arXiv subjects

Xue-Bing Wu

Publications and source records attributed to Xue-Bing Wu.

At least 163 records · Page 9Linked to original sources

arXiv2006

Accretion Disk Spectra of the Brightest Ultra-luminous X-ray Source in M82

Emission spectra of hot accretion disks characteristic of advection dominated accretion flow (ADAF) models are investigated for comparison with the brightest ultra-luminous source, X-1, in the galaxy M82. If the spectral state of the source is similar to the low luminosity hard state of stellar mass black holes in our Galaxy, a fit to the {\it Chandra} X-ray spectrum and constraints from the radio and infrared upper limits, require a black hole mass in the range of $9 \times 10^4 - 5 \times 10^5 \msun$. Lower black hole masses ($\la 10^4 \msun$) are possible if M82 X-1 corresponds to the high luminosity hard state of Galactic black hole X-ray binary sources. Both of these spectrally degenerate hot accretion disk solutions lead to an intermediate mass black hole interpretation for M82 X-1. Since these solutions have different spectral variability with X-ray luminosity and predict different infrared emission, they can be distinguished by future off axis {\it Chandra} observations or simultaneous sensitive infrared detections.

astro-ph↗

arXiv2006

Harmonic QPOs and Thick Accretion Disk Oscillations in BL Lac Object AO 0235+164

Periodic outbursts are observed in many AGNs and usually explained with a supermassive black hole binary (SMBHB) scenario. However, multiple periods are observed in some AGNs and cannot be explained with it. Here we analyze the periodicity of the radio light curves of AO 0235+164 at multi-frequencies and report the discovery of six QPOs in integer ratio 1:2:3:4:5:6 of QPO frequencies, of which the second with period $P_2 = (5.46 \pm 0.47) {\rm yr}$ is the strongest. We fit the radio light curves and show that the initial phases of six QPOs have zero or $π$ differences relative to each other. We suggest a harmonic relationship of QPOs. The centroid frequency, relative strength, harmonic relationship and relative initial phases of QPOs are independent of radio frequency. The harmonic QPOs are likely due to the quasi-periodic injection of plasma from an oscillating accretion disk into the jet. We estimate the supermassive black hole mass $M_{\rm BH} \simeq (4.72\pm 2.04) \times 10^8 M_\odot$ and the accretion rate $\dot{m}\simeq 0.007$. With the knowledge of accretion disk, it implies that the inner region of accretion disk of AO 0235+164 is a radiatively inefficient accretion flow. The oscillation accretion is due to the p-mode oscillation of the thick disk probably excited by a SMBHB. The theoretical predications of fundamental oscillation frequency and the harmonics are well consistent with the observations. Harmonic QPOs would be absent when the thick disk becomes geometrically thin due to the increase of accretion rate. We discuss the observations of AO 0235+164 basing on the SMBHB-thick disk oscillation scenario.

astro-ph↗

arXiv2006

The non-constant slope of the CIV Baldwin effect in NGC 4151

The relationship between the emission-line equivalent width and the continuum luminosity, so called the Baldwin effect, plays an important role in studying the physics of the broad line region of AGNs. Using the archived ultraviolet spectra obtained by IUE, HST, and HUT in 1978-2002, we investigated the intrinsic CIV Baldwin effect of a well-studied Seyfert 1 galaxy NGC 4151. Both its continuum flux and CIV emission-line flux varied about two orders of magnitude in more than two decades, making it one of the best targets for studying the slope variation of the Baldwin effect. We fitted the CIV line profile of the 490 archived UV spectra of NGC 4151 with the same model consisting of a few Gaussian components, and derived the slope in the log-log plot for the total flux of CIV emission line against the UV continuum flux in different observation epochs.We found that the slope is not constant for NGC 4151, as it varies from 0.58 in the highest flux epoch to 0.83 in the lowest flux epoch. The slope evidently decreases as the continuum flux increases, which reinforces the previous findings of the non-constant slope in the H_beta Baldwin effect of NGC 5548 and the CIV Baldwin effect of Fairall 9. Our result suggests that such a non-constant slope may not be unusual for AGNs. Its physical origin is probably related to the different non-linear responses of the emission line to the variable ionizing continuum caused by the different accretion modes at different luminosity levels. We briefly discuss the effects of various absorption components in the CIV line profile of NGC 4151 and argue that the slope variation is not driven mainly by the absorption effect. Intensive, long-term and high resolution spectral observations of strongly variable AGNs are needed in the future to confirm our result.

