Accretion Discs Research Papers - Academia.edu (original) (raw)

The solidification behavior of liquid metal in a container under rapid cooling process is one of the major concerns to be analyzed. In order to analyze its fundamental behavior, a three- dimensional (3D) fluid dynamics simulation was... more

The solidification behavior of liquid metal in a container under rapid cooling process is one of the major concerns to be analyzed. In order to analyze its fundamental behavior, a three- dimensional (3D) fluid dynamics simulation was developed using a particle-based method, known as the smoothed particle hydrodynamics (SPH) method. Governing equations that determine the fluid motion and heat transfer involving phase change process are solved by discretizing their gradient and Laplacian term with the moving particles and calculating the interaction with its neighboring particles. The results demonstrate that the SPH mehod can successfully reproduce the behavior and defect prediction of liquid metal solidification process.

The standard accretion discs are known to be thermally and viscously unstable over a certain range of temperatures. In the inner disc regions there may develop radiation pressure driven instability, which is possibly related to the rapid... more

The standard accretion discs are known to be thermally and viscously unstable over a certain range of temperatures. In the inner disc regions there may develop radiation pressure driven instability, which is possibly related to the rapid variability detected in AGNs in the UV range. In the outer disc develops the ionization instability, similar to that in the cataclysmic variables,

In this paper, we applied an improved Smoothing Particle Hydrodynamics (SPH) method by using gradient kernel renormalization in three-dimensional cases. The purpose of gradient kernel renormalization is to improve the accuracy of... more

In this paper, we applied an improved Smoothing Particle Hydrodynamics (SPH) method by using gradient kernel renormalization in three-dimensional cases. The purpose of gradient kernel renormalization is to improve the accuracy of numerical simulation by improving gradient kernel approximation. This method is implemented for simulating free surface flows, in particular dam break case with rigid ball structures and the propagation of waves towards a slope in a rectangular tank.

Using mathematical formalism borrowed from dynamical systems theory, a complete analytical investigation of the critical behaviour of the stationary flow configuration for the low angular momentum axisymmetric black hole accretion... more

Using mathematical formalism borrowed from dynamical systems theory, a complete analytical investigation of the critical behaviour of the stationary flow configuration for the low angular momentum axisymmetric black hole accretion provides valuable insights about the nature of the phase trajectories corresponding to the transonic accretion in the steady state, without taking recourse to the explicit numerical solution commonly performed in the literature to study the multi-transonic black hole accretion disc and related astrophysical phenomena. Investigation of the accretion flow around a non rotating black hole under the influence of various pseudo-Schwarzschild potentials and forming different geometric configurations of the flow structure manifests that the general profile of the parameter space divisions describing the multi-critical accretion is roughly equivalent for various flow geometries. However, a mere variation of the polytropic index of the flow cannot map a critical solution from one flow geometry to the another, since the numerical domain of the parameter space responsible to produce multi-critical accretion does not undergo a continuous transformation in multi-dimensional parameter space. The stationary configuration used to demonstrate the aforementioned findings is shown to be stable under linear perturbation for all kind of flow geometries, black hole potentials, and the corresponding equations of state used to obtain the critical transonic solutions. Finally, the structure of the acoustic metric corresponding to the propagation of the linear perturbation studied are discussed for various flow geometries used.

We describe a new algorithm for including the dynamical effects of ionizing radiation in SPH simulations, and we present several examples of how the algorithm can be applied to problems in star formation. We use the HEALPix software to... more

We describe a new algorithm for including the dynamical effects of ionizing radiation in SPH simulations, and we present several examples of how the algorithm can be applied to problems in star formation. We use the HEALPix software to tessellate the sky and to solve the equation of ionization equilibrium along a ray towards each of the resulting tesserae. We exploit the hierarchical nature of HEALPix to make the algorithm adaptive, so that fine angular resolution is invoked only where it is needed, and the computational cost is kept low. We present simulations of (i) the spherically symmetric expansion of an HII region inside a uniform-density, non--self-gravitating cloud; (ii) the spherically symmetric expansion of an HII region inside a uniform-density, self-gravitating cloud; (iii) the expansion of an off-centre HII region inside a uniform-density, non--self-gravitating cloud, resulting in rocket acceleration and dispersal of the cloud; and (iv) radiatively driven compression and ablation of a core overrun by an HII region. The new algorithm provides the means to explore and evaluate the role of ionizing radiation in regulating the efficiency and statistics of star formation.

