arXiv subjects

Xinwu Cao

Publications and source records attributed to Xinwu Cao.

At least 91 records · Page 5Linked to original sources

arXiv2009

The anti-correlation between the hard X-ray photon index and the Eddington ratio in LLAGNs

We find a significant anti-correlation between the hard X-ray photon index and the Eddington ratio L_Bol/L_Edd for a sample of Low-Ionization Nuclear Emission-line Regions (LINERs) and local Seyfert galaxies, compiled from literatures with Chandra or XMM-Newton observations. This result is in contrast with the positive correlation found in luminous active galactic nuclei (AGNs), while it is similar to that of X-ray binaries (XRBs) in 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 (LLAGNs) may originate from the Comptonization process in ADAF, and the accretion process in LLAGNs may be similar to that of XRBs in the low/hard state, which is different from that in luminous AGNs.

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arXiv2009

SDSS spectroscopy for blazars in the $Fermi$ LAT bright AGN sample

We have collected all available spectra and photometric data from SDSS catalogue for bright AGNs complied from the first three months of the Fermi large area telescope all-sky survey. Based on the 106 high-confidence and 11 low-confidence association bright AGN list, the photometry data are collected from SDSS DR7 for 28 sources (12 BL Lacs and 16 FSRQs), two of which are low-confidence association bright AGNs. Among these 28 SDSS photometric sources, SDSS spectra are available for 20 sources (6 BL Lacs and 14 FSRQs). The black hole mass M_BH and the broad line region (BLR) luminosity were obtained for 14 FSRQs by measuring the line-width and strength of broad emission lines from SDSS spectra. The broad emission lines measurements of five FSRQs are presented for the first time in this work. The optical continuum emission of these 14 FSRQs is found to be likely dominated by the nonthermal jet emission through comparing the relationship between the broad Mg II line and continuum luminosity to that of radio quiet AGNs. The black hole mass of 14 FSRQs ranges from 10^8.2 to 10^9.9 solar mass, with most of sources larger than 10^9 solar mass. The Eddington ratio L_bol/L_Edd ranges from 10^-1.5 to ~ 1. This implies that the optically thin, geometrically thick accretion disk may exist in these FSRQs.

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arXiv2009

M_BH - sigma relation in SDSS flat-spectrum radio quasars

The relationship between the black hole mass and velocity dispersion indicated with [O III] line width is investigated for a sample of 87 flat-spectrum radio quasars (FSRQs) selected from SDSS DR3 quasar catalogue. We found the M_bh - sigma_[O III] relation is deviated from Tremaine et al. relation for nearby inactive galaxies, with a larger black hole mass at given velocity dispersion. There is no strong evidence of cosmology evolution in M_bh - sigma_[O III] relation up to z~0.8. A significant correlation between the [O III] luminosity and Broad Line Region (BLR) luminosity is found. When transferring the [O III] luminosity to Narrow Line Region (NLR) luminosity, the BLR luminosity is, on average, larger than NLR one by about one order of magnitude. We found a strong correlation between the synchrotron peak luminosity and NLR luminosity, which implies a tight relation between the jet physics and accretion process.

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arXiv2009

Images of the radiatively inefficient accretion flow surrounding a Kerr black hole: application in Sgr A*

In fully general relativity, we calculate the images of the radiatively inefficient accretion flow (RIAF) surrounding a Kerr black hole with arbitrary spins, inclination angles, and observational wavelengths. For the same initial conditions, such as the fixed accretion rate, it is found that the intrinsic size and radiation intensity of the images become larger, but the images become more compact in the inner region, while the size of the black hole shadow decreases with the increase of the black hole spin. With the increase of the inclination angles, the shapes of the black hole shadows change and become smaller, even disappear at all due to the obscuration by the thick disks. For median inclination angles, the radial velocity observed at infinity is larger because of both the rotation and radial motion of the fluid in the disk, which results in the luminous part of the images is much brighter. For larger inclination angles, such as the disk is edge on, the emission becomes dimmer at longer observational wavelengths (such as at 7.0mm and 3.5mm wavelengths), or brighter at shorter observational wavelengths (such as at 1.3 mm wavelength) than that of the face on case, except for the high spin and high inclination images. These complex behaviors are due to the combination of the Lorentz boosting effect and the radiative absorption in the disk. We hope our results are helpful to determine the spin parameter of the black hole in low luminosity sources, such as the Galactic center. A primary analysis by comparison with the observed sizes of Sgr A* at millimeters strongly suggests that the disk around the central black hole at Sgr A* is highly inclined or the central black hole is rotating fast.