astro-ph↗

arXiv2006

Black hole mass and accretion rate of Active Galactic Nuclei

The determination of the central black hole mass is crucial to the understanding of AGN physics. In this paper we briefly review some methods that are currently used to estimate the black hole mass of AGNs. Particularly we demonstrate the importance of two correlations: one between the black hole mass and the stellar velocity dispersion and another one between the broad line region (BLR) size and the optical continuum luminosity. Besides applying these relations in deriving black hole masses of various types of AGNs, we also employed the fundamental plane of elliptical galaxies to estimate the central velocity dispersions of AGN host galaxies, and then the black hole masses of AGNs including BL Lac objects. In addition, we derived another empirical relation between the BLR size and H_beta emission line luminosity from AGNs with the BLR size measured by reverberation mapping studies, and argued that more accurate black hole masses of extremely radio-loud AGNs could be obtained with it than using the usual R-L_{5100Å} relation because of the jet contribution to the optical continuum. Finally we pointed out that black hole mass estimation is very much helpful to determine the accretion rate and understand the accretion process in AGNs.

astro-ph↗

arXiv2006

The black hole fundamental plane from a uniform sample of radio and X-ray emitting broad line AGNs

We derived the black hole fundamental plane relationship among the 1.4GHz radio luminosity (L_r), 0.1-2.4keV X-ray luminosity (L_X), and black hole mass (M) from a uniform broad line SDSS AGN sample including both radio loud and radio quiet X-ray emitting sources. We found in our sample that the fundamental plane relation has a very weak dependence on the black hole mass, and a tight correlation also exists between the Eddington luminosity scaled X-ray and radio luminosities for the radio quiet subsample. Additionally, we noticed that the radio quiet and radio loud AGNs have different power-law slopes in the radio--X-ray non-linear relationship. The radio loud sample displays a slope of 1.39, which seems consistent with the jet dominated X-ray model. However, it may also be partly due to the relativistic beaming effect. For radio quiet sample the slope of the radio--X-ray relationship is about 0.85, which is possibly consistent with the theoretical prediction from the accretion flow dominated X-ray model. We briefly discuss the reason why our derived relationship is different from some previous works and expect the future spectral studies in radio and X-ray bands on individual sources in our sample to confirm our result.

astro-ph↗

arXiv2005

Estimate black hole masses of AGNs using ultraviolet emission line properties

Based on the measured sizes of broad line region of the reverberation-mapping AGN sample, two new empirical relations are introduced to estimate the central black hole masses of radio-loud high-redshift ($z > 0.5$) AGNs. First, using the archival $IUE/HST$ spectroscopy data at UV band for the reverberation-mapping objects, we obtained two new empirical relations between the BLR size and \Mg/\C emission line luminosity. Secondly, using the newly determined black hole masses of the reverberation-mapping sample for calibration, two new relationships for determination of black hole mass with the full width of half maximum and the luminosity of \Mg/\C line are also found. We then apply the relations to estimate the black hole masses of AGNs in Large Bright Quasar Surveyq and a sample of radio-loud quasars. For the objects with small radio-loudness, the black hole mass estimated using the $R_{\rm BLR} - L_{\eMg/\eC}$ relation is consistent with that from the $R_{BLR} - L_{3000Å/1350 Å}$ relation. But for radio-loud AGNs, the mass estimated from the $R_{BLR} - L_{\eMg/\eC}$ relation is systematically lower than that from the continuum luminosity $L_{3000Å/1350Å}$. Because jets could have significant contributions to the UV/optical continuum luminosity of radio-loud AGNs, we emphasized again that for radio-loud AGNs, the emission line luminosity may be a better tracer of the ionizing luminosity than the continuum luminosity, so that the relations between the BLR size and UV emission line luminosity should be used to estimate the black hole masses of high redshift radio-loud AGNs.

