Properties Of Accretion Disks Research Articles

EMERGENT SPECTRA FROM DISKS SURROUNDING KERR BLACK HOLES: EFFECT OF PHOTON TRAPPING AND DISK SELF-SHADOWING

Based on a new estimation of their thickness, the global properties of relativistic slim accretion disks are investigated in this work. The resulting emergent spectra are calculated using the relativistic ray-tracing method, in which we neglect the self-irradiation of the accretion disk. The angular dependence of the disk luminosity, the effects of the heat advection and the disk thickness on the estimation of the black hole spin are discussed. Compare to the previous works, our improvements are that we use the self-consistent disk equations and we consider the disk self-shadowing effect. We find that at the moderate accretion rate, the radiation trapped in the outer region of the accretion disks will escape in the inner region of the accretion disk and contribute to the emergent spectra. At the high accretion rate, for the large inclination and large black hole spin, both the disk thickness and the heat advection have significant influence on the emergent spectra. Consequently, these effects will influence the measurement of the black hole spin based on the spectra fitting and influence the angular dependence of the luminosity. For the disks around Kerr black holes with $a=0.98$, if the disk inclination is greater than $60^\circ$, and their luminosity is beyond 0.2 Eddington luminosity, the spectral model which is based on the relativistic standard accretion disk is no longer applicable for the spectra fitting. We also confirm that the effect of the self-shadowing is significantly enhanced by the light-bending, which implies that the non-relativistic treatment of the self-shadowing is inaccurate. According to our results, the observed luminosity dependence of the measured spin suggests that the disk self-shadowing significantly shapes the spectra of GRS 1915+105, which might lead to the underestimation of the black hole spin for the high luminosity states.

On the accretion disc properties in eclipsing dwarf nova EM Cyg

In this paper we analyze the behavior of the unusual dwarf nova EM Cyg using the data obtained in April–October, 2007, at the Vyhorlat observatory (Slovak Republic) and in September, 2006, at the Crimean Astrophysical Observatory (Ukraine). During our observations, EM Cyg has shown outbursts in every 15–40 days. Because on the light curves of EM Cyg a partial eclipse of the accretion disc is observed, we applied the eclipse mapping technique to a reconstruction of the temperature distribution in the eclipsed parts of the disc. Calculations of the accretion rate for the system were made for the quiescent and the outburst states of activity for various distances.

HOT HIGH-MASS ACCRETION DISK CANDIDATES

To better understand the physical properties of accretion disks in high-mass star formation, we present a study of a 12 high-mass accretion disk candidates observed at high spatial resolution with the Australia Telescope Compact Array (ATCA) in the NH3 (4,4) and (5,5) lines. Almost all sources were detected in NH3, directly associated with CH3OH Class II maser emission. From the remaining eleven sources, six show clear signatures of rotation and/or infall motions. These signatures vary from velocity gradients perpendicular to the outflows, to infall signatures in absorption against ultracompact HII regions, to more spherical infall signatures in emission. Although our spatial resolution is ~1000AU, we do not find clear Keplerian signatures in any of the sources. Furthermore, we also do not find flattened structures. In contrast to this, in several of the sources with rotational signatures, the spatial structure is approximately spherical with sizes exceeding 10^4 AU, showing considerable clumpy sub-structure at even smaller scales. This implies that on average typical Keplerian accretion disks -- if they exist as expected -- should be confined to regions usually smaller than 1000AU. It is likely that these disks are fed by the larger-scale rotating envelope structure we observe here. Furthermore, we do detect 1.25cm continuum emission in most fields of view.