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arXiv2009

Flat-Spectrum Radio Quasars from SDSS DR3 Quasar Catalogue

We constructed a sample of 185 Flat Spectrum Radio Quasars (FSRQs) by cross-correlating the Shen et al.'s SDSS DR3 X-ray quasar sample with FIRST and GB6 radio catalogues. From the spectrum energy distribution (SED) constructed using multi-band (radio, UV, optical, Infrared and X-ray) data, we derived the synchrotron peak frequency and peak luminosity. The black hole mass and the broad line region (BLR) luminosity (then the bolometric luminosity) were obtained by measuring the line-width and strength of broad emission lines from SDSS spectra. We define a subsample of 118 FSRQs, of which the nonthermal jet emission is thought to be dominated over the thermal emission from accretion disk and host galaxy. For this subsample, we found 25 FSRQs having synchrotron peak frequency > 10^{15} Hz, which is higher than the typical value for FSRQs. While only a weak anti-correlation is found between the synchrotron peak frequency and peak luminosity, it becomes significant when combining with the Wu et al.'s sample of 170 BL Lac objects. At similar peak frequency, the peak luminosity of FSRQs with $\nupeak > 10^{15}$ Hz is systematically higher than that of BL Lac objects, with some FSRQs out of the range covered by BL Lac objects. Although high $\nupeak$ are found in some FSRQs, they do not reach the extreme value of BL Lacs. For the subsample of 118 FSRQs, we found significant correlations between the peak luminosity and black hole mass, the Eddington ratio, and the BLR luminosity, indicating that the jet physics may be tightly related with the accretion process.

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arXiv2009

The bulk kinetic power of radio jets in active galactic nuclei

Based on the Königl's inhomogeneous jet model, we estimate the jet parameters, such as bulk Lorentz factor $Γ$, viewing angle $θ$ and electron number density $n_{\rm e}$ from radio VLBI 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 are tightly linked with the radio jets, though how the disk 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 are closely related with the energy flux transported through jets from the central part of AGNs.

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arXiv2009

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, and the hard X-ray bolometric correction factor L_bol/L_x increases with the Eddington ratio. The Compton reflection is also found to be correlated with the hard X-ray spectral index. 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 αp_gas fails to explain all these observational features, while αp_tot always leads to constant L_bol/L_x independent of the Eddington ratio. The resulted spectra of the disc-corona systems with α(p_gas p_tot)^1/2 show that both the hard X-ray spectral index and the hard X-ray bolometric correction factor L_bol/L_x 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, which may imply the different accretion mode 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.

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arXiv2009

On the BL Lacertae objects/radio quasars and the FRI/II dichotomy

In the frame of unification schemes for radio-loud active galactic nuclei (AGNs), FR I radio galaxies are believed to be BL Lacertae (BL Lac) objects with the relativistic jet misaligned to our line of sight, and FR II radio galaxies correspond to misaligned radio quasars. The Ledlow-Owen dividing line for FR I/FR II dichotomy in the optical absolute magnitude of host galaxy-radio luminosity (M_R-L_Rad) plane can be translated to the line in the black hole mass-jet power (M_bh-Q_jet) plane by using two empirical relations: Q_jet-L_Rad and M_bh}-M_R. We use a sample of radio quasars and BL Lac objects with measured black hole masses to explore the relation of the jet power with black hole mass, in which the jet power is estimated from the extended radio emission. It is found that the BL Lac objects are clearly separated from radio quasars by the Ledlow & Owen FR I/II dividing line in the M_bh-Q_jet plane. This strongly supports the unification schemes for FR I/BL Lac object and FR II/radio quasar. We find that the Eddington ratios L_bol/L_Edd of BL Lac objects are systematically lower than those of radio quasars in the sample with a rough division at L_bol/L_Edd 0.01, and the distribution of Eddington ratios of BL Lac objects/quasars exhibits a bimodal nature, which imply that the accretion mode of BL Lac objects may be different from that of radio quasars.