The magnetorotational instability originates from the elastic coupling of fluid elements in orbit around a gravitational well. Since inertial accelerations play a fundamental dynamical role in the process, one may expect substantial... more

The magnetorotational instability originates from the elastic coupling of fluid elements in orbit around a gravitational well. Since inertial accelerations play a fundamental dynamical role in the process, one may expect substantial modifications by strong gravity in the case of accretion on to a black hole. In this paper, we develop a fully covariant, Lagrangian displacement vector field formalism with the aim of addressing these issues for a disk embedded in a stationary geometry with negligible radial flow. This construction enables a transparent connection between particle dynamics and the ensuing dispersion relation for MHD wave modes. The MRI--in its incompressible variant-- is found to operate virtually unabated down to the marginally stable orbit; the putative inner boundary of standard accretion disk theory. To get a qualitative feel for the dynamical evolution of the flow below rrmmsr_{\rm ms}rrmms, we assume a mildly advective accretion flow such that the angular velocity profile departs slowly from circular geodesic flow. This exercise suggests that the turbulent eddies will occur at spatial scales approaching the radial distance while tracking the surfaces of null angular velocity gradients. The implied field topology, namely large-scale horizontal field domains, should yield strong mass segregation at the displacement nodes of the non-linear modes when radiation stress dominates the local disk structure (an expectation supported by quasi-linear arguments and by the non-linear behavior of the MRI in a non-relativistic setting). Under this circumstance, baryon-poor flux in horizontal field domains will be subject to radial buoyancy and to the Parker instability, thereby promoting the growth of poloidal field.

In a stationary, general relativistic, axisymmetric, inviscid and rotational accretion flow, described within the Kerr geometric framework, transonicity has been examined by setting up the governing equations of the flow as a first-order... more

In a stationary, general relativistic, axisymmetric, inviscid and rotational accretion flow, described within the Kerr geometric framework, transonicity has been examined by setting up the governing equations of the flow as a first-order autonomous dynamical system. The consequent linearized analysis of the critical points of the flow leads to a comprehensive mathematical prescription for classifying these points, showing that the only possibilities are saddle points and centre-type points for all ranges of values of the fixed flow parameters. The spin parameter of the black hole influences the multitransonic character of the flow, as well as some of its specific critical properties. The special case of a flow in the space-time of a non-rotating black hole, characterized by the Schwarzschild metric, has also been studied for comparison and the conclusions are compatible with what has been seen for the Kerr geometric case.

We find a very strong correlation between the intrinsic spectral slope in X-rays and the amount of Compton reflection from a cold medium in Seyfert AGNs and in hard state of X-ray binaries with either black holes or weakly-magnetized... more

We find a very strong correlation between the intrinsic spectral slope in X-rays and the amount of Compton reflection from a cold medium in Seyfert AGNs and in hard state of X-ray binaries with either black holes or weakly-magnetized neutron stars. Objects with soft intrinsic spectra show much stronger reflection than ones with hard spectra. We find that at a given spectral slope, black-hole binaries have similar or more reflection than Seyferts whereas neutron-star binaries in our sample have reflection consistent with that in Seyferts. The existence of the correlation implies a dominant role of the reflecting medium as a source of seed soft photons for thermal Comptonization in the primary X-ray source.