astro-ph↗

arXiv2004

Black hole mass and accretion rate of active galactic nuclei with double-peaked broad emission lines

(Abridged) Using an empirical relation between the broad line region size and optical continuum luminosity, we estimated the black hole mass and accretion rate for 135 AGNs with double-peaked broad emission lines in two samples. With black hole masses from $3\times 10^7M_\odot$ to $5\times 10^9M_\odot$, these AGNs have the dimensionless accretion rates (Eddington ratios) between 0.001 and 0.1, and the bolometric luminosity between $10^{43}erg/s$ and $10^{46}erg/s$, both being significantly larger than those of several previously known low-luminosity double-peaked AGNs. The optical-X-ray spectra indices, $α_{OX}$, of these high-luminosity double-peaked AGNs is between 1 and 1.9. Modest correlations of the $α_{OX}$ value with the Eddington ratio and bolometric luminosity indicate that double-peaked AGNs with higher Eddington ratio or higher luminosity tend to have larger $α_{OX}$ value. Therefore we suggested that the accretion process in some high-luminosity double-peaked AGNs is probably different from that of low-luminosity objects where an ADAF-like accretion flow was thought to exist. This is also supported by the presence of possible big blue bumps in the spectra of some double-peaked AGNs with higher Eddington ratios. We noticed that the prototype double-peaked emission line AGN, Arp 102B, may be an ``intermediate'' object between the high and low luminosity double-peaked AGNs. In addition, we found an apparent strong anti-correlation between the peak separation of double-peaked profile and Eddington ratio. If it is real, it may provide us a clue to understand why double-peaked broad emission lines were hardly found in luminous AGNs with Eddington ratio larger than 0.1.

astro-ph↗

arXiv2004

Black hole mass estimation with a relation between the BLR size and emission line luminosity of AGN

An empirical relation between the broad line region (BLR) size and optical continuum luminosity is often adopted to estimate the BLR size and then the black hole mass of AGNs. However, optical luminosity may not be a good indicator of photoionizing luminosity for extremely radio-loud AGNs because the jets usually contribute significantly to the optical continuum. Therefore, the black hole masses derived for blazar-type AGNs with this method are probably overestimated. Here we first derived a tight empirical relation between the BLR size and the H$_β$ emission line luminosity, $R(\rm{light-days})= 24.05(L_{H_β}/10^{42} ergs s^{-1})^{0.68}$, from a sample of 34 AGNs with the BLR size estimated with the reverberation mapping technique. Then we applied this relation to estimate the black hole masses of some AGNs and found that for many extremely radio-loud AGNs the black hole masses obtained with the $R-L_{H_β}$ relation are systematically lower than those derived previously with the $R-L_{5100Å}$ relation, while for radio-quiet and slightly radio-loud AGNs the results obtained with these two methods are almost the same. The difference of black hole masses estimated with these two relations increases with the radio-loudness for extremely radio-loud AGNs, which is consistent with the fact that their equivalent widths of H$_β$ emission line become smaller at higher radio-loudness. If the small H$_β$ equivalent widths of extremely radio-loud AGNs are indeed caused by the beaming effect, we argue that the optical emission line luminosity may be a better tracer of ionizing luminosity for blazar-type AGNs and the black hole mass derived with the $R-L_{H_β}$ relation are probably more accurate.