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arXiv2008

Rapidly spinning massive black holes in active galactic nuclei: evidence from the black hole mass function

The comparison of the black hole mass function (BHMF) of active galactic nuclei (AGN) relics with the measured mass function of the massive black holes in galaxies provides strong evidence for the growth of massive black holes being dominated by mass accretion. We derive the Eddington ratio distributions as functions of black hole mass and redshift from a large AGN sample with measured Eddington ratios given by Kollmeier et al. We find that, even at the low mass end, most black holes are accreting at Eddington ratio ~0.2, which implies that the objects accreting at extremely high rates should be rare or such phases are very short. Using the derived Eddington ratios, we explore the cosmological evolution of massive black holes with an AGN bolometric luminosity function (LF). It is found that the resulted BHMF of AGN relics is unable to match the measured local BHMF of galaxies for any value of (constant) radiative efficiency. Motivated by Volonteri, Sikora & Lasota's study on the spin evolution of massive black holes, we assume the radiative efficiency to be dependent of black hole mass, i.e., it is low for M<10^8 solar masses and it increases with black hole mass for black holes with >10^8 solar masses. We find that the BHMF of AGN relics can roughly reproduce the local BHMF of galaxies if the radiative efficiency ~0.08 for the black holes with <10^8 solar masses and it increases to ~0.18 for black holes with >10^9 solar masses, which implies that most massive black holes (>10^9 solar masses) are spinning very rapidly.

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arXiv2008

Jet power extracted from ADAF and the applications to X-ray binaries and radio galaxy FR dichotomy

We calculate the jet power of the classical Blandford-Znajek(BZ) model and hybrid model developed by Meier based on the global solutions of advection dominated accretion flows (ADAFs) surrounding Kerr black holes. We find that the jet power of the hybrid model is larger than that of the pure BZ model. The jet power will dominate over the accretion power, and the objects will enter into "jet-power-dominated advective systems", when the accretion rate is less than a critical value mdot_c=Mdot_c/Mdot_Edd, where 3*10^-4 < mdot_c < 5*10^-3 is a function of black hole spin parameter. The accretion power will be dominant when mdot<mdot_c and the objects will enter into "accretion-power-dominated advective systems." This is roughly consistent with that constrained from the low/hard-state black hole X-ray binaries (e.g., Fender et al.). We calculate the maximal jet power as a function of black hole mass with the hybrid jet formation model, and find it can roughly reproduce the dividing line of the Ledlow-Owen relation for FR I/FR II dichotomy in the jet power-black hole(BH) mass plane (Q_jet-M_BH) if the dimensionless accretion rate mdot~0.01 and BH spin parameter j~0.9-0.99 are adopted. This accretion rate mdot~0.01 is consistent with that of the critical accretion rate for the accretion mode transition of a standard disk to an ADAF constrained from the state transition of X-ray binaries. Our results imply that most FR I galaxies may be in the ADAF accretion mode similar to the low/hard-state XRBs.

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arXiv2008

Origin of the X-ray Emission in the Nuclei of FR Is

We investigate the X-ray origin in FRIs using the multi-waveband high resolution data of eight FR I sources, which have very low Eddington ratios. We fit their multi-waveband spectrum using a coupled accretion-jet model. We find that X-ray emission in the source with the highest L_X (~1.8*10^-4 L_Edd) is from the advection-dominated accretion flow (ADAF). Four sources with moderate L_X(~several*10^-6 L_Edd) are complicated. The X-ray emission of one FR I is from the jet, and the other three is from the sum of the jet and ADAF. The X-ray emission in the three least luminous sources (L_X<1.0*10^-6L_Edd) is dominated by the jet. These results roughly support the predictions of Yuan and Cui(2005) where they predict that when the X-ray luminosity of the system is below a critical value, the X-radiation will not be dominated by the emission from the ADAF any longer, but by the jet. We also find that the accretion rates in four sources must be higher than the Bondi rates, which implies that other fuel supply (e.g., stellar winds) inside the Bondi radius should be important.

