Quasar Disk Research Articles

Microlensing analysis of 14.5-year light curves in SDSS J1004+4112: Quasar accretion disk size and intracluster stellar mass fraction

Context. The gravitational lens system SDSS J1004+4112 was the first known example of a quasar lensed by a galaxy cluster. The interest in this system has been renewed following the publication of r-band light curves spanning 14.5 years and the determination of the time delays between the four brightest quasar images. Aims. We constrained the quasar accretion disk size and the fraction of the lens mass in stars using the signature of microlensing in the quasar image light curves. Methods. We built the six possible histograms of microlensing magnitude differences between the four quasar images and compared them with simulated model histograms, using a χ2 test to infer the model parameters. Results. We infer a quasar disk half-light radius of R1/2 = (0.70 ± 0.04)RE = (6.4 ± 0.4) √M/0.3M⊙ light-days at 2407 Å in the rest frame and stellar mass fractions at the quasar image positions of αA > 0.059, αB = 0.056+0.021-0.027, αC = 0.030+0.031-0.021, and αD = 0.072+0.034-0.016. Conclusions. The inferred disk size is broadly compatible with most previous estimates, and the stellar mass fractions are within the expected ranges for galaxy clusters. In the region where image C lies, the stellar mass fraction is compatible with a stellar contribution from the brightest cluster galaxy, galaxy cluster members, and intracluster light, but the values at images B, D, and especially A are slightly larger, possibly suggesting the presence of extra stellar components.

Negative Lags on the Viscous Timescale in Quasar Photometry and Prospects for Detecting More with LSST

The variability of quasar light curves can be used to study the structure of quasar accretion disks. For example, continuum reverberation mapping uses delays between variability in short and long wavelength bands (short lags) to measure the radial extent and temperature profile of the disk. Recently, a potential reverse lag, where variations in shorter wavelength bands lag the longer wavelength bands at the much longer viscous timescale, was detected for Fairall 9. Inspired by this detection, we derive a timescale for these long negative lags from fluctuation propagation models and recent simulations. We use this timescale to forecast our ability to detect long lags using the Vera Rubin Legacy Survey of Space and Time (LSST). After exploring several methods, including the interpolated cross-correlation function, a Von-Neumann estimator, javelin, and a maximum-likelihood Fourier method, we find that our two main methods, javelin and the maximum-likelihood method, can together detect long lags of up to several hundred days in mock LSST light curves. Our methods work best on proposed LSST cadences with long season lengths, but can also work for the current baseline LSST cadence, especially if we add observations from other optical telescopes during seasonal gaps. We find that LSST has the potential to detect dozens to hundreds of additional long lags. Detecting these long lags can teach us about the vertical structure of quasar disks and how it scales with different quasar properties.

Metallicity ceiling in quasars from recycled stellar winds

Context. Optically luminous quasars are metal rich across all redshifts. Surprisingly, there is no significant trend in the broad line region (BLR) metallicity with different star formation rates (SFR), and the average N V/C IV metallicity does not appear to exceed 9.5 Z⊙. Combined, these observations may suggest a metallicity ceiling. Aims. Here, we conduct an exploratory study on scenarios relating to the evolution of embedded stars that may lead to a metallicity ceiling in quasar disks. Methods. We developed a simple model that starts with gas in a “closed box,” which is enriched by cycles of stellar evolution until eventually newly formed stars may undergo significant mass loss before they reach the supernova stage and further enrichment is halted. Using the MESA code, we created a grid over a parameter space of masses (> 8 M⊙) and metallicities (1 − 10 Z⊙), and located portions of the parameter space where mass loss via winds occurs on a timescale shorter than the lifetime of the stars. Results. Based upon reasonable assumptions about stellar winds, we found that sufficiently massive (8 − 22 M⊙) and metal-rich (∼9 Z⊙) stars lose significant mass via winds and may no longer evolve to the supernovae stage, thereby failing to enrich and increase the metallicity of their surroundings. This suggests that a metallicity ceiling is the final state of a closed-box system of gas and stars.