astro-ph↗

arXiv2003

Double-double radio galaxies: remnants of merger of supermassive binary black holes

The activity of active galaxy may be triggered by the merge of galaxies and present-day galaxies are probably the product of successive minor mergers. The frequent galactic merges at high redshift imply that active galaxy harbors supermassive unequal-mass binary black holes in its center at least once during its life time. In this paper, we showed that the recently discovered double-lobed FR II radio galaxies are the remnants of such supermassive binary black holes. The inspiraling secondary black hole opens a gap in the accretion disk and removes the inner accretion disk when it merges into the primary black hole, leaving a big hole of about several hundreds of Schwarzschild radius in the vicinity of the post-merged supermassive black hole and leading to an interruption of jet formation. When the outer accretion disk slowly refills the big hole on a viscous time scale, the jet formation restarts and the interaction of the recurrent jets and the inter-galactic medium forms a secondary pair of lobes. We applied the model to a particular double-lobed radio source B1834+620 and showed that the orbit of the secondary is elliptical with a typical eccentricity $e \simeq 0.68$ and the mass ratio $q$ of the secondary and the primary is $0.01 \la q \la 0.4$. The accretion disk is a standard $α$-disk with $0.01 \la α\la 0.04$ and the ratio of disk half height $H$ and radius $r$ is $δ\simeq 0.01$. The model predicates that double-lobed radio structure forms only in FR II radio galaxies.

astro-ph↗

arXiv2003

Color-redshift Relations and Photometric Redshift Estimations of Quasars in Large Sky Surveys

With a recently constructed composite quasar spectrum and the χ^2 minimization technique, we demonstrated a general method to estimate the photometric redshifts of a large sample of quasars by deriving the theoretical color-redshift relations and comparing the theoretical colors with the observed ones. We estimated the photometric redshifts from the 5-band SDSS photometric data of 18678 quasars in the first major data release of SDSS and compare them with the spectroscopic redshifts. The redshift difference is smaller than 0.1 for 47% of quasars and 0.2 for 68 % of them. Based on the calculation of the theoretical color-color diagrams of stars, galaxies and quasars in both the SDSS and BATC photometric systems, we expected that with the BATC system of 15 intermediate filters we would be able to select candidates of high redshift quasars more efficiently than in the SDSS, provided the BATC survey could detect objects with magnitude fainter than 21.

astro-ph↗

arXiv2002

Black hole mass and binary model for BL Lac object OJ 287

Recent intensive observations of the BL Lac object OJ 287 raise a lot of questions on the models of binary black holes, processing jets, rotating helical jets and thermal instability of slim accretion disks. After carefully analyzing their radio flux and polarization data, Valtaoja et al. (\cite{valtaoja00}) propose a new binary model. Based on the black hole mass of $4 \times 10^8 {\rm M_\odot}$ estimated with the tight correlations of the black hole masses and the bulge luminosity or central velocity dispersion of host galaxies, we computed the physical parameters of the new binary scenario. The impact of the secondary on the accretion disk around the primary black hole causes strong shocks propagating inwards and outwards, whose arrival at the jet roots is identified with the rapid increase of optical polarization and the large change of polarization angle at about 0.30 yr after the first main optical flare. An increase of optical polarization, a large rotation of positional angle and a small synchrotron flare at 2007.05 between the optical outbursts at 2006.75 and 2007.89 are expected by the model. With the estimated parameters, we predicated an increase of $γ$-ray flux appearing about 5 days after the first optical/IR peak, which is consistent with the EGRET observations.