The Galactic transient X-ray binary MAXI J1836-194 was discovered on 29th August 2011. Here we make a detailed study of the spectral and timing properties of its 2011 outburst using archival data of RXTE Proportional Counter Array... more

The Galactic transient X-ray binary MAXI J1836-194 was discovered on 29th August 2011. Here we make a detailed study of the spectral and timing properties of its 2011 outburst using archival data of RXTE Proportional Counter Array instrument. The evolution of accretion flow dynamics of the source during the outburst through spectral analysis with Chakrabarti-Titarchuk's two-component advective flow (TCAF) solution as a local table model in XSPEC. We also fitted spectra with combined disk blackbody and power-law models and compared it with the TCAF model fitted results. The source is found to be in hard and hard-intermediate spectral states only during entire phase of this outburst. No soft or soft-intermediate spectral states are observed. This could be due to the fact that this object belongs to a special class of sources (e.g., MAXI J1659-152, Swift J1753.5-0127, etc.) that have very short orbital periods and that companion is profusely mass-losing or the disk is immersed inside an excretion disk. In these cases, flows in the accretion disk is primarily dominated by low viscous sub-Keplerian flow and the Keplerian rate is not high enough to initiate softer states. Low-frequency quasi-periodic oscillations (QPOs) are observed sporadically although as in normal outbursts of transient black holes, monotonic evolutions of QPO frequency during both rising and declining phases are observed. From the TCAF fits, we find mass of the black hole in the range of 7.5 − 11 M ⊙ and from time differences between peaks of the Keplerian and sub-Keplerian accretion rates we obtain viscous timescale for this particular outburst ∼ 10 days. Subject headings: X-Rays:binaries – stars individual: (MAXI J1836-194) – stars:black holes – accretion, accretion disks – shock waves – radiation:dynamics

Some of the progress in understanding the variability of active galactic nuclei (AGNs) from the optical to X-ray regions is reviewed. Although where there is a clear correlation between variations in the two regions, the optical lags the... more

Some of the progress in understanding the variability of active galactic nuclei (AGNs) from the optical to X-ray regions is reviewed. Although where there is a clear correlation between variations in the two regions, the optical lags the X-rays, simple reprocessing of the X-ray radiation to produce significant amounts of longer-wavelength continua seems to be ruled out. In a couple of objects where there has been correlated X-ray and optical variability, the amplitude of the optical variability has exceeded the amplitude of the X-ray variability. We suggest that the factor linking the X-ray and optical regions might not be irradiation, but accelerated particles striking matter (as in activity in the solar chromosphere). The diversity in optical/X-ray relationships at different times in the same object, and between different objects, could be explained by evolving differences in local geometry, and by changing directions of motion relative to our line of sight. Linear shot-noise models of the variability are ruled out; instead there must be large-scale organization of variability. Variability occurs on light-crossing timescales rather than viscous timescales and this probably rules out the standard Shakura-Sunyaev accretion disk. Instead, we believe that the main energy-generation mechanism is probably electromagnetic. The overall average continuum shape appears to be the same in both radio-loud and radio-quiet AGNs, strongly suggesting a similar origin to the continua. Radio-loud and radio-quiet AGNs have quite similar optical variability properties, and this suggests a common variability mechanism. Beaming effects could be significant in all types of AGN. Despite their extreme X-ray variability properties, our observations show that narrow-line Seyfert 1s (NLS1s) do not show extreme optical variability, and that their optical variability properties could well be similar to those of non-NLS1s.

Cool magnetohydrodynamics (MHD) disc wind physics is reviewed by means of a self-similar analytical model, putting special emphasis on the mathematical aspects of the solution. It is found that the key parameter of the theory (μ) measures... more

