigate the fuel supply in these sources. We find that the accretion rate in four of the five sources for which we have good constraints must be higher than the Bondi rate. This implies that another fuel supply, such as gas released by the stellar population inside the Bondi radius, should be important.