astro-ph↗

arXiv2002

Supermassive Black Hole Masses of AGNs with Elliptical Hosts

The recently discovered tight correlation between supermassive black hole mass and central velocity dispersion suggests a possibility to estimate the SMBH mass from the measured central velocity dispersion. However, for most AGNs it is difficult to measure the central velocity dispersions of their host galaxies directly. In this paper we adopt the fundamental plane for ellipticals to estimate the central velocity dispersion and SMBH mass for a number of AGNs with morphology parameters of their elliptical host galaxies obtained by the HST imaging observations. The estimated SMBH masses of 63 BL Lac objects, 10 radio galaxies, 10 radio-loud quasars and 9 radio-quiet quasars are mostly in the range of $10^{7.5} M_\odot$ to $10^{9} M_\odot$. No significant difference in black hole mass is found for high-frequency peaked BL Lacs and low-frequency peaked BL Lacs, as well as for radio galaxies and radio-loud quasars. The Eddington ratios of radio galaxies are substantially smaller than those of quasars. This suggests that the different observational features of radio-loud AGNs may be mainly dominated by accretion rate rather than by the black hole mass, which is in agreement with some evolutionary scenarios recently proposed for radio-loud AGNs. Different from some previous claims, we found that the derived mean SMBH mass for radio-loud quasars is only slightly larger than that of radio-quiet quasars. Though their SMBH mass distributions are statistically different, their Eddington ratio distributions are probably from the same population. In addition, we noted that the relation between black hole mass and host galaxy luminosity obtained using the fundamental plane provides further arguments for a nonlinear scaling law between SMBH and galactic bulge mass.

astro-ph↗

arXiv2001

On black hole masses, radio-loudness and bulge luminosities of Seyfert galaxies

We estimated black hole masses for 9 Seyfert 1 and 13 Seyfert 2 galaxies in the Palomar and CfA bright Seyfert samples using the tight correlation between black hole mass and bulge velocity dispersion. Combining other 13 Seyfert 1s and 2 Seyfert 2s in these samples but with black hole masses measured recently by reverberation mapping and stellar/gas dynamics, we studied the correlations of black hole masses with radio loudness and bulge luminosities for a sample of 37 Seyfert galaxies. We found that if radio-loudness is measured using the optical and radio luminosities of the nuclear components, the black hole masses of radio-loud Seyfert 1s tend to increase with the radio-loudness. The black hole masses of all Seyfert galaxies increase with the radio power, but Seyfert galaxies have larger radio powers than nearby galaxies with the same black hole masses. In addition, the correlation between black hole masses and bulge V-band luminosities for Seyfert galaxies is consistent with that found for quasars and normal galaxies. The combined sample of 37 Seyfert galaxies, 15 quasars and 30 normal galaxies suggests a possible universal nonlinear relation between black hole and bulge masses, $M(BH) \propto M(bulge)^{1.74\pm0.14}$, which is slightly steeper than that found recently by Laor (2001) for a smaller sample. This nonlinear relation is supported by a larger sample including 65 Seyfert galaxies. The different M(BH)/M(bulge) ratio for galaxies with different bulge luminosities or different black hole masses may be explained by this relation. These results are consistent with some theoretical implications and are important for understanding the nature of radio emissions and the formation and evolution of supermassive black holes and galaxies.

astro-ph↗

arXiv2001

Inclinations and black hole masses of Seyfert 1 galaxies

A tight correlation of black hole mass and central velocity dispersion has been found recently for both active and quiescent galaxies. By applying this correlation, we develop a simple method to derive the inclination angles for a sample of 11 Seyfert 1 galaxies that have both measured central velocity dispersions and black hole masses estimated by reverberation mapping. These angles, with a mean value of 36 degree that agrees well with the result obtained by fitting the iron K$α$ lines of Seyfert 1s observed with ASCA, provide further support to the orientation-dependent unification scheme of AGN. A positive correlation of the inclinations with observed FWHMs of H$β$ line and a possible anti-correlation with the nuclear radio-loudness have been found. We conclude that more accurate knowledge on inclinations and broad line region dynamics is needed to improve the black hole mass determination of AGN with the reverberation mapping technique.

astro-ph↗

arXiv2001

Testing the transition layer model of quasi-periodic oscillations in neutron star X-ray binarie