Cool magnetohydrodynamics (MHD) disc wind physics is reviewed by means of a self-similar analytical model, putting special emphasis on the mathematical aspects of the solution. It is found that the key parameter of the theory (μ) measures the relation between magnetic and tidal forces. The generation of MHD winds from accretion discs requires a subtle tuning between both stresses because only a narrow range of μ values is allowed; this range is, indeed, close to the cut-off of the magnetic turbulence induced by the development of the Balbus-Hawley instability. The space of solutions can be separated into two quite distinct classes: low-μsolutions generate magnetically dominated outflows and display a characteristic density change from horizontal to vertical stratification, while in high-μsolutions the density decreases without any intermediate enhancement as the rotation axis is approached. These theoretical (dynamical) results have been used to study the properties of the base of the wind. Density and velocity laws have been derived directly from the dynamics. The effect of the propagation of the stellar X-ray radiation through the wind has been analysed to determine the temperature law at the base of the wind (polar angles θ > 45°). It is shown that a cocoon of photoionized gas is generated around the star. The extent of the photoionized region is small (tenths of au) in dense outflows and close to the disc plane; however, it may cover the whole wind extent in diffuse winds, e.g. disc winds generated by small accretion rates (<=10-9 Msolar yr-1). Photoionization also modifies the electron density in the plasma. As a consequence, the ambipolar diffusion heating decreases in the inner part of the wind by roughly one order of magnitude with respect to that derived by other authors. In fact, radiative heating controls the thermal properties of the inner 0.3 and 1 au of the disc wind for accretion rates of 10-7 and 10-8 Msolar yr-1, respectively. The temperature of the densest region (base) of the wind is, at most, ~=10 000 K. Therefore, although densities as high as ~109 cm-3 can be achieved by disc winds, the temperature is significantly smaller than the ~5 × 105-8 × 105 K derived from the ultraviolet (UV) observations of the base of the optical jets. Also, it is shown that densities as high as ~109 cm-3 cannot be achieved at the jet recollimation point for the accretion rates observed in the T Tauri stars. In summary, we conclude that the flow traced by the UV semiforbidden lines is not associated with cold disc winds but, most likely, it is tracing the hot inner jet, postulated in cold disc wind theory, which prevents the radial collapse of the wind.

Context: Ionizing radiation plays a crucial role in star formation at all epochs. In contemporary star formation, ionization abruptly raises the pressure by more than three orders of magnitude; the temperature increases from ~10~K to... more

Context: Ionizing radiation plays a crucial role in star formation at all epochs. In contemporary star formation, ionization abruptly raises the pressure by more than three orders of magnitude; the temperature increases from ~10~K to ~10^4~K, and the mean molecular weight decreases by a factor of more than 3. This may result in positive feedback, either by compressing pre-existing clouds and rendering them unstable, or by sweeping up gravitationally unstable shells. It may also result in negative feedback (by dispersing residual dense gas). Ionizing radiation from OB stars is also routinely invoked as a means of injecting kinetic energy into the interstellar medium and as a driver of sequential self-propagating star formation in galaxies. Aims: We describe a new algorithm for including the dynamical effects of ionizing radiation in SPH simulations, and we present several examples of how the algorithm can be applied to problems in star formation. Methods: We use the HEALPix software to tessellate the sky and to solve the equation of ionization equilibrium along a ray towards each of the resulting tesserae. We exploit the hierarchical nature of HEALPix to make the algorithm adaptive, so that fine angular resolution is invoked only where it is needed, and the computational cost is kept low. Results: We present simulations of (i) the spherically symmetric expansion of an Hii region inside a uniform-density, non-self-gravitating cloud; (ii) the spherically symmetric expansion of an Hii region inside a uniform-density, self-gravitating cloud; (iii) the expansion of an off-centre Hii region inside a uniform-density, non-self-gravitating cloud, resulting in rocket acceleration and dispersal of the cloud; and (iv) radiatively driven compression and ablation of a core overrun by an Hii region. Conclusions: The new algorithm provides the means to explore and evaluate the role of ionizing radiation in regulating the efficiency and statistics of star formation.

We present high quality near infrared imaging polarimetry of HL Tau at 0.4 to 0.6 arcsec resolution, obtained with Subaru/CIAO and UKIRT/IRCAM. 3-D Monte Carlo modelling with aligned oblate grains is used to probe the structure of the... more

We present high quality near infrared imaging polarimetry of HL Tau at 0.4 to 0.6 arcsec resolution, obtained with Subaru/CIAO and UKIRT/IRCAM. 3-D Monte Carlo modelling with aligned oblate grains is used to probe the structure of the circumstellar envelope and the magnetic field, as well as the dust properties. At J band the source shows a centrosymmetric pattern dominated by scattered light. In the H and K bands the central source becomes visible and its polarisation appears to be dominated by dichroic extinction, with a position angle inclined by ~40 degrees to the disc axis. The polarisation pattern of the environs on scales up to 200 AU is consistent with the same dichroic extinction signature superimposed on the centrosymmetric scattering pattern. These data can be modelled with a magnetic field which is twisted on scales from tens to hundreds of AU, or alternatively by a field which is globally misaligned with the disc axis. A unique solution to the field structure will require spatially resolved circular polarisation data. The best fit Monte Carlo model indicates a shallow near infrared extinction law. When combined with the observed high polarisation and non-negligible albedo these constraints can be fitted with a grain model involving dirty water ice mantles in which the largest particles have radii slightly in excess of 1 um. The best fit model has an envelope structure which is slightly flattened on scales up to several hundred AU. Both lobes of the bipolar outflow cavity contain a substantial optical depth of dust (not just within the cavity walls). Curved, approximately parabolic, cavity walls fit the data better than a conical cavity. The small inner accretion disc observed at millimetre wavelengths is not seen at this spatial resolution.