Hotspots and a clumpy disc: variability of brown dwarfs and stars in the young σ Ori cluster

The properties of accretion disks around stars and brown dwarfs in the SOri cluster (age 3 Myr) are studied based on NIR time series photometry supported by MIR spectral energy distributions. We monitor ~30 young low-mass sources over 8 nights in the J- and K-band using the duPont telescope at Las Campanas. We find three objects showing variability with J-band amplitudes >0.5 mag; five additional objects exhibit low-level variations. All three highly variable sources have been previously identified as highly variable; thus we establish the long-term nature of their flux changes. The lightcurves contain periodic components with timescales of ~0.5-8 days, but have additional irregular variations superimposed -- the characteristic behaviour for classical T Tauri stars. Based on the colour variability, we conclude that hot spots are the dominant cause of the variations in two objects, including one likely brown dwarf, with spot temperatures in the range of 6000-7000 K. For the third one (#2), a brown dwarf or very low mass star, inhomogenities at the inner edge of the disk are the likely origin of the variability. Based on mid-infrared data from Spitzer, we confirm that the three highly variable sources are surrounded by circum-(sub)-stellar disks. They show typical SEDs for T Tauri-like objects. Using SED models we infer an enhanced scaleheight in the disk for the object #2, which favours the detection of disk inhomogenities in lightcurves and is thus consistent with the information from variability. In the SOri cluster, about every fifth accreting low-mass object shows persistent high-level photometric variability. We demonstrate that estimates for fundamental parameters in such objects can be significantly improved by determining the extent and origin of the variations.

Can stellar mass black holes be quark stars?

We investigate the possibility that stellar mass black holes, with masses in the range of $3.8M_{\odot}$ and $6M_{\odot}$, respectively, could be in fact quark stars in the Color-Flavor-Locked (CFL) phase. Depending on the value of the gap parameter, rapidly rotating CFL quark stars can achieve much higher masses than standard neutron stars, thus making them possible stellar mass black hole candidates. Moreover, quark stars have a very low luminosity and a completely absorbing surface - the infalling matter on the surface of the quark star is converted into quark matter. A possibility of distinguishing CFL quark stars from stellar mass black holes could be through the study of thin accretion disks around rapidly rotating quark stars and Kerr type black holes, respectively. Furthermore, we show that the radiation properties of accretion disks around black holes and CFL quark stars are also very similar. However, strange stars exhibit a low luminosity, but high temperature bremsstrahlung spectrum, which, in combination with the emission properties of the accretion disk, may be the key signature to differentiate massive strange stars from black hole.

Observational properties of accretion discs: Precessing and warped?

Over the last 40 years, X-ray astronomy missions have revealed long-term, superorbital periods in a variety of X-ray binaries. These modulations can provide significant constraints on the physical properties of accretion discs. Some of these modulations are Her X-1-like and are interpreted as irradiation-driven, tilted, precessing accretion discs. Others show more complex light curves, with the period changing on timescales >1000 d, and are interpreted in terms of the Ogilvie and Dubus [Ogilvie, G.I., Dubus, G., 2001, MNRAS 320, 485 (OD01)] disc stability criteria. We suggest a categorisation of superorbital periods into six different types, based on their observed characteristics.

On the X-ray spectra of luminous, inhomogeneous accretion flows

We discuss the expected X-ray spectral and variability properties of black hole accretion discs at high luminosity, under the hypothesis that radiation pressure dominated discs are subject to violent clumping instabilities and, as a result, have a highly inhomogeneous two-phase structure. After deriving the full accretion disc solutions explicitly in terms of the parameters of the model, we study their radiative properties both with a simple two-zones model, treatable analytically, and with radiative transfer simulations which account simultaneously for energy balance and Comptonisation in the hot phase, together with reflection, reprocessing, ionization and thermal balance in the cold phase. We show that, if not only the density, but also the heating rate within these flows is inhomogeneous, then complex reflection-dominated spectra can be obtained for a high enough covering fraction of the cold phase. In general, large reflection components in the observed X-ray spectra should be associated with strong soft excesses, resulting from the combined emission of ionized atomic emission lines. The variability properties of such systems are such that, even when contributing to a large fraction of the hard X-ray spectrum, the reflection component is less variable than the power-law like emission originating from the hot Comptonising phase, in agreement with what is observed in many Narrow Line Seyfert 1 galaxies and bright Seyfert 1. Our model falls within the family of those trying to explain the complex X-ray spectra of bright AGN with ionized reflection, but presents an alternative, specific, physically motivated, geometrical setup for the complex multi-phase structure of the inner regions of near-Eddington accretion flows.

