arXiv subjects

Xue-Bing Wu

Publications and source records attributed to Xue-Bing Wu.

At least 145 records · Page 8Linked to original sources

arXiv2011

On the structure of Accretion Disks with Outflows

In order to study the outflows from accretion disks, we solve the set of hydrodynamic equations for accretion disks in the spherical coordinates ($rθϕ$) to obtain the explicit structure along the $θ$ direction. Using self-similar assumptions in the radial direction, we change the equations to a set of ordinary differential equations (ODEs) about the $θ$-coordinate, which are then solved with symmetrical boundary conditions in the equatorial plane, and the velocity field is obtained. The $α$ viscosity prescription is applied and an advective factor $f$ is used to simplify the energy equation.The results display thinner, quasi-Keplerian disks for Shakura-Sunyaev Disks (SSDs) and thicker, sub-Keplerian disks for Advection Dominated Accretion Flows (ADAFs) and slim disks, which are consistent with previous popular analytical models. However, an inflow region and an outflow region always exist, except when the viscosity parameter $α$ is too large, which supports the results of some recent numerical simulation works. Our results indicate that the outflows should be common in various accretion disks and may be stronger in slim disks, where both advection and radiation pressure are dominant. We also present the structure dependence on the input parameters and discuss their physical meanings. The caveats of this work and possible improvements in the future are discussed.

astro-ph.HE↗

arXiv2011

Stellar populations of ultraluminous infrared galaxies

Ultraluminous infrared galaxies (ULIRGs) have several types according to dominance of starburst or AGN component. We made stellar population analysis for a sample of 160 ULIRGs to study the evolution of ULIRGs. We found that the dominance of intermediate-age and old stellar populations increases along the sequence of HII-like ULIRGs, Seyfert-HII composite ULIRGs, and Seyfert 2 ULIRGs. Consequently the typical mean stellar age and the stellar mass increase along the sequence. Comparing the gas mass estimated from the CO measurements with the stellar mass estimated from the optical spectra, we found that gas fraction is anti-correlated with the stellar mass. HII-like ULIRGs with small stellar masses do not possess enough gas and the total mass, and therefore have no evolution connections with massive Seyfert 2 ULIRGs. Only massive ULIRGs may follow the evolution sequence toward AGNs, and massive HII-like ULIRGs are probably in an earlier stage of the sequence.

astro-ph.CO↗

arXiv2010

Estimating black hole masses of blazars

Estimating black hole masses of blazars is still a big challenge. Because of the contamination of jets, using the previously suggested size -- continuum luminosity relation can overestimate the broad line region (BLR) size and black hole mass for radio-loud AGNs, including blazars. We propose a new relation between the BLR size and $H_β$ emission line luminosity and present evidences for using it to get more accurate black hole masses of radio-loud AGNs. For extremely radio-loud AGNs such as blazars with weak/absent emission lines, we suggest to use the fundamental plane relation of their elliptical host galaxies to estimate the central velocity dispersions and black hole masses, if their velocity dispersions are not known but the host galaxies can be mapped. The black hole masses of some well-known blazars, such as OJ 287, AO 0235+164 and 3C 66B, are obtained using these two methods and the M - $σ$ relation. The implications of their black hole masses on other related studies are also discussed.

astro-ph.CO↗

arXiv2010

Eight new quasars discovered by LAMOST in one extragalactic field

We report the discovery of eight new quasars in one extragalactic field (five degree centered at RA=$08^h58^m08.2^s$, Dec=$01^o32'29.7''$) with the LAMOST commissioning observations on December 18, 2009. These quasars, with $i$ magnitudes from 16.44 to 19.34 and redshifts from 0.898 to 2.773, were not identified in the SDSS spectroscopic survey, though six of them with redshifts less than 2.5 were selected as quasar targets in SDSS. Except one source without near-IR $Y$-band data, seven of these eight new quasars meet a newly proposed quasar selection criterion involving both near-IR and optical colors. Two of them were found in the 'redshift desert' for quasars ($z$ from 2.2 to 3), indicating that the new criterion is efficient for recovering the missing quasars with similar optical colors as stars. Although LAMOST met some problems during the commissioning observations, we were still able to identify other 38 known SDSS quasars in this field, with $i$ magnitudes from 16.24 to 19.10 and redshifts from 0.297 to 4.512. Our identifications imply that a substantial fraction of quasars may be missing in the previous quasar surveys. The implication of our results to the future LAMOST quasar survey is discussed.

