We find a significant anticorrelation between the hard X‐ray photon index Γ and the Eddington ratio Lbol/LEdd for a sample of low‐ionization nuclear emission‐line regions and local Seyfert galaxies, compiled from literature with Chandra or XMM–Newton observations. This result is in contrast with the positive correlation found in luminous active galactic nuclei (AGN), while it is similar to that of X‐ray binaries (XRBs) in the low/hard state. Our result is qualitatively consistent with the spectra produced from advection‐dominated accretion flows (ADAFs). It implies that the X‐ray emission of low‐luminosity active galactic nuclei (LLAGN) may originate from the Comptonization process in ADAF, and the accretion process in LLAGN may be similar to that of XRBs in the low/hard state, which is different from that in luminous AGN.
The hard X-ray emission of active galactic nuclei (AGN) is believed to originate from the hot coronae above the cold accretion discs. The hard X-ray spectral index is found to be correlated with the Eddington ratio L bol /L Edd , and the hard X-ray bolometric correction factor L bol /L X,2−10 keV increases with the Eddington ratio. The Compton reflection is also found to be correlated with the hard X-ray spectral index for Seyfert galaxies and X-ray binaries. These observational features provide very useful constraints on the accretion disc-corona model for AGN. We construct an accretion disc-corona model and calculate the spectra with different magnetic stress tensors in the cold discs, in which the corona is assumed to be heated by the reconnection of the magnetic fields generated by buoyancy instability in the cold accretion disc. Our calculations show that the magnetic stress tensor τ rϕ = αp gas fails to explain all these observational features, while the disc-corona model with τ rϕ = αp tot always leads to constant L bol /L X,2−10keV independent of the Eddington ratio. The resulted spectra of the disc-corona systems with τ rϕ = α √ p gas p tot show that both the hard X-ray spectral index and the hard X-ray bolometric correction factor L bol /L X,2−10 keV increase with the Eddington ratio, which are qualitatively consistent with the observations. We find that the disc-corona model is unable to reproduce the observed very hard X-ray continuum emission from the sources accreting at low rates (e.g. ∼ 1 for L bol /L Edd ∼ 0.01), which may imply the different accretion modes in these low-luminosity sources. We suggest that the disc-corona system transits to an advectiondominated accretion flow+disc corona system at low accretion rates, which may be able to explain all the above-mentioned correlations.
The central black hole masses of a sample of radio‐loud quasars are estimated by using the data of Hβ linewidth and the optical continuum luminosity. The vast majority of the quasars in this sample have black hole masses larger than 108 M⊙, while a few quasars may contain relatively smaller black holes. We found a significant anti‐correlation between the radio loudness and the central black hole mass. It might imply that the jet formation is governed by the black hole mass.
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 L X /L Edd $ 10 À4 to 10 À8 (with L X 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 L X ($1.8 ; 10 À4 L Edd ) is from the ADAF. The results for the four sources with moderate L X (several times 10 À6 L Edd ) 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 (L X P 1.0 ; 10 À6 L Edd ) 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.
Large scale magnetic field threading an accretion disk is a key ingredient in the jet formation model. The most attractive scenario for the origin of such a large scale field is the advection of the field by the gas in the accretion disk from the interstellar medium or a companion star. However, it is realized that outward diffusion of the accreted field is fast compared to the inward accretion velocity in a geometrically thin accretion disk if the value of the Prandtl number P m is around unity. In this work, we revisit this problem considering the angular momentum of the disk is removed predominantly by the magnetically driven outflows. The radial velocity of the disk is significantly increased due to the presence of the outflows. Using a simplified model for the vertical disk structure, we find that even moderately weak fields can cause sufficient angular momentum loss via a magnetic wind to balance outward diffusion. There are two equilibrium points, one at low field strengths corresponding to a plasma-beta at the midplane of order several hundred, and one for strong accreted fields, β ∼ 1. We surmise that the first is relevant for the accretion of weak, possibly external, fields through the outer parts of the disk, while the latter one could explain the tendency, observed in full 3D numerical simulations, of strong flux bundles at the centers of disk to stay confined in spite of strong MRI turbulence surrounding them.
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