A disc wind model for blueshifts in quasar broad emission lines

ABSTRACT Blueshifts – or, more accurately, blue asymmetries – in broad emission lines such as C iv λ1550 are common in luminous quasars and correlate with fundamental properties such as Eddington ratio and broad absorption line (BAL) characteristics. However, the formation of these blueshifts is still not understood, and neither is their physical connection to the BAL phenomenon or accretion disc. In this work, we present Monte Carlo radiative transfer and photoionization simulations using parametrized biconical disc-wind models. We take advantage of the azimuthal symmetry of a quasar and show that we can reproduce C iv blueshifts provided that (i) the disc-mid-plane is optically thick out to radii beyond the line formation region, so that the receding wind bicone is obscured; and (ii) the system is viewed from relatively low (that is, more face-on) inclinations (≲40°). We show that C iv emission-line blueshifts and BALs can form in the same wind structure. The velocity profile of the wind has a significant impact on the location of the line formation region and the resulting line profile, suggesting that the shape of the emission lines can be used as a probe of wind-driving physics. While we are successful at producing blueshifts/blue asymmetries in outflows, we struggle to match the detailed shape or skew of the observed emission-line profiles. In addition, our models produce redshifted emission-line asymmetries for certain viewing angles. We discuss our work in the context of the C iv λ1550 emission blueshift versus equivalent-width space and explore the implications for quasar disc wind physics.

A Broad-line Quasar with Unexplained Extreme Velocity Offsets: Post-shock Outflow?

The quasar SDSS 0956 + 5128 exhibits three distinct velocity components with large offsets in emission: the systemic velocity of [O ii], [O iii], and [Ne iii] narrow lines have redshift z = 0.7142; the broad Mg ii line is shifted by −1200 km s−1 with respect to the narrow lines; the broad Hα and Hβ lines are at −4100 km s−1. We present new Hubble Space Telescope spectra of Lyα and C iv emission lines and high-resolution images of the quasar. The offsets of these lines are consistent with the velocity component of the Balmer emission, and the photometry in optical and near-infrared wavelengths does not show any signs of recent mergers in the host galaxy or irregularities in the location of the quasar. The data do not confirm predictions of the previous most likely hypotheses involving a special orientation and morphology of the quasar disk, such as in the recoiling black hole scenario, neither it is consistent with accretion disk winds. Instead, based on the cumulative evidence, we propose a new scenario, in which the broad-line region is in the state of outflow caused by a strong shock wave, with a supernova as a possible event for producing the shock ejecta.

New evidence for a cosmological distribution of stellar mass primordial black holes

ABSTRACT In this paper, we show that to explain the observed distribution of amplitudes in a large sample of quasar light curves, a significant contribution from microlensing is required. This implies the existence of a cosmologically distributed population of stellar mass compact bodies making up a large fraction of the dark matter. Our analysis is based on the light curves of a sample of over 1000 quasars, photometrically monitored over a period of 26 yr. The intrinsic variations in quasar luminosity are derived from luminous quasars where the quasar accretion disc is too large to be microlensed by stellar mass bodies, and then synthetic light curves for the whole sample are constructed with the same statistical properties. We then run microlensing simulations for each quasar with convergence in compact bodies appropriate to the quasar redshift assuming a Lambda cold dark matter cosmology. The synthetic light curve is then superimposed on the amplification pattern to incorporate the effects of microlensing. The distribution of the resulting amplitudes can then be compared with observation, giving a very close match. This procedure does not involve optimizing parameters or fitting to the data, as all inputs such as lens mass and quasar disc size come from independent observations in the literature. The overall conclusion of the paper is that to account for the distribution of quasar light curve amplitudes it is necessary to include the microlensing effects of a cosmologically distributed population of stellar mass compact bodies, most plausibly identified as stellar mass primordial black holes.

Quasar microlensing models with constraints on the Quasar light curves

Quasar microlensing analyses implicitly generate a model of the variability of the source quasar. The implied source variability may be unrealistic yet its likelihood is generally not evaluated. We used the damped random walk (DRW) model for quasar variability to evaluate the likelihood of the source variability and applied the revised algorithm to a microlensing analysis of the lensed quasar RX J1131-1231. We compared the estimates of the source quasar disk and average lens galaxy stellar mass with and without applying the DRW likelihoods for the source variability model and found no significant effect on the estimated physical parameters. The most likely explanation is that unreliastic source light curve models are generally associated with poor microlensing fits that already make a negligible contribution to the probability distributions of the derived parameters.

Apparent quasar disc sizes in the “bird’s nest” paradigm

Quasar microlensing effects make it possible to measure the accretion disc sizes around distant supermassive black holes that are still well beyond the spatial resolution of contemporary instrumentation. The sizes measured with this technique appear inconsistent with the standard accretion disc model. Not only are the measured accretion disc sizes larger, but their dependence on wavelength is in most cases completely different from the predictions of the standard model. We suggest that these discrepancies may arise not from non-standard accretion disc structure or systematic errors, as it was proposed before, but rather from scattering and reprocession of the radiation of the disc. In particular, the matter falling from the gaseous torus and presumably feeding the accretion disc may at certain distances become ionized and produce an extended halo that is free from colour gradients. A simple analytical model is proposed assuming that a geometrically thick translucent inflow acts as a scattering mirror changing the apparent spatial properties of the disc. This inflow may be also identified with the broad line region or its inner parts. Such a model is able to explain the basic properties of the apparent disc sizes, primarily their large values and their shallow dependence on wavelength. The only condition required is to scatter significant portion of the luminosity of the disc. This can easily be fulfilled if the scattering inflow has large geometrical thickness and clumpy structure.