astro-ph.CO↗

arXiv2010

A very bright i=16.44 quasar in the `redshift desert' discovered by LAMOST

The redshift range from 2.2 to 3, is known as the 'redshift desert' of quasars because quasars with redshift in this range have similar optical colors as normal stars and are thus difficult to be found in optical sky surveys. A quasar candidate, SDSS J085543.40-001517.7, which was selected by a recently proposed criterion involving near-IR $Y-K$ and optical $g-z$ colors, was identified spectroscopically as a new quasar with redshift of 2.427 by the LAMOST commissioning observation in December 2009 and confirmed by the observation made with the NAOC/Xinglong 2.16m telescope in March 2010. This quasar was not targeted in the SDSS spectroscopic survey because it locates in the stellar locus of the optical color-color diagrams, while it is clearly separated from stars in the $Y-K$ vs. $g-z$ diagram. Comparing with other SDSS quasars we found this new quasar with $i$ magnitude of 16.44 is apparently the brightest one in the redshift range from 2.3 to 2.7. From the spectral properties we derived its central black hole mass as $(1.4\sim3.9) \times 10^{10} M_\odot$ and the bolometric luminosity as $3.7\times 10^{48}$ \ergs, which indicates that this new quasar is intrinsically very bright and belongs to the most luminous quasars in the universe. Our identification supports that quasars in the redshift desert can be found by the quasar selection criterion involving the near-IR colors. More missing quasars are expected to be recovered by the future LAMOST spectroscopic surveys, which is important to the study of the cosmological evolution of quasars at redshift higher than 2.2.

astro-ph.CO↗

arXiv2010

Quasar candidate selection and photometric redshift estimation based on SDSS and UKIDSS data

We present a sample of 8498 quasars with both SDSS $ugriz$ optical and UKIDSS $YJHK$ near-IR photometric data. With this sample, we obtain the median colour-z relations based on 7400 quasars with magnitude uncertainties less than 0.1mag in all bands. By analyzing the quasar colours, we propose an empirical criterion in the $Y-K$ vs. $g-z$ colour-colour diagram to separate stars and quasars with redshift $z<4$, and two other criteria for selecting high-z quasars. Using the SDSS-UKIDSS colour-z relations, we estimate the photometric redshifts of 8498 SDSS-UKIDSS quasars, and find that 85.0% of them are consistent with the spectroscopic redshifts within $|Δz|<0.2$, which leads to a significant increase of the photometric redshift accuracy than that based on the SDSS colour-z relations only. We compare our colour selection criterion with a small UKIDSS/EDR quasar/star sample and a sample of 4671 variable sources in the SDSS Stripe 82 region with both SDSS and UKIDSS data, and find that they can be clearly divided into two classes (quasars and stars) by our criterion in the $Y-K$ vs. $g-z$ plot. We select 3834 quasar candidates from the variable sources with $g<20.5$ in Stripe 82, 826 of them being SDSS quasars and the rest without SDSS spectroscopy. We demonstrate that even at the same spectroscopy limit as SDSS, with our criterion we can at least partially recover the missing quasars with $z\sim2.7$ in SDSS. The SDSS identified quasars only take a small fraction (21.5%) of our quasar candidates selected from the variable sources in Stripe 82, indicating that a deeper spectroscopy is very promising in producing a larger sample of quasars than SDSS. The implications of our results to the future Chinese LAMOST quasar survey are also discussed.

astro-ph.CO↗

arXiv2009

Ultra-luminous Infrared Galaxies in Sloan Digital Sky Survey Data Release 6

Ultra-luminous infrared galaxies (ULIRGs) are interesting objects with dramatic properties. Many efforts have been made to understand the physics of their luminous infrared emission and evolutionary stages. However, a large ULIRG sample is still needed to study the properties of their central black holes (BHs), the BH-host galaxy relation, and their evolution. We identified 308 ULIRGs from the Sloan Digital Sky Survey Data Release 6, and classified them into the NL ULIRGs (with only narrow emission lines) and the Type I ULIRGs (with broad emission lines). About 56% of ULIRGs in our total sample show interaction features, and this percentage is 79% for redshift z < 0.2. Optical identifications of these ULIRGs show that the AGN percentage is at least 49%, and the percentage increases with the infrared luminosity. We found 62 Type I ULIRGs, and estimated their BH masses and velocity dispersions from their optical spectra. Together with known Type I ULIRGs in the literature, a sample of 90 Type I ULIRGs enables us to make a statistical study. We found that the BH masses of Type I ULIRGs are typically smaller than those of PG QSOs, and most Type I ULIRGs follow the M_{BH}-sigma relation. However, some ULIRGs with larger Eddington ratio deviate from this relation, even the line width of the [OIII] narrow line (NL) core or the [SII] line was used as the surrogate of velocity dispersion. This implies that at least some ULIRGs are probably still in the early evolution stage toward QSOs. The anti-correlation between the mass deviation from the M_{BH}-sigma relation and the Eddington ratio supports that the evolution of Type I ULIRGs is probably followed by the building up of the M_{BH}-sigma relation and the evolution to the QSO phase.