We compare the theoretical predictions of the transition layer model with some observational features of quasi-periodic oscillations (QPOs) in neutron star X-ray binaries. We found that the correlation between horizontal branch oscillation (HBO) frequencies and kilohertz (kHz) QPO frequencies, the difference between the low-frequency QPOs in atoll sources and HBOs in Z sources, and the correlation between the frequencies of low-frequency QPOs and break frequencies can be well explained by the transition layer model, provided the neutron star mass is around 1.4 solar mass and the angle between magnetosphere equator and accretion disk plane is around 6 degree. The observed decrease of peak separation between two kHz QPO frequencies with the increase of kHz QPO frequencies and the increase of QPO frequencies with the increase of inferred mass accretion rate are also consistent with the theoretical predictions of transition layer model. In addition, we derive a simple equation that can be adopted to estimate the angle ($δ$) between magnetosphere equator and accretion disk plane by use of the simultaneously observed QPO frequency data. We estimate these angles, in the range of 4 to 8 degrees, for five Z sources and two atoll sources. The nearly constant $δ$ value for each source, derived from the different sets of simultaneously observed QPO frequency data, provides a strong test of the theoretical model. Finally, we suggest that the similar transition layer oscillations may be also responsible for the observed QPOs in accretion-powered millisecond X-ray pulsar and Galactic black hole candidates.

astro-ph↗

arXiv2000

Trapped Disk Oscillations and Stable QPOs in Microquasars

The observed quasi-periodic oscillations (QPOs) in low-mass X-ray binaries have been suggested to be probably related to the characteristic oscillation modes in the inner part of a relativistic accretion disk. In this paper we study the trapped feature of these oscillation modes by considering the radial perturbations and viscosity effects. We find that the trapped oscillations are amplified significantly when the viscosity parameter increases and the trapped size varies when the radial wavenumber and Mach number change. Our results support that the stable hundred-hertz QPOs and their low fractional rms amplitudes found in two Galactic microquasars may be explained by some trapped non-damping oscillation modes in the inner part of accretion disks around rotating black holes.

astro-ph↗

arXiv2000

Three-Layered Atmospheric Structure in Accretion Disks Around Stellar-Mass Black Holes

Modeling of the x-ray spectra of the Galactic superluminal jet sources GRS 1915+105 and GRO J1655-40 reveal a three-layered atmospheric structure in the inner region of their accretion disks. Above the cold and optically thick disk of a temperature 0.2-0.5 keV, there is a warm layer with a temperature of 1.0-1.5 keV and an optical depth around 10. Sometimes there is also a much hotter, optically thin corona above the warm layer, with a temperature of 100 keV or higher and an optical depth around unity. The structural similarity between the accretion disks and the solar atmosphere suggest that similar physical processes may be operating in these different systems.

astro-ph↗

arXiv1999

X-ray luminous radio-quiet high redshift QSOs in the ROSAT All-Sky Survey

X-ray luminous radio-quiet high redshift QSOs are rare and can be used for the investigation of several important astronomical questions. We have conducted a large area survey for radio-quiet high redshift QSOs in the ROSAT All-Sky Survey, from which QSO candidates are selected using the digitized objective prism spectra from the Hamburg Quasar Survey. The 22 candidates with Galactic latitudes larger than $35^o$ were observed with the 2.16m telescope at Xinglong station of Beijing Astronomical Observatory. Among the 19 new QSOs in our sample, six are radio-quiet QSOs with redshifts larger than 1.3 and three of them have redshifts larger than 2. Thus we have doubled the number of known X-ray luminous, radio-quiet high redshift QSOs in the surveyed sky area. The distribution of $f_{1.4GHz}/f_{1keV}$ of radio-loud and radio-quiet QSOs in this area shows two well separated peaks. This can be taken as an evidence for different emission mechanisms of the observed X-rays in the two subgroups.

astro-ph↗