Influence of the black hole rotation on accretion discs and jets in active galaxies: the case of NGC 1068

Although we do not observe directly black holes in the Universe, their presence has been inferred in several astrophysical systems, from galactic to extragalactic scales. In this work, we will restrict our focus in the possible signatures of the presence of Kerr black holes in active galactic nuclei, one of the most powerful sources in the Universe. Particularly, we will discuss how jet/accretion disc precession and their directional stability can be used to trace the accretion disc properties as well as the black hole spin in the Seyfert galaxy NGC 1068.

A Study of Synthetic and Observed Hα Spectra of TT Hydrae

The formation and properties of accretion disks and circumstellar material in Algol-type systems are not very well understood. In order to study the underlying physics of these structures, we have calculated synthetic Hα spectra of TT Hya, which is an Algol-type eclipsing binary with an accretion disk. Both the primary and secondary stars were considered in the calculations, as well as a disk surrounding the primary. The Roche model for the secondary star was assumed. The synthetic spectra cover all the phases, including primary eclipse, and are compared with the observed spectra. The influence of various effects and free parameters of the disk on the emerging spectrum was studied. This enabled us to put some constraints on the geometry, temperature, density, and velocity fields within the disk. Differences found between the observed and synthetic spectra unravel the existence of a gas stream, as well as a hotter disk-gas interaction region. An additional cooler circumstellar region between the C1 and C2 Roche surfaces is suggested to account for various observed effects. A new computer code called SHELLSPEC was created for this purpose and is briefly described in this work as well. It is designed to solve simple radiative transfer along the line of sight in three-dimensional moving media. The scattered light from a central object is taken into account assuming an optically thin environment. Output intensities are then integrated through the two-dimensional projection surface of a three-dimensional object. The assumptions of the code include LTE and optional known state quantities and velocity fields in three dimensions.

INFLUENCE OF THE RELICT COSMOLOGICAL CONSTANT ON ACCRETION DISCS

Surprisingly, the relict cosmological constant has a crucial influence on properties of accretion discs orbiting black holes in quasars and active galactic nuclei. We show it by considering basic properties of both the geometrically thin and thick accretion discs in the Kerr–de Sitter black hole (naked-singularity) spacetimes. Both thin and thick discs must have an outer edge allowing outflow of matter into the outer space, located nearby the so-called static radius, where the gravitational attraction of a black hole is balanced by the cosmological repulsion. Jets produced by thick discs can be significantly collimated after crossing the static radius. Extension of discs in quasars is comparable with extension of the associated galaxies, indicating a possibility that the relict cosmological constant puts an upper limit on extension of galaxies.

The Connection between Jets, Accretion Disks, and Black Hole Mass in Blazars

The properties of relativistic radio jets are thought to be closely connected with the properties of accretion disks in active galactic nuclei. We explore this issue using a sample of 35 blazars with very-long baseline observations, for which we can estimate the kinetic powers of their relativistic jets, the bolometric luminosity of their accretion disks, and the masses of their black holes. Contrary to previous claims, we find that the jet kinetic power is significantly correlated with the disk luminosity. This supports the notion that the disk is somehow coupled to the jet, even on parsec scales. Moreover, we show that the correlation improves by including the black hole mass as a second parameter. The dominance of the jet, parameterized as the ratio of the jet kinetic power to the disk luminosity, is largely controlled by and is inversely correlated with the Eddington ratio of the accretion disk. This empirical relation should serve as a useful guide for theoretical models for jet formation.

High-energy spectra from black-hole candidates

Our knowledge of the high-energy emission spectra from candidates has increased enormously with missions like BeppoSAX and RossiXTE, thanks to their broad-band window. I present the main and most solid points of this current knowledge, both in terms of detailed spectral modeling of X-ray broad-band spectra, and more generally in relation to the black-hole state paradigm, which links spectral and timing properties of accretion disks. Finally, through the discussion of recent results on two systems, XTE J1650-500 and GX 339-4, I present evidence for a new view of the state paradigm.