astro-ph.CO↗

arXiv2008

The black hole fundamental plane: revisit with a larger sample of radio and X-ray emitting broad-line AGNs

We use a recently released SDSS catalog of X-ray emitting AGNs in conjunction with the FIRST 20cm radio survey to investigate the black hole fundamental plane relationship between the 1.4GHz radio luminosity ($L_r$), 0.1-2.4 keV X-ray luminosity ($L_X$), and the black hole mass ($M$), namely, $log L_r=ξ_{RX}log L_X+ξ_{RM}log M +const$. For this purpose, we have compiled a large sample of 725 broad-line AGNs, which consists of 498 radio-loud sources and 227 radio-quiet sources. Our results are generally consistent with those in our previous work based on a smaller sample of 115 SDSS AGNs. We confirm that radio-loud objects have a steeper slope ($ξ_{RX}$) in the radio-X-ray relationship with respect to radio-quiet objects, and the dependence of the black hole fundamental plane on the black hole mass ($ξ_{RM}$) is weak. We also find a tight correlation with a similar slope between the soft X-ray luminosity and broad emission line luminosity for both radio-loud and radio-quiet AGNs, which implies that their soft X-ray emission is unbeamed and probably related to the accretion process. With the current larger sample of AGNs, we are able to study the redshift evolution of the black hole fundamental plane relation for both radio-loud and radio-quiet subsamples. We find that there is no clear evidence of evolution for radio-quiet AGNs, while for radio-loud ones there is a weak trend where $ξ_{RM}$ decreases as the redshift increases. This may be understood in part as due to the observed evolution of the radio spectral index as a function of redshift. Finally, we discuss the relativistic beaming effect and some other uncertainties related to the black hole fundamental plane (Abridged).

astro-ph↗

arXiv2007

Modeling the Broadband Spectral Energy Distribution of the Microquasars XTE J1550-564 and H 1743-322

We report results from a systematic study of the spectral energy distribution (SED) and spectral evolution of XTE J1550--564 and H 1743--322 in outburst. The jets of both sources have been directly imaged at both radio and X-ray frequencies, which makes it possible to constrain the spectrum of the radiating electrons in the jets. We modelled the observed SEDs of the jet `blobs' with synchrotron emission alone and with synchrotron emission plus inverse Compton scattering. The results favor a pure synchrotron origin of the observed jet emission. Moreover, we found evidence that the shape of the electron spectral distribution is similar for all jet `blobs' seen. Assuming that this is the case for the jet as a whole, we then applied the synchrotron model to the radio spectrum of the total emission and extrapolated the results to higher frequencies. In spite of significant degeneracy in the fits, it seems clear that, while the synchrotron radiation from the jets can account for nearly 100% of the measured radio fluxes, it contributes little to the observed X-ray emission, when the source is relatively bright. In this case, the X-ray emission is most likely dominated by emission from the accretion flows. When the source becomes fainter, however, the jet emission becomes more important, even dominant, at X-ray energies. We also examined the spectral properties of the sources during outbursts and the correlation between the observed radio and X-ray variabilities. The implication of the results is discussed.

astro-ph↗

arXiv2007

How to form bulges/ellipticals in dark halos as fast as central black holes?