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arXiv2008

The advection-dominated accretion flow+thin accretion disk model for two low-luminosity active galactic nuclei: M81 and NGC4579

It was found that the advection-dominated accretion flow (ADAF)+thin disk model calculations can reproduce the observed spectral energy distributions (SED) of the two low luminosity AGN, provided they are accreting at ~0.01-0.03 Eddington rates and the thin disks are truncated to ADAFs at ~100 R_s (Schwarzschild radii) for M81 and NGC4579 (Quataert et al., 1999). However, the black hole masses adopted in their work are about one order of magnitude lower than recent measurements on these two sources. Adopting the well estimated black hole masses, our ADAF+thin disk model calculations can reproduce the observed SEDs of these two LLAGN, if the black hole is accreting at 2.5e-4 Eddington rates with the thin disk truncated at 120 R_s for M81 (3.3e-3 and R_tr = 80R_s are required for NGC4579). The observed widths of the thermal X-ray iron lines at 6.8 keV are consistent with the Doppler broadening by the Keplerian motion of the gases in the transition zones at ~100R_s. The observed thermal X-ray lines provide a useful diagnosis on the physical properties of the transition zones. We calculate the thermal X-ray line emission from the transition zone between the ADAF and the thin disk with standard software package Astrophysical Plasma Emission Code (APEC), and the physical implications on the models of the transition zones are discussed.

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arXiv2008

The relation between black hole masses and Lorentz factors of the jet components in blazars

We explore the relation between black hole mass ($M_{\rm BH}$) and the motion of the jet components for a sample of blazars. The Very Long Baseline Array (VLBA) 2cm Survey and its continuation: Monitoring of Jets in active galactic nuclei (AGNs) with VLBA Experiments (MOJAVE) have observed 278 radio-loud AGNs, of which 146 blazars have reliable measurements on their apparent velocities of jet components. We calculate the minimal Lorentz factors for these sources from their measured apparent velocities, and their black hole masses are estimated with their broad-line widths. A significant intrinsic correlation is found between black hole masses and the minimal Lorentz factors of the jet components, which the Eddington ratio is only weakly correlated with the minimal Lorentz factor, which may imply that the Blandford-Znajek (BZ) mechanism may dominate over the Blandford-Payne (BP) mechanism for the jet acceleration (at least) in blazars.

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arXiv2008

An accretion disc model for quasar optical variability

Some different correlations between optical-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 disk model can explain these correlations, provided the optical-UV variability is triggered by the change of accretion rate. The disk 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_dot (m_dot = M_dot / M_dotEdd). 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_dot0 ~ 0.1 with variation of m_delta ~ 0.4 - 0.5 m_dot0. We also found that the observed correlations of optical-UV variability amplitude with luminosity or rest-frame wavelength can be qualitatively explained by this accretion disc model.

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arXiv2007

How many radio-loud quasars can be detected by the Gamma-Ray Large Area Space Telescope?

In the unification scheme, radio quasars and FR II radio galaxies come from the same parent population, but viewed at different angles. Based on the Comptonization models for the gamma-ray emission from active galactic nuclei (AGNs), we estimate the number of radio quasars and FR II radio galaxies to be detected by the Gamma-Ray Large Area Space Telescope (GLAST) using the luminosity function (LF) of their parent population derived from the flat-spectrum radio quasar (FSRQ) LF. We find that ~1200 radio quasars will be detected by GLAST, if the soft seed photons for Comptonization come from the regions outside the jets. We also consider the synchrotron self-Comptonization (SSC) model, and find it unlikely to be responsible for gamma-ray emission from radio quasars. We find that no FR II radio galaxies will be detected by GLAST. Our results show that most radio AGNs to be detected by GLAST will be FSRQs (~99 % for the external Comptonization model, EC model), while the remainder (~1 %) will be steep-spectrum radio quasars (SSRQs). This implies that FSRQs will still be good candidates for identifying gamma-ray AGNs even for the GLAST sources. The contribution of all radio quasars and FR II radio galaxies to the extragalactic gamma-ray background (EGRB) is calculated, which accounts for ~30 % of the EGRB.

