Quasar Accretion Disk Research Articles

The First Precise Determination of an Optical–Far‐Ultraviolet Extinction Curve Beyond the Local Group ( \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape $z=0.83$ \end{document} )

We present the optical-far-ultraviolet extinction curve of the dust in the lens galaxy of the gravitational lens system SBS 0909+532 (z = 0.83). Extending our previous optical-UV estimate (from λ ~ 2 to 5 μm-1) into the far ultraviolet (from λ ~ 5 to 8 μm-1) is crucial for comparing with the extinction curves measured for Local Group galaxies in the spectral region where the differences are greatest. The SBS 0909+532 curve is similar to that of the LMC2 supershell, with a weaker 2175 A feature and a steeper rise into the UV than that observed in the Milky Way. The shapes of the extinction curve inferred from the quasar continuum and emission lines are in very good agreement. There is, however, a 0.6 ± 0.1 mag offset in the implied magnification of the source that can be interpreted as differential magnification due to microlensing of the compact accretion disk producing the continuum emission by the stars in the lens galaxy. There is no evidence for a wavelength dependence on the microlensing effect except for a ~2 σ shift near the Mg II emission line. If this difference were confirmed, it could be used to probe the emission profile of the quasar accretion disk.

Close stars and accretion in low-luminosity active galactic nuclei

Quasar accretion discs are believed to form stars by self-gravity. Low-luminosity active galactic nuclei (LLAGNs) are much dimmer galactic centres, and are often believed to be quasars that ran out of gaseous fuel. LLAGN accretion discs should thus coexist with thousands to millions of stars or protostars left from the previous stronger accretion activity. In principle, these stars may produce several important effects: (i) contribute to the optical/ultraviolet spectra of some LLAGNs; (ii) the dusty discs could reprocess stellar radiation in the infrared frequencies and then dominate the LLAGN spectra in that region; (iii) deplete the (accretion) gas disc much faster than it can accrete on to the supermassive black hole; (iv) stars, individually or in groups, may slow down and modulate the accretion flow significantly due to their inertia. In this way they may produce the LLAGN cut-off discs; (v) alternatively, frequent enough stellar collisions and resulting stellar disruptions could keep the inner disc empty. Here we explore these ideas. We find that, despite 'low' luminosities of LLAGNs, unrealistically high stellar densities are required to make a sizable radiative contribution to the Hubble Space Telescope optical/ultraviolet spectra of these galactic nuclei. Stellar contribution to the infrared spectrum is more likely. Further, if LLAGNs are in a quasi-steady state for as long as 10 7 yr or more, too high stellar densities would again be required to significantly affect the dynamics of accretion flow. However, if LLAGNs are 'short-lived' phenomena, e.g. t ≤ 10 5 yr, the low-activity states of quiescence-outburst cycles, then embedded stars may be much more important through the mass effects (iii)-(v). With observations of LLAGNs becoming progressively better, it will be more and more difficult to neglect the presence of close stars in and around nuclear accretion discs.

Supermassive Stars in Quasar Disks

We propose that supermassive stars may form in quasar accretion disks, and we discuss possible observational consequences. The structure and stability of very massive stars are reviewed. Because of high accretion rates, quasar disks are massive, and the fringes of their optically luminous parts are prone to fragmentation. Starting from a few hundred solar masses, a dominant fragment will grow to the isolation mass, which is a significant fraction of the disk mass, more quickly than the fragment contracts onto the stellar main sequence. A gap will form in the disk, and the star will migrate inward on the accretion timescale, which is comparable to the star's main-sequence lifetime. By interrupting the gas supply to the inner disk, the gap may temporarily dim and redden the quasar. The final stages of stellar migration will be a strong source of low-frequency gravitational waves.

A Study of Quasar Evolution in the X‐Ray Band with the Aid of Gravitational Lensing