Turbulence in Accretion Disks

An accretion disk is an inevitable part of the star forming process. Recent years have witnessed dramatic progress in our understanding of how turbulence arises and transports angular momentum in astrophysical accretion disks. The key conceptual point is that the combination of a subthermal magnetic field and outwardly decreasing differential rotation is subject to the magnetorotational instability. This rapidly generates magnetohydrodynamical (MHD) turbulence, leading to greatly enhanced angular momentum transport. Purely hydrodynamic disks, on the other hand, are stable. Disks that are too cool to couple effectively to the magnetic field will not be turbulent. Fully global three dimensional MHD simulations are now beginning to probe the properties of accretion disks from first principles.

Accretion disks: The magnetohydrodynamic powerhouse

Accretion disks orbiting around compact objects such as white dwarfs, neutron stars and black holes are a central paradigm of high energy astrophysics. Accretion disks are believed to power systems such as x-ray binaries, active galactic nuclei, and quasars through the release of gravitational potential energy as gas spirals in. This infall requires the transfer of angular momentum outward from one fluid element to another. Thus the dynamics and evolution of accretion disks depend fundamentally upon this angular momentum transport process. Because accretion disks are composed of highly ionized conducting plasma with very high Reynolds number, the equations governing their evolution are those of (inviscid) magnetohydrodynamics (MHD). In disks the combination of a subthermal magnetic field and outwardly decreasing differential rotation leads to a powerful linear instability (the magnetorotational instability) which, in turn, generates MHD turbulence that produces a net Maxwell stress and the required angular momentum transport. The turbulence generates heat through dissipation at the viscous and resistive length-scales. Thus, accretion disks are systems where the power generation mechanism derives directly from a magnetohydrodynamic process. Fully global three-dimensional MHD simulations are now beginning to probe the properties of accretion disks from first principles.

Long-term properties of accretion discs in X-ray binaries -- II. Stability of radiation-driven warping

A significant number of X-ray binaries (XRBs) are now known to exhibit long-term 'superorbital' periodicities on time-scales of ∼10-100 d. Several physical mechanisms have been proposed that give rise to such periodicities, in particular warping and/or precession of the accretion disc. Recent theoretical work predicts the stability to disc warping of XRBs as a function of the mass ratio, binary radius, viscosity and accretion efficiency, and here we examine the constraints that can be placed on such models by current observations. In Paper I we used a dynamic power spectrum (DPS) analysis of long-term X-ray data sets (Compton Gamma Ray Observatory, Rossi X-Ray Timing Explorer), focusing on the remarkable, smooth variations in the superorbital period exhibited by SMC X-1. Here we use a similar DPS analysis to investigate the stability of the superorbital periodicities in the neutron star XRBs Cyg X-2, LMC X-4 and Her X-1, and thereby confront stability predictions with observation. We find that the period and nature of superorbital variations in these sources are consistent with the predictions of warping theory. We also use a dynamic light-curve analysis to examine the behaviour of Her X-1 as it enters and leaves the 1999 anomalous low state (ALS). This reveals a significant phase shift some 15 cycles before the ALS, which indicates a change in the disc structure or profile leading into the ALS.

Studying the Properties of Accretion Disks and Coronae in Black Hole X‐Ray Binaries with Monte Carlo Simulation

Understanding the properties of the hot corona is important for studying the accretion disks in black hole X-ray binary systems. Using the Monte Carlo technique to simulate the inverse Compton scattering between photons emitted from the cold disk and electrons in the hot corona, we have produced two table models in the XSPEC format for the spherical corona case and the disklike (slab) corona case. All parameters in our table models are physical properties of the system and can be derived from data fitting directly. Applying the models to broadband spectra of the black hole candidate XTE J2012+381 observed with BeppoSAX, we estimate the size of the corona and the inner radius of the disk. The size of the corona in this system is several tens of gravitational radii, and the substantial increase of the inner disk radius during the transit from the hard state to the soft state is not found.