Structure of radiation-dominated gravitoturbulent quasar discs

Self-gravitating accretion discs in a gravitoturbulent state, including radiation and gas pressures, are studied using a set of new analytical solutions. While the Toomre parameter of the disc remains close to its critical value for the onset of gravitational instability, the dimensionless stress parameter is uniquely determined from the thermal energy reservoir of the disc and its cooling rate. Our solutions are applicable to the accretion discs with dynamically important radiation pressure like in the quasars discs. We show that physical quantities of a gravitoturbulent disc in the presence of radiation are significantly modified compared to solutions with only gas pressure. We show that the dimensionless stress parameter is an increasing function of the radial distance so that its steepness strongly depends on the accretion rate. In a disc without radiation its slope is 4.5, however, we show that in the presence of radiation, it varies between 2 and 4.5 depending on the accretion rate and the central mass. As for the surface density, we find a shallower profile with an exponent -2 in a disc with sub-Eddington accretion rate compared to a similar disc, but without radiation, where its surface density slope is -3 independent of the accretion rate. We then investigate gravitational stability of the disc when the stress parameter reaches to its critical value. In order to self-consistently determine the fragmentation boundary, however, it is shown that the critical value of the stress parameter is a power-law function of the ratio of gas pressure and the total pressure and its exponent is around 1.7. We also estimate the maximum mass of the central black hole using our analytical solutions.

Optical polarization of quasars and the Balmer edge feature revealed by ultraviolet and polarized visible to near-infrared emissions

Polarized emission from a quasar is produced by wavelength-independent electron scattering surrounding its accretion disc, and thus avoid the contamination from its host galaxy and reveal the intrinsic emission spectrum of the accretion disc. Ultra-violet (UV) emission from a quasar is normally free from the contamination from its host galaxy. Polarization fraction of the quasar's disc emission can therefore be determined by comparing total UV emission with polarized visible to near-infrared (NIR) emission; and the resulting continuum spectrum from UV to infrared can reveal the theoretically expected Balmer edge absorption feature. We fit the polarized spectra in visible and NIR bands together with the total UV spectra of two type-1 quasars (3C 95, 4C 09.72), to an extended geometrically thin and optically thick accretion disc model. In addition to the standard model, we include the Balmer edge absorption due to co-rotational neutral gas on a narrow annulus of the accretion disc. We find that the extended thin accretion disc model provides adequate description on the continuum spectra of the two quasars from UV to NIR wavelengths. A Monte-Carlo-Markov-Chain fitting to the continuum spectra is able to well constrain the true polarization fraction of the disk emission, which allows the Balmer edge feature to be completely revealed from polarized visible to UV continua. The Balmer edge feature is prominent in both quasars' spectra, and is significantly broadened due to the orbital motion of gas in the accretion disc. The broadening of the Balmer edge feature is therefore related to the quasar's inclination. This work proves the concept of determining quasar's inclination from the Balmer edge feature in their continuum spectra.

Cooling of Young Stars Growing by Disk Accretion

In the initial formation stages young stars must acquire a significant fraction of their mass by accretion from a circumstellar disk that forms in the center of a collapsing protostellar cloud. Throughout this period mass accretion rates reach 10−6 to 10−5 M☉ yr−1, allowing the disk to extend all the way to the stellar surface unimpeded by the protostellar magnetic field. We study the effect of irradiation of the stellar surface produced by the hot inner disk on properties of accreting fully convective low-mass stars and also look at objects such as young brown dwarfs and giant planets. At high irradiation raises the surface temperature of the equatorial region above the photospheric temperature T0 that a star would have in the absence of accretion. In these regions an almost isothermal outer radiative zone forms on top of the fully convective interior, leading to a suppression of the local internal cooling flux derived from stellar contraction (similar suppression occurs in irradiated ``hot Jupiters''). The high-latitude (polar) parts of the stellar surface, where disk irradiation is weak, preserve their temperature at the level of T0. The total intrinsic luminosity integrated over the whole stellar surface is reduced compared to the nonaccreting case, by up to a factor of several for some objects (young brown dwarfs, stars in quasar disks, and forming giant planets), potentially leading to the retardation of stellar contraction.