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arXiv2007

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

We investigate the X-ray origin in FR Is using the radio, submillimetre, optical, and {\em Chandra} X-ray data of a small sample consisting of eight FR I sources. These sources are very dim, with X-ray luminosities $L_{\rm X}/L_{\rm Edd} \sim 10^{-4}-10^{-8}$ ($L_{\rm X}$ is the X-ray luminosity between 2-10 keV). We try to fit the multiwaveband spectrum using a coupled accretion-jet model. In this model, the accretion flow is described by an advection-dominated accretion flow (ADAF) while in the innermost region of ADAF a fraction of accretion flow is transferred into the vertical direction and forms a jet. We find that X-ray emission in the source with the highest $L_{\rm X}$ ($\sim 1.8 \times 10^{-4}L_{\rm Edd}$) is from the ADAF. The results for the four sources with moderate $L_{\rm X}$ ($\sim$ several $\times 10^{-6}L_{\rm Edd}$) are complicated. Two are mainly from the ADAFs, one from the jet, and the other from the sum of the jet and ADAF. The X-ray emission in the three least luminous sources ($L_{\rm X} \lesssim 1.0\times 10^{-6}L_{\rm Edd}$) is dominated by the jet although for one source it can also be interpreted by the ADAF since the quality of X-ray data is low. We conclude that these results roughly support the predictions of Yuan & Cui (2005) where they predict that when the X-ray luminosity of the system is below a 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 rates in four sources among the five in which we can have good constraints to their accretion rates must be higher than the Bondi rates. This implies that other fuel supply, such as the gas released by the stellar population inside the Bondi radius, should be important.

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arXiv2007

Growth of massive black holes during radiatively inefficient accretion phases

The massive black holes in most faint active galactic nuclei (AGNs) and even normal galaxies are still accreting gases, though their accretion rates are very low. Radiatively inefficient accretion flows (RIAFs) are supposed in these faint sources, which should radiate mostly in the hard X-ray band. We calculate the contribution to the X-ray background from both the bright AGNs and the RIAFs in faint AGNs/normal galaxies. Our calculations show that both the observed intensity and spectral shape of the XRB with an energy peak at ~30$ keV can be well reproduced without including the emission of Compton-thick AGNs, if the massive black holes in faint AGNs/normal galaxies are spinning rapidly with a~0.9 and accreting at rates ~1.0-3.0\times 10^{-4}. It indicates that less than ~5 per cent of local massive black hole mass density was accreted during radiatively inefficient accretion phases, which is obviously only an upper limit, because Compton-thick AGNs have not been considered. If the same number of the Compton-thick AGNs with log N_H=24-25 as those with log N_H=23-24 is considered, the fraction of local black hole mass density accumulated during inefficient accretion phases should be lower than ~2 per cent.

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arXiv2007

Constraints on radiatively inefficient accretion history from Eddington ratio distribution of active galactic nuclei

The transition of a standard thin disk to a radiatively inefficient accretion flow (RIAF) is expected to occur, when the accretion rate is close to the critical rate. The radiative efficiencies of accretion flows accreting at rates lower than the critical accretion rate become significantly lower than that of standard thin disks. It is believed that the initial transition radius is small just after the accretion mode transition, and then the transition radius increases with decreasing accretion rate, as suggested by some theoretical models and observations. Based on such variable transition radius models, we derive how the dimensionless accretion rate evolves with time from the observed Eddington ratio distribution for a sample of low-luminosity active galactic nuclei in the local universe. The derived time-dependent accretion rates show a rapid decrease after the transition of the standard thin accretion disk to a RIAF, which is consistent with that derived from the hard X-ray background.

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