Astrophysical jets are ubiquitous throughout the universe. They can be observed to emerge from protostellar objects, stellar x-ray binaries and supermassive black holes located at the center of active galaxies, and they are believed to... more

Astrophysical jets are ubiquitous throughout the universe. They can be observed to emerge from protostellar objects, stellar x-ray binaries and supermassive black holes located at the center of active galaxies, and they are believed to originate from a central object that is surrounded by a magnetized accretion disc. With the motivations to understand whether hypersonic Newtonian jets produce any similarity to the morphologies observed in jets from young stellar objects (YSOs) and whether numerical codes, based on Godunov-type schemes, capture the basic physics of shocked flows, we have conceived a laboratory experiment and performed three-dimensional (3D) numerical simulations that reproduce the mid-to-long-term evolution of hypersonic jets. Here we show that these jets propagate, maintaining their collimation over long distances, in units of the jet initial radius. The jets studied are quasi-isentropic, are both lighter and heavier than the ambient and meet the two main scaling parameter requirements for proto-stellar jets: the ejection Mach number and the ambient/jet density ratio.

We study the disc emission component hidden in the single-peaked broad emission lines (BELs) of active galactic nuclei. We compare the observed broad lines from a sample of 90 Seyfert 1 spectra taken from the Sloan Digital Sky Survey with... more

We study the disc emission component hidden in the single-peaked broad emission lines (BELs) of active galactic nuclei. We compare the observed broad lines from a sample of 90 Seyfert 1 spectra taken from the Sloan Digital Sky Survey with simulated line profiles. We consider a two-component broad line region model where an accretion disc and a surrounding non-disc region with isotropic cloud velocities generate the simulated BEL profiles. The analysis is mainly based on the measurements of the full widths (at 10 per cent, 20 per cent and 30 per cent of the maximum intensity) and on the asymmetries of the line profiles. Comparing these parameters for the simulated and observed Hα broad lines, we found that the hidden disc emission may be present in BELs even if the characteristic of two-peaked-line profiles is absent. For the available sample of objects (Seyfert 1 galaxies with single-peaked BELs), our study indicates that, in the case of the hidden disc emission in single-peaked bro...

We study the stability of accretion disc around magnetised stars. Starting from the equations of magnetohydrodynamics we derive equations for linearized perturbation of geometrically thin, optically thick axisymmetric accretion disc with... more

We study the stability of accretion disc around magnetised stars. Starting from the equations of magnetohydrodynamics we derive equations for linearized perturbation of geometrically thin, optically thick axisymmetric accretion disc with an internal dynamo around magnetized stars. The structure and evolution of such discs are governed by an evolution equation for matter surface density ∑(R,T). Using the time-dependent equations for an accretion disc we do a linear stability analysis of our steady disc solutions in the presence of the magnetic field generated due to an internal dynamo.

We argue that the narrow-line regions (NLRs) of Seyfert galaxies are powered by the transport of energy and momentum by the radio-emitting jets. This implies that the ratio of the radio power to jet energy —ux is much smaller than is... more

We argue that the narrow-line regions (NLRs) of Seyfert galaxies are powered by the transport of energy and momentum by the radio-emitting jets. This implies that the ratio of the radio power to jet energy —ux is much smaller than is usually assumed for radio ...