Lens‐Aided Multi‐Angle Spectroscopy (LAMAS) Reveals Small‐Scale Outflow Structure in Quasars

Spectral differences between lensed quasar image components are common. Since lensing is intrinsically achromatic, these differences are typically explained as the effect of either microlensing, or as light path time delays sampling intrinsic quasar spectral variability. Here we advance a novel third hypothesis: some spectral differences are due to small line-of-sight differences through quasar disk wind outflows. In particular, we propose that variable spectral differences seen only in component A of the widest separation lens SDSS J1004+4112 are due to differential absorption along the sight lines. The absorber properties required by this hypothesis are akin to known broad absorption line (BAL) outflows but must have a broader, smoother velocity profile. We interpret the observed C IV emission-line variability as further evidence for spatial fine structure transverse to the line of sight. Since outflows are likely to be rotating, such absorber fine structure can consistently explain some of the UV and X-ray variability seen in AGNs. The implications are many: (1) Spectroscopic differences in other lensed objects may be due to this lens-aided multi-angle spectroscopy (LAMAS). (2) Outflows have fine structure on size scales of arcseconds, as seen from the nucleus. (3) Assuming either broad absorption line region sizes proposed in recent wind models, or typically assumed continuum emission region sizes, LAMAS and/or variability provide broadly consistent absorber size scale estimates of ~1015 cm. (4) Very broad smooth absorption may be ubiquitous in quasar spectra, even when no obvious troughs are seen.

Extremely Luminous Water Vapor Emission from a Type 2 Quasar at Redshift z = 0.66

A search for water masers in 47 Sloan Digital Sky Survey type 2 quasars using the Green Bank Telescope has yielded a detection at a redshift of z = 0.660. This maser is more than an order of magnitude higher in redshift than any previously known and, with a total isotropic luminosity of 23,000 L☉, also the most powerful. The presence and detectability of water masers in quasars at z ~ 0.3-0.8 may provide a better understanding of quasar molecular tori and disks, as well as fundamental quasar and galaxy properties such as black hole masses. Water masers at cosmologically interesting distances may also eventually provide, via direct distance determinations, a new cosmological observable for testing the reality and properties of dark energy, currently inferred primarily through Type 1a supernova measurements.

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.

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.

Quasar discs – III. Line and continuum correlations

We analyse a very large sample of Active Galactic Nuclei (AGNs) and discuss several of the observed line and continuum correlations. We show that the luminosity of the Ly α and C IVλ1549 lines do not increase as fast as the continuum luminosity, and that the slope of the Baldwin relationship is a function of the luminosity range of the sample under study. Thin AGN accretion discs can provide one explanation for those correlations, as we show by computing the continuum emitted by such systems and combining it with photoionization model calculations for the BLR. The agreement between the observed and the theoretical line correlations is very good, especially for Ly α. In particular, there is no need to invoke a luminosity-dependent covering factor, since higher luminosity discs are predicted to emit softer ionizing continuum. The alternative explanation involves line and continuum variability. While a complete theory of thin AGN discs is not yet available, the observational tests investigated in this paper may be a very promising method to look for the existence of such systems.

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.

Quasar discs – II. A composite model for the broad-line region

The possibility of geometrically thin accretion discs in active galactic nuclei (AGNs) was discussed in Paper I of this series (H. Netzer, Mon. Not. R.astr. Soc. (1985) 216, 63). The apparent luminosity of such discs depends on the viewing angle and this may be the reason for the observed correlation of continuum brightness and line equivalent width. This idea is taken one step further in the present work and the emission line spectrum of a gas exposed to the anisotropic ionizing radiation of a disc is investigated. One example which is studied in detail is that of a disc UV continuum combined with an isotropic X-ray source. It is characterized by (a) a larger ionization parameter and a smaller broad-line region compared with standard photoionization models, (b) an angular dependence of emission line intensities and (c) a large covering factor at low disc latitudes. Such models can explain both the high- and low-excitation lines, as well as the strong Balmer continuum. They offer an explanation of the Hα/Hβ and CIVλ1549/CIII] λ1909 profile differences and indicate a way of obtaining a symmetric Lα from an expanding cloud ensemble. Analysis of the L– M relationship in AGNs, within the framework of the new model, shows that these objects have smaller central masses and higher accretion efficiencies compared with previous estimates. There are important consequences for the two-phase model and cloud formation, and specific predictions of the observed |${L}_\text{UV}/{L}_\text{X}$|⁠, which is angle dependent.