We present results from a minisurvey of relatively high redshift (1.7 < z < 4), gravitationally lensed, radio-quiet quasars observed with the Chandra X-Ray Observatory and with XMM-Newton. The lensing magnification effect allows us to search for changes with redshift in quasar spectroscopic and flux variability properties, ranging over 3 orders of magnitude in intrinsic X-ray luminosity. It extends the study of quasar properties to unlensed X-ray flux levels as low as a few times 10-15 ergs cm-2 s-1 in the observed 0.4-8 keV band. For the first time, these observations of lensed quasars have provided medium to high signal-to-noise ratio X-ray spectra of a sample of relatively high redshift and low X-ray luminosity quasars. We find a possible correlation between the X-ray power-law photon index and the X-ray luminosity of the gravitationally lensed, radio-quiet quasar sample. The X-ray spectral slope steepens as the X-ray luminosity increases. This correlation is still significant when we combine our data with other samples of radio-quiet quasars with z > 1.5, especially in the low-luminosity range between 1043 and 1045.5 ergs s-1. This result is surprising, considering that such a correlation is not found for quasars with redshifts below 1.5. We suggest that this correlation can be understood in the context of the hot-corona model for X-ray emission from quasar accretion disks, under the hypothesis that the quasars in our sample accrete very close to their Eddington limits and that the observed luminosity range is set by the range of black hole masses (this hypothesis is consistent with recent predictions of semianalytic models for quasar evolution). The upper limits of X-ray variability for our relatively high redshift sample of lensed quasars are consistent with the known correlation between variability and luminosity observed in Seyfert 1 galaxies when this correlation is extrapolated to the larger luminosities of our sample.

Quantitative Interpretation of Quasar Microlensing Light Curves

We develop a general method for analyzing the light curves of microlensed quasars and apply it to the OGLE light curves of the four-image lens Q2237+0305. We simultaneously estimate the effective source velocity, the average stellar mass, the stellar mass function, and the size and structure of the quasar accretion disk. The light curves imply an effective source-plane velocity of 10,200 km s-1 ≲ veh⟨M/M☉⟩-1/2 ≲ 39,600 km s-1 (68% confidence). Given an independent estimate for the source velocity, found by combining estimates for the peculiar velocity of the lens galaxy with its measured stellar velocity dispersion, we obtain a mean stellar mass of ⟨M⟩ ≃ 0.037 h2 M☉ (0.0059 h2 M☉ ≲ ⟨M⟩ ≲ 0.20 h2 M☉). We were unable to distinguish a Salpeter mass function from one in which all stars had the same mass, but we do find a strong lower bound of κ*/κ ≳ 0.5 on the fraction of the surface mass density represented by the microlenses. Our models favor a standard thin accretion disk model as the source structure over a simple Gaussian source. For a face-on, thin disk radiating as a blackbody with temperature profile Ts ∝ R-3/4, the radius rs where the temperature matches the filter pass band [2000 Å or Ts(rs) ≃ 7 × 104 K] is 1.4 × 1015 h-1 cm ≲ rs ≲ 4.5 × 1015 h-1 cm. The flux predicted by the disk model agrees with the observed flux of the quasar, so nonthermal or optically thin emission processes are not required. From the disk structure we estimate a black hole mass of MBH ≃ 1.1 × 109 h-3/2η(L/LE)-1/2 M☉, consistent with the mass estimated under the assumption that the quasar is radiating at the Eddington luminosity (L/LE = 1).

An Accretion Model for the Growth of the Central Black Hole Associated with Ionization Instability in Quasars

A possible accretion model associated with the ionization instability of quasar disks is proposed to address the growth of the central black hole (BH) harbored in the host galaxy. The evolution of quasars in cosmic time is assumed to change from a highly active state to a quiescent state triggered by the S-shaped ionization instability of the quasar accretion disk. For a given external mass transfer rate supplied by the quasar host galaxy, ionization instability can modify the accretion rate in the disk and separate the accretion flows of the disk into three different phases, like an S-shape. We suggest that the bright quasars observed today are those quasars with disks in the upper branch of the S-shaped instability, and the faint or 'dormant' quasars are simply these systems in the lower branch. The middle branch is the transition state, which is unstable. We assume the quasar disk evolves according to the advection-dominated inflow-outflow solution (ADIOS) configuration in the stable lower branch of the S-shaped instability, and the Eddington accretion rate is used to constrain the accretion rate in the highly active phase. The mass ratio between a BH and its host galactic bulge is a natural consequence of an ADIOS. Our model also demonstrates that a seed BH approx. 2 x 10(exp 6) solar masses similar to those found in spiral galaxies today is needed to produce a BH with a final mass of approx. 2 x 10(exp 8) solar masses.

Reconstruction of the strip brightness distribution in a quasar accretion disk from gravitational microlensing data

A technique is proposed for the successive reconstruction of the branches of the strip brightness distribution for a quasar accretion disk via the analysis of observations of high magnification flux events in the multiple quasar images produced by a gravitational lens. The distribution branches are searched for on compact sets of nonnegative, monotonically nonincreasing, convex downward functions. The results of numerical simulations and application of the technique to real observations show that the solution obtained is stable against random noise. Analysis of the light curve of a high magnification event in image C of the gravitational lens QSO 2237+0305 observed by the OGLE group in summer 1999 has yielded the form of the strip brightness distribution in the accretion disk of the lensed quasar. The results are consistent with the hypothesis that the quasar disk was scanned by a fold caustic. The form of the strip distribution is consistent with the expected appearance of an accretion disk rotating around a supermassive black hole.