Long-term properties of accretion discs in X-ray binaries -- I. The variable third period in SMC X-1

Long-term X-ray monitoring data from the Rossi X-Ray Timing Explorer (RXTE) All-Sky Monitor (ASM) and the Compton Gamma Ray Observatory (CGRO) Burst And Transient Source Experiment (BATSE) reveal that the third (superorbital) period in SMC X-1 is not constant, but varies between 40 and 60 d. A dynamic power spectrum analysis indicates that the third period has been present continuously throughout the five years of ASM observations. This period changed smoothly from 60 to 45 d and then returned to its former value, on a time-scale of approximately 1600 d. During the nearly 4 yr of overlap between the CGRO and RXTE missions, the simultaneous BATSE hard X-ray data confirm and extend this variation in SMC X-1. Our discovery of such an instability in the superorbital period of SMC X-1 is interpreted in the context of recent theoretical studies of warped, precessing accretion discs. We find that the behaviour of SMC X-1 is consistent with a radiation-driven warping model.

Thermal stability of self-gravitating, optically thin accretion disks

In the dynamics of accretion disks, the presence of collective eects associated with the self-gravity of the disk is expected to aect not only the momentum transport, but also the relevant energy balance equations, which could dier substantially from the non-self-gravitating case. Here we follow the model that, when the disk is suciently cold, the stirring due to Jeans-related instabilities acts as a source of eective heating. The corresponding reformulation of the energy equations allows us to: (i) demonstrate how self-regulation can be established, so that the stability parameter Q is maintained close to a threshold value, with weak dependence on radius; (ii) rediscuss the opacity properties in the self-gravitating regime. In particular, we show that, if cooling is dominated by bremsstrahlung, an optically thin stationary accretion solution is thermally stable, even in the non-advective case, provided the disk is self-gravitating. The details of the cooling function have little eect on the structure of such accretion disk, which is in any case induced by self-gravity to self-regulate. This condition of self-gravitating accretion is expected to be appropriate for the outer regions of many disks of astrophysical interest. With the reformulation of the energy equations described in this paper, we have also secured: (iii )a starting point for the study of the emission properties of self-gravitating accretion disks; (iv) a tool to analyze the structure of the transition region, where the disk becomes self-gravitating.

Basic Properties of Supercritical Accretion Disks

Abstract At the turn of the millennium, supercritical accretion disks (superdisks), where the mass-accretion rate highly exceeds the Eddington's one, are being resurrected under recent observational and theoretical progress. We examined the effects and properties of the self-irradiation, self-occultation, continuous and line spectra of superdisks, and found various distinct features of superdisks. Due to self-irradiation, the effective flux is enhanced by a factor of 1.1 to 1.8 all over the disk, depending on the relative disk thickness. For a superdisk with finite thickness, the flux at infinity is enhanced in the poleward direction, while it is zero in the equatorial direction. The continuous spectra are generally flat (vSv ∼ v0), as long as we see the disk pole-on. It, however, strongly depends on the inclination angle: for a larger inclination angle the central part of the disk is self-occulted and the flat nature of the continuum disappears. The line spectra are also greatly modified from those of the standard disk. In the non-relativistic standard model the line spectra have symmetrical double peaks. In the superdisk, however, the line spectra become asymmetric with strong blue peaks and weak red peaks, even for the non-relativistic case. This is due to the projection effect. The relations between the observables and model parameters are derived.

Self-Gravitating Accretion Disks

We consider a class of fully self-gravitating accretion disks, for which efficient cooling mechanisms are assumed to maintain the disk close to the margin of Jeans stability. For such self-regulated disks the equations become very simple in the outer regions, where the angular momentum convective transport approximately balances the viscous transport. These latter equations have been shown (G Bertin 1997 Astrophys. J. 478 L71) to lead naturally to a self-similar solution with flat rotation curve, with circular velocity proportional to (Ṁ)1/3 and fixed opening angle. Here we report on a recent extensive investigation (G Bertin and G Lodato 1999 Astron. Astrophys. 350 694) of the general properties of self-regulated accretion disks.