Gravity is nearly a universal constant in the cusp of an NFW galaxy halo. Inside this external field an isothermal gas sphere will collapse and trigger a starburst if above a critical central pressure. Thus formed spheroidal stellar systems have Sersic-profile and satisfy the Faber-Jackson relation. The process is consistent with observed starbursts. We also recover the M_BH vs. velocity dispersion relation, if the gas collapse is regulated or resisted by the feedback from radiation from the central BH.

astro-ph↗

arXiv2007

The Nature of Linearly Polarized Millimeter and Sub-millimeter Emission in Sagittarius A*

The linearly polarized millimeter and sub-millimeter emission in Sagittarius A* is produced within 10 Schwarzschild radii of the supermassive black hole at the Galactic Center and may originate from a hot magnetized accretion disk, where electrons are heated efficiently by turbulent plasma waves. In such a scenario, the flux density and polarization are very sensitive to the electron heating rate and the inclination angle of disk, respectively, and the major axis of the sub-millimeter intrinsic polarization, which is aligned with the rotation axis of the disk, is perpendicular to the major axis of the polarized near-infrared emission. In combination with MHD simulations, which study the properties of the magnetic field and viscous stresses, the current spectral and polarization measurements give tight constraints on the model parameters. Simultaneous observations will be able to test the model.

astro-ph↗

arXiv2007

Disk-Evaporation Fed Corona: Structure and Evaporation Feature with Magnetic Field

The disk-corona evaporation model naturally interprets many observational phenomena in black hole X-ray binaries, such as the truncation of an accretion disk and the spectral state transitions. On the other hand, magnetic field is known to play an important role in transporting angular momentum and producing viscosity in accretion flows. In this work, we explicitly take the magnetic field in the accretion disk corona into account and numerically calculate the coronal structure on the basis of our two-temperature evaporation code. We show that the magnetic field influences the coronal structure by its contribution to the pressure, energy and radiative cooling in the corona and by decreasing the vertical heat conduction. We found that the maximal evaporation rate keeps more or less constant ($\sim 0.03$ Eddington rate) while the strength of magnetic fields changes, but that the radius corresponding to the maximal evaporation rate decreases with increasing magnetic field. This predicts that the spectral state transition always occurs at a few percent of Eddington accretion rate, while the inner edge of thin disk can be at $\sim 100 R_{\rm S} $ or even less in the hard state before the transition to the soft state. These results alleviate the problem that previous evaporation models predict too large a truncation radius, and are in better agreement with the observational results of several black hole X-ray binaries, though discrepancies remain.

astro-ph↗

arXiv2007

A feedback compression star formation model and the black hole - bulge relations

We present a "feedback compression" model to describe the galactic spheroid formation and its relation with the central nuclear activity. We suggest that the star formation itself can serve as the "positive feedback" in some extremely dense region to trigger the starburst. The star formation rate as well as the related stellar feedback-induced turbulence will be maximized under the regulation of the background dark halo's gravity. There is also stellar feedback acting inward to confine and obscure the central black hole (BH) till the BH grows sufficiently large to satisfy a balance condition between the accretion disk wind and the inward stellar feedback. The extremely vigorous star formation activity, the BH - bulge relations, the maximum velocity dispersion as well as the maximum BH mass are investigated based on such scenario, and are found to be consistent with observations.

astro-ph↗

arXiv2007

The mass and luminosity functions and the formation rate of DA white dwarfs in the Sloan Digital Sky Survey

The SDSS Data Release 1 includes 1833 DA white dwarfs (WDs) and forms the largest homogeneous sample of WDs. This sample provides the best opportunity to study the statistical properties of WDs. We adopt a recently established theoretical model to calculate the mass and distance of each WD using the observational data. Then we adopt a bin-correction method to correct for selection effects and use the 1/V weight-factor method to calculate the luminosity function, the continuous mass function and the formation rate of these WDs. The SDSS DA WD sample is incomplete and suffers seriously from selection effects. After corrections for the selection effects, only 531 WDs remain. From this final sample we derive the most up-to-date luminosity function and mass function, in which we find a broad peak of WD masses centered around 0.58$M_{\odot}$. The DA WD space density is calculated as $8.81\times10^{-5}pc^{-3}$ and the formation rate is $2.579\times 10^{-13}pc^{-3}yr^{-1}$. The statistical properties of the SDSS DA WD sample are generally in good agreement with previous observational and theoretical studies, and provide us information on the formation and evolution of WDs. However, a larger and more complete all-sky WD sample is still needed to explain some subtle disagreements and unresolved issues.