The emission line--radio correlation for radio sources using the 7C Redshift Survey

We have used narrow emission line data from the new 7C Redshift Survey to investigate correlations between the narrow-line luminosities and the radio properties of radio galaxies and steep-spectrum quasars. The 7C Redshift Survey is a low-frequency (151 MHz) selected sample with a flux-density limit about 25-times fainter than the 3CRR sample. By combining these samples, we can for the first time distinguish whether the correlations present are controlled by 151 MHz radio luminosity L_151 or redshift z. We find unequivocal evidence that the dominant effect is a strong positive correlation between narrow line luminosity L_NLR and L_151, of the form L_NLR proportional to L_151 ^ 0.79 +/- 0.04. Correlations of L_NLR with redshift or radio properties, such as linear size or 151 MHz (rest-frame) spectral index, are either much weaker or absent. We use simple assumptions to estimate the total bulk kinetic power Q of the jets in FRII radio sources, and confirm the underlying proportionality between jet power and narrow line luminosity first discussed by Rawlings & Saunders (1991). We make the assumption that the main energy input to the narrow line region is photoionisation by the quasar accretion disc, and relate Q to the disc luminosity, Q_phot. We find that 0.05 < Q / Q_phot < 1 so that the jet power is within about an order of magnitude of the accretion disc luminosity. The most powerful radio sources are accreting at rates close to the Eddington limit of supermassive black holes (~ 10^9 - 10^10 solar masses), whilst lower power sources are accreting at sub-Eddington rates.

Imaging a Quasar Accretion Disk with Microlensing

We show how analysis of a quasar high-magnification microlensing event may be used to construct a map of the frequency-dependent surface brightness of the quasar accretion disk. The same procedure also allows determination of the disk inclination angle, the black hole mass (modulo the caustic velocity), and possibly the black hole spin. This method depends on the validity of one assumption: that the optical and ultraviolet continuum of the quasar is produced on the surface of an azimuthally symmetric, flat equatorial disk, whose gas follows prograde circular orbits in a Kerr spacetime (and plunges inside the marginally stable orbit). Given this assumption, we advocate using a variant of first-order linear regularization to invert multifrequency microlensing light curves to obtain the disk surface brightness as a function of radius and frequency. The other parameters can be found by minimizing χ2 in a fashion consistent with the regularized solution for the surface brightness. We present simulations for a disk model appropriate to the Einstein Cross quasar, an object uniquely well suited to this approach. These simulations confirm that the surface brightness can be reconstructed quite well near its peak and that there are no systematic errors in determining the other model parameters. We also discuss the observational requirements for successful implementation of this technique.

On the Origin of Polarization near the Lyman Edge in Quasars

Optical/UV radiation from accretion disks in quasars is likely to be partly scattered by a hot plasma enveloping the disk. We investigate whether the scattering may produce the steep rises in polarization observed blueward of the Lyman limit in some quasars. We suggest and assess two models. In the first model, primary disk radiation with a Lyman edge in absorption passes through a static ionized covering the disk, which has a temperature about 3 keV and a Thomson optical depth about unity. Electron scattering in the skin smears out the edge and produces a steep rise in polarization at lambda < 912 A. In the second model, the scattering occurs in a hot coronal plasma outflowing from the disk with a mildly relativistic velocity. We find that the second model better explains the data. The ability of the models to fit the observed rises in polarization is illustrated with the quasar PG 1630+377.

Polarization near the Lyman Edge in Accretion Disk Atmosphere Models of Quasars

We calculate self-consistently the structure, spectrum, and polarization of optically thick atmospheres under the conditions expected in quasar accretion disks. We consider a pure-hydrogen composition and neglect the effects of external illumination. We show that, blueward of the Lyman edge, the polarization rises above the value expected from an optically thick, pure electron-scattering atmosphere. This arises because of the wavelength dependence of the absorption opacity, combined with the fact that the source function in this region of the spectrum has a steep variation with optical depth. We suggest that this effect may help explain the recent Hubble Space Telescope observations of steep rises in polarization blueward of the Lyman edge in several quasars. We predict that the polarization should continue to rise blueward of the wavelength at which the current observations end, that cooler spectra should be more polarized, and that the polarization angle in this region should be parallel to the disk plane.