At present, J1819–1458 is the only rotating radio transient (RRAT) detected in X-rays. We have studied the long-term evolution of this source in the fallback disc model. The model can reproduce the period, period derivative and X-ray... more

At present, J1819–1458 is the only rotating radio transient (RRAT) detected in X-rays. We have studied the long-term evolution of this source in the fallback disc model. The model can reproduce the period, period derivative and X-ray luminosity of J1819–1458 simultaneously in the accretion phase at ages ∼ 2 × 10^5 yr. We obtained reasonable model curves with a magnetic dipole field strength B_0 ∼ 5 × 10^11 G on the pole of the neutron star, which is much weaker than the field inferred from the dipole-torque formula. With this B0 and the measured period, we find J1819–1458 below and close to the radio pulsar death line. Our results are not sensitive to initial period, and the source properties can be produced with a large range of disc masses. Our simulations indicate that J1819–1458 is evolving towards the properties of dim isolated neutron stars at later phases of evolution. This implies a close evolutionary link between RRATs and dim isolated neutron stars. For other RRATs with measured period derivatives and unknown X-ray luminosities, we have estimated the lower limits on the B_0 values in the fallback disc model. These limits allow a dipole field distribution for RRATs that could fill the B_0 gap between the estimated B_0 ranges of dim thermal isolated neutron stars and central compact objects in the same model.

UV and optically selected candidates for stellar tidal disruption events (TDE) often exhibit broad spectral features (HeII emission, Hα emission, or absorption lines) on a blackbody-like continuum (1e4 K<T<1e5 K). The lines presumably... more

UV and optically selected candidates for stellar tidal disruption events (TDE) often exhibit broad spectral features (HeII emission, Hα emission, or absorption lines) on a blackbody-like continuum (1e4 K<T<1e5 K). The lines presumably emit from TDE debris or circumnuclear clouds photoionized by the flare. Line velocities however are much lower than expected from a stellar disruption by supermassive black hole (SMBH), and are somewhat faster than expected for the broad line region (BLR) clouds of a persistently active galactic nucleus (AGN). The distinctive spectral states are not strongly related to observed luminosity and velocity, nor to SMBH mass estimates. We use exhaustive photoionization modelling to map the domain of fluxes and cloud properties that yield (e.g.) a He-overbright state where a large HeII(4686Å)/Hα line-ratio creates an illusion of helium enrichment. Although observed line ratios occur in a plausible minority of cases, AGN-like illumination can not reproduce the observed equivalent widths. We therefore propose to explain these properties by a light-echo photoionization model: the initial flash of a hot blackbody (detonation) excites BLR clouds, which are then seen superimposed on continuum from a later, expanded, cooled stage of the luminous source. The implied cloud mass is substellar, which may be inconsistent with a TDE. Given these and other inconsistencies with TDE models (e.g. host-galaxies distribution) we suggest to also consider alternative origins for these nuclear flares, which we briefly discuss (e.g. nuclear supernovae and starved/subluminous AGNs).

This is the second in a series of papers investigating the oscillation properties of relativistic, non-self-gravitating tori orbiting around black holes. Extending the work done in a Schwarzschild background, here we consider the... more

This is the second in a series of papers investigating the oscillation properties of relativistic, non-self-gravitating tori orbiting around black holes. Extending the work done in a Schwarzschild background, here we consider the axisymmetric oscillations of vertically integrated tori in a Kerr space-time. The tori are modelled with a number of different non-Keplerian distributions of specific angular momentum, and we discuss how the oscillation properties depend on these and on the rotation of the central black hole. We first consider a local analysis to highlight the relations between acoustic and epicyclic oscillations in a Kerr space-time, and subsequently perform a global eigenmode analysis to compute the axisymmetric p modes. In analogy with what has been found in a Schwarzschild background, these modes behave as sound waves that are modified by rotation and are globally trapped in the torus. For constant distributions of specific angular momentum, the eigenfrequencies appear in a sequence 2:3:4:..., which is essentially independent of the size of the disc and of the black hole rotation. For non-constant distributions of angular momentum, on the other hand, the sequence depends on the properties of the disc and on the spin of the black hole, becoming harmonic for sufficiently large tori. We also comment on how p modes could explain the high-frequency quasi-periodic oscillations observed in low-mass X-ray binaries with a black hole candidate and the properties of an equivalent model in Newtonian physics.