Xinwu Cao

On the masses of black holes in radio-loud quasars

The central black hole masses of a sample of radio-loud quasars are estimated by using the data of H beta linewidth and the optical continuum luminosity. The vast majority of the quasars in this sample have black hole masses larger than 10(8) M(circle dot), 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.

The anticorrelation between the hard X-ray photon index and the Eddington ratio in low-luminosity active galactic nuclei

We find a significant anticorrelation between the hard X-ray photon indexand 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

On the origin of X-ray emission in some FR I galaxies: ADAF or jet?

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) similar to 10(-4) to 10(-8) (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) (similar to 1.8 x 10(-4) L(Edd)) is from the ADAF. The results for the four sources with moderate L(X) (several times 10(-6) 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) less than or similar to 1.0 x 10(-6) 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.

Correlation between radio and broad line emission in radio loud quasars

We present a correlation between radio and broad-line emission for a sample of radio-loud quasars that supports a close Link between accretion processes and relativistic jets. BL Lac objects seem to follow the statistical behaviour of quasars, but with fainter broad-line emission.

An accretion disc-corona model for X-ray spectra of active galactic nuclei

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,2-10keV) 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 tau(r phi) = alpha p(gas) fails to explain all these observational features, while the disc-corona model with tau(r phi) = alpha p(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 tau(r phi) = alpha root 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,2-10keV) 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. Gamma similar to 1 for L(bol)/L(Edd) similar to 0.01), which may imply the different accretion modes in these low-luminosity sources. We suggest that the disc-corona system transits to an advection-dominated accretion flow+disc corona system at low accretion rates, which may be able to explain all the above-mentioned correlations.

An accretion disc model for quasar optical variability

Some different correlations between optical-ultraviolet (UV) variability and other quasar properties, such as luminosity, black hole mass and rest-frame wavelength, were discovered. The positive correlation between optical-UV variability amplitude and black hole mass was first found by Wold et al., and this was confirmed by Wilhite et al. We suggest that the accretion disc model can explain these correlations, provided the optical-UV variability is triggered by the change of accretion rate. The disc temperature of accretion discs decreases with increasing black hole mass, which leads to systematical spectral shape difference with black hole mass even if the black hole is accreting at the same rate m (m = M/M(Edd)). The observed positive correlation between optical-UV variability and black hole mass can be well reproduced by our model calculations, if the mean accretion rate m(0) similar to 0.1 with variation of delta m similar to 0.4-0.5m(0). We also found that the observed correlations of optical-UV variability mplitude with luminosity or rest-frame wavelength can be qualitatively explained by this accretion disc model.

THE LARGE-SCALE MAGNETIC FIELDS OF ADVECTION-DOMINATED ACCRETION FLOWS

We calculate the advection/diffusion of the large-scale magnetic field threading an advection-dominated accretion flow (ADAF) and find that the magnetic field can be dragged inward by the accretion flow efficiently if the magnetic Prandtl number P-m = eta/nu similar to 1. This is due to the large radial velocity of the ADAF. It is found that the magnetic pressure can be as high as similar to 50% of the gas pressure in the inner region of the ADAF close to the black hole horizon, even if the external imposed homogeneous vertical field strength is less than or similar to 5% of the gas pressure at the outer radius of the ADAF, which is caused by the gas in the ADAF plunging rapidly to the black hole within the marginal stable circular orbit. In the inner region of the ADAF, the accretion flow is significantly pressured in the vertical direction by the magnetic fields, and therefore its gas pressure can be two orders of magnitude higher than that in the ADAF without magnetic fields. This means that the magnetic field strength near the black hole is underestimated by assuming equipartition between magnetic and gas pressure with the conventional ADAF model. Our results show that the magnetic field strength of the flow near the black hole horizon can be more than one order of magnitude higher than that in the ADAF at similar to 3R(g) (R-g = 2GM/c(2)), which implies that the Blandford-Znajek mechanism could be more important than the Blandford-Payne mechanism for ADAFs. We find that the accretion flow is decelerated near the black hole by the magnetic field when the external imposed field is strong enough or the gas pressure of the flow is low at the outer radius, or both. This corresponds to a critical accretion rate, below which the accretion flow will be arrested by the magnetic field near the black hole for a given external imposed field. In this case, the gas may accrete as magnetically confined blobs diffusing through field lines in the region very close to the black hole horizon, similar to those in compact stars. Our calculations are also valid for the case that the inner ADAF connects to the outer cold thin disk at a certain radius. In this case, the advection of the external fields is quite inefficient in the outer thin disk due to its low radial velocity, and the field lines thread the disk almost vertically, while these field lines can be efficiently dragged inward by the radial motion of the inner ADAF.

The bulk kinetic power of radio jets in active galactic nuclei

Based on the Konigls inhomogeneous jet model, we estimate the jet parameters, such as bulk Lorentz factor Gamma, viewing angle theta and electron number density n(e) from radio very long-baseline interferometry and X-ray data for a sample of active galactic nuclei (AGNs) assuming that the X-rays are from the jet rather than the intracluster gas. The bulk kinetic power of jets is then calculated using the derived jet parameters. We find a strong correlation between the total luminosity of broad emission lines and the bulk kinetic power of the jets. This result supports the scenario that the accretion process is tightly linked with the radio jets, though how the disc and jet are coupled is not revealed by present correlation analysis. Moreover, we find a significant correlation between the bulk kinetic power and radio extended luminosity. This implies that the emission from the radio lobes is closely related with the energy flux transported through jets from the central part of AGNs.

Jet Formation in BL Lacertae Objects with Different Accretion Modes

We estimate the masses of massive black holes in BL Lac objects from their host galaxy luminosity. The power of jets and central optical ionizing luminosity for a sample of BL Lac objects are derived from their extended radio emission and the narrow-line emission, respectively. The maximal jet power that can be extracted from a standard thin accretion disk/spinning black hole is calculated as a function of dimensionless accretion rate (m) over dot ((m) over dot = (M) over dot/(M) over dot (Edd)). Comparing with the derived jet power, we find that the accretion disks in most BL Lac objects should not be standard accretion disks. For a pure advection-dominated accretion flow (ADAF), there is an upper limit on its optical continuum luminosity because of the existence of an upper limit (m) over dot(crit) on the accretion rate. It is found that a pure ADAF is too faint to produce the optical ionizing luminosity of BL Lac objects derived from their narrow-line luminosity. We propose that an ADAF is present in the inner region of the disk, and it becomes a standard thin disk in the outer region in most BL Lac objects, i.e., ADAF+SD (standard disk) scenario. This ADAF+SD scenario can explain both the jet power and optical ionizing continuum emission of these BL Lac objects. The inferred transition radii between the inner ADAF and outer SD are in the range of 40-150 GM(bh)/c(2), if the disks are accreting at the rate (m) over dot = 0.01.

The relation between extended radio and line emission for radio-loud quasars

We explore the relationship between the extended radio and line emission for a radio-loud quasar sample including both core-dominated and lobe-dominated quasars. A strong correlation is present between the extended radio and broad-line emission. The core emission is also correlated with the broad-line emission for core-dominated quasars in the sample. The statistical behaviour of the core emission of lobe-dominated quasars is rather different from that of core-dominated quasars. The extended radio luminosity is a good tracer for jet power, while the core luminosity can only be a jet power tracer for core-dominated quasars.