Non-LTE effects on the strength of the Lyman edge in quasar accretion disks

We have calculated UV/EUV (300 A which is less than or equal to lambda which is less than or equal to 1500 A) continuous energy distributions of accretion disks in the centers of active galactic nuclei (AGNs) for disk luminosities in the range 0.1 L(sub Edd) less than or equal to L(sub acc) less than 1.0 L(sub Edd) and central masses ranging from 10(exp 8) solar mass to 10(exp 9) solar mass. The vertical gas pressure structure of the disk and the disk height are obtained analytically; the temperature stratification and the resulting continuum radiation fields are calculated numerically. We have included non-Local Thermodynamic Equilibrium (LTE) effects of both the ionization equilibrium and the level populations of hydrogen and helium. We show that these non-LTE effects reduce the strength of the Lyman edge when comapred to the LTE case. In non-LTE we find that the edge can be weakly in emission or absorption for disks seen face-on, depending on the disk parameters.

Steady-state shocks in an accretion disc around a Kerr black hole

Results of numerical simulations of shock solutions in a geometrical thin accretion disk around a Kerr black hole (BH) are presented. Using the smoothed particle hydrodynamics (SPH) technique, the influence of the central object is included by means of an effective potential, We first present the theory of standing shock formation in accretion disks around a Kerr black hole, and show that the results of our numerical simulation agree very well with the theoretical results. We find that the shocks in an inviscid flow are very stable. We also remove the ambiguity prevalent regarding the location and stability of shocks in adiabatic flows. Finally we sketch some of the astrophysical consequences of our findings in relation to accretion disks in Active Galactic Nuclei (AGN) and Quasars.

Evolution of the luminosity-color distribution of quasar accretion disks

We investigate the luminosity and color evolutions of a sample of ∼550 quasars with redshifts 0.4<z<2.2. The flux and color distributions are compared with those expected for thin accretion disk spectra, and it is found that fost sources are adequately described as thin disks. A significant number of sources, however, have colors too blue to be standard thin disks, and we postulate that these blue spectra represent young, super-Eddington sources with modified accretion structures such as radiation tori, filaments, or clouds. Using the thin disk model, we map the observed flux-color distribution onto the mass-accretion rate parameter space

Constraints on quasar accretion disks from the optical/ultraviolet/soft X-ray big bump

The simplest accretion-disk models have difficulties explaining the optical/UV/soft-X-ray 'big bump' in quasars. Here more realistic models are investigated, incorporating opacity and inclination effects while retaining an analytic form for ease of computation. It is found that opacity effects can explain the uniform 20,000-30,000 K 'maximum disk temperature' derived by previous workers. The observed spectral turndown would be the result of the onset of electron scattering and should not be identified with the hottest part of the disk. The frequency at which this spectral turndown occurs is only weakly dependent on the accretion-disk parameters. These opacity effects also allow high-frequency EUV and soft-X-ray extensions of the big bump without exceeding the Eddington limit strongly. For a wide range of disk parameters, the EUV spectra of quasar disks should have spectral slopes near -1.

Radiation of accretion disks in quasars and galactic nuclei

The continuum spectra of accretion disks around supermassive Schvarzshild black hole are calculated for a set of accretion disk parameters. In every point of the disk surface the atmosphere is considered to be similar to the atmosphere of the star with the same values of the surface gravity and effective temperature. All important effects of general relativity affected the spectrum shape are taken into account.

Compton heated winds and coronae above accretion disks. I Dynamics

X rays emitted in the inner part of an accretion disk system can heat the surface of the disk farther out, producing a corona and possibly driving off a strong wind. The dynamics of Compton-heated coronae and winds are analyzed using an approximate two-dimensional technique to estimate the mass loss rate as a function of distance from the source of X rays. The findings have important dynamical implications for accretion disks in quasars, active galactic nuclei, X ray binaries, and cataclysmic variables. These include: mass loss from the disk possibly comparable with or exceeding the net accretion rate onto the central compact object, which may lead to unstable accretion; sufficient angular momentum loss in some cases to truncate the disk in a semidetached binary at a smaller radius than that predicted by tidal truncation theories; and combined static plus ram pressure in the wind adequate to confine line-emitting clouds in quasars and Seyfert galaxies.

Thermal continuum from accretion disks in quasars

ACCRETION onto black holes in galactic nuclei is a promising power source for quasars1,2, but observational proof has been elusive. I argue here that supermassive accretion in disks may explain the flat optical continuum observed from some QSOs and Seyfert galaxies.