astro-ph↗

arXiv2007

CO line width and the black hole -- bulge relationship at high redshift

Recently, it has been suggested that the CO line width (FWHM(CO)) is a surrogate for the bulge velocity dispersion ($σ$) of the host galaxies of high-redshift quasars, and the black hole -- bulge ($M_{BH}-σ$) relation obtained with this assumption departs significantly from the $M_{BH}-σ$ relation in the local universe. In this study, we first present an investigation of the correlation between the CO line width and the bulge velocity dispersion using a sample of 33 nearby Seyfert galaxies. We find that the formula adopted in previous studies, $σ=\rm{FWHM(CO)}/2.35$, is generally not a good approximation. Using it, one may underestimate the value of bulge velocity dispersion significantly when the CO line is narrower than 400 $km s^{-1}$. By involving the galactic inclination angle $i$ as an additional parameter, we obtain a tight correlation between the inclination-corrected CO line width and the bulge velocity dispersion, namely, $\rm {FWHM(CO)}/\sin i=-67.16\pm80.18+(3.62\pm0.68)σ$. Using this new relation, we can better estimate the bulge velocity dispersion from the CO line width if the galactic inclination is known. We apply this new relation to nine high-redshift quasars with CO line detections and find that they are consistent with the local $M_{BH}-σ$ relation if their inclination angles are around $15^o$. The possible smaller inclinations of the high-redshift quasars are preferred because of their relatively greater likelihood of detection, and are also consistent with their relatively smaller CO line widths compared to submillimeter galaxies (SMGs) at high redshift having a similar total amount of molecular gas. Future observations are needed to confirm these results.

astro-ph↗

arXiv2007

Galaxy Bulge Formation: Interplay with Dark Matter Halo and Central Supermassive Black Hole

We present a simple scenario where the formation of galactic bulges was regulated by the dark halo gravity and regulated the growth of the central supermassive black hole. Assuming the angular momentum is low, we suggest that bulges form in a runaway collapse due to the "gravothermal" instability once the central gas density or pressure exceeds certain threshold (Xu & Zhao 2007). We emphasize that the threshold is nearly universal, set by the background NFW dark matter gravity $g_{DM} \sim 1.2 \times 10^{-8}{\rm cm} {\rm sec}^{-2}$ in the central cusps of halos. Unlike known thresholds for gradual formation of galaxy disks, we show that the universal "halo-regulated" star formation threshold for spheroids matches the very high star formation rate and star formation efficiency shown in high-redshift observations of central starburst regions. The starburst feedback also builds up a pressure shortly after the collapse. This large pressure could both act outward to halt further infall of gas from larger scale, and act inward to counter the Compton-thick wind launched from the central black hole in an Eddington accretion. Assuming the feedback balancing inward and outward forces, our scenario naturally gives rise to the black hole-bulge relationships observed in the local universe.

astro-ph↗

arXiv2007

Weighing black holes in radio-loud AGNs

Because of the contamination of jets, using the size -- continuum luminosity relation can overestimate the broad line region (BLR) size and black hole mass for radio-loud AGNs. We propose a new relation between the BLR size and $H_β$ emission line luminosity and present evidences for using it to get more accurate black hole masses of radio-loud AGNs. For extremely radio-loud AGNs such as blazars with weak/absent emission lines, we suggest to use the fundamental plane relation of their elliptical host galaxies to estimate the central velocity dispersions and black hole masses, if their host galaxies can be mapped.

astro-ph↗

arXiv2006

Modeling the hard states of XTE J1550--564 during its 2000 outburst

We study hard states of the black-hole binary XTE J1550--564 during its 2000 outburst. In order to explain those states at their highest luminosities, $L\sim 10%$ of the Eddington luminosity, $L_{\rm E}$, we propose a specific hot accretion flow model. We point out that the highest values of the hard-state $L$ are substantially above the $L$ an advection-dominated accretion flow (ADAF) can produce, $\sim 0.4α^2 L_{\rm E}$, which is only $\sim (3$--$4)%L_{\rm E}$ even for $α$ as high as 0.3. On the other hand, we successfully explain the hard states with $L\sim (4$--$10)%$ using the luminous hot accretion flow (LHAF) model. As $10%L_{\rm E}$ is also roughly the highest luminosity an LHAF can produce, such an agreement between the predicted and observed highest luminosities provides by itself strong support for this model. Then, we study multi-waveband spectral variability during the 2000 outburst. In addition to the primary maxima in the optical light curves, secondary maxima were detected after the transition from the very high state to the hard state. We show that the secondary maxima are well modeled by synchrotron emission from a jet formed during the state transition. We argue that the absence of the corresponding secondary peak in the X-ray light curve indicates that the X-ray jet emission, regardless of its radiative process, synchrotron or its Comptonization, is not important in the hard state compared to the emission from the accretion flow.

astro-ph↗