Part Of Accretion Disk Research Articles

Testing Relativistic Reflection Models with GRMHD Simulations of Accreting Black Holes

X-ray reflection spectroscopy is currently one of the leading techniques for studying the inner part of accretion disks around black holes, measuring black hole spins, and even testing fundamental physics in strong gravitational fields. However, the accuracy of these measurements depends on the reflection models employed for the spectral analysis, which are sometimes questioned. In this work, we use a general relativistic magnetohydrodynamic code to generate a thin accretion disk in Kerr spacetime and ray-tracing techniques to calculate its relativistically broadened reflection spectrum. We simulate NuSTAR observations and we test the capability of current reflection models to recover the correct input parameters. Our study shows that we can measure the correct input parameters in the case of high inclination angle sources, while we find some minor discrepancies when the inclination angle of the disk is low.

Implementation of a radial disk ionization profile in the relxill_nk model

Very steep reflection emissivity profiles in the inner part of accretion disks are commonly found in the analysis of X-ray observations of black hole binaries and AGN, but there is some debate about their exact origin. While steep reflection emissivity profiles can be naturally produced by compact coronae close to black holes, the measured radial emissivity parameter can be further increased by the radial disk ionization profile when the theoretical model assumes a disk with constant ionization. In this paper, we implement the possibility of a radial disk ionization profile in the reflection model RELXILL_NK, which is a package designed to calculate reflection spectra of "deformed" Kerr black holes. We analyze a NuSTAR observation of the black hole binary EXO 1846-031, which was previously found to have a very high inner emissivity index. We find that the model with a radial disk ionization profile improves the fit, but the impact on the estimate of the black hole spin parameter and on the constraint of the deformation parameter is modest. However, we show that the analysis of future observations of Athena and eXTP will necessarily require models with a radial disk ionization profile to have accurate constraints of the deformation parameters.

Can the magneto rotational instability be performed in the presence of strong magnetic field?

According to previous studies, it is known that Magneto-Rotational-Instability (MRI) can produce turbulent in accretion disks and it can be the origin of viscosity in these systems. These studies showed that MRI can not perform in systems with strong magnetic field. So, the origin of viscosity is as open question in accretion disks with strong magnetic field yet. In this paper, we try to review the instability of accretion plasma disks in the presence of the term including electron pressure in Ohm law. This term can be important in the inner region of accretion disks where the temperature is high and density is low. Some additional terms such as time dependent electrical field and Nernst coefficients are added to equations also. Noted, these terms are neglected in previous studies. Then, the local instability analysis is applied and solutions show that the presence of electron pressure term in Ohm law, can affect the stability of system. According to results, in a system with strong magnetic field \((\beta \leq 0.1)\) and without this term, \((A=0)\), the growth rate of MRI mode is so small, \(\gamma <0.2\). This result is same as previous result and means that MRI can not perform in accretion systems with strong magnetic field. However, in the presence of this term, \(A\neq 0\), result becomes difference. Low \(A\) parameter, \(A\leq 0.12\), can decrease the growth rate of unstable mode to zero and leads to more stability in system. On the other hands, big one, \(A\succeq 0.2\) increases the growth rate of unstable mode and leads to more instability in system. So, a system with strong magnetic field, (\(\beta =0.1\)) can be unstable, \(\gamma =0.7\), when the fraction of electron pressure term in Ohm law is big, \(A=0.9\). These results show that the MRI can be amplified by electron pressure in the inner part of accretion disks and so a system with strong magnetic field can be magneto rotationally unstable in the presence of big electron pressure.

3D simulations of disc winds extending radially self-similar MHD models

Disc-winds originating from the inner parts of accretion discs are considered as the basic component of magnetically collimated outflows. The only available analytical MHD solutions to describe disc-driven jets are those characterized by the symmetry of radial self-similarity. However, radially self-similar MHD jet models, in general, have three geometrical shortcomings, (i) a singularity at the jet axis, (ii) the necessary assumption of axisymmetry, and (iii) the non-existence of an intrinsic radial scale, i.e. the jets formally extend to radial infinity. Hence, numerical simulations are necessary to extend the analytical solutions towards the axis, by solving the full three-dimensional equations of MHD and impose a termination radius at finite radial distance. We focus here on studying the effects of relaxing the (ii) assumption of axisymmetry, i.e. of performing full 3D numerical simulations of a disc-wind crossing all magnetohydrodynamic critical surfaces. We compare the results of these runs with previous axisymmetric 2.5D simulations. The structure of the flow in all simulations shows strong similarities. The 3D runs reach a steady state and stay close to axisymmetry for most of the physical quantities, except for the poloidal magnetic field and the toroidal velocity which slightly deviate from axisymmetry. The latter quantities show signs of instabilities, which, however, are confined to the region inside the fast magnetosonic separatrix surface. The forces present in the flow, both of collimating and accelerating nature, are in good agreement in both the 2.5D and the 3D runs. We conclude that the analytical solution behaves well also after relaxing the basic assumption of axisymmetry.

Shapes of Fe K α lines from annuli near black holes

Recent x-ray observations of microquasars and Seyfert galaxies reveal broad emission lines in their spectra, which can arise in the innermost parts of accretion disks. We consider a radiating annulus model to simulate spectral line shapes. That is a natural approximation for narrow emitting circular rings without extra astrophysical assumptions about emissivity laws. Recently, Muller and Camenzind (Astron. Astrophys. 413, 861 (2004)) presented results of their calculations and classified different types of spectral line shapes and described their origin. We clarified their hypothesis about an origin of double peaked and double horned line shapes. On the basis of results of numerical simulations, we showed that double peaked spectral lines arise almost for any locations of narrow emission rings (annuli), although Muller and Camenzind claimed that such profiles arise for relatively flat spacetimes and typical radii for emission region of about 25r g. We showed that triangular spectral lines could arise for nearest annuli and high inclination angles. We discuss a possibility of appearance of narrow spectral line shapes as a result of spiraling evolution of matter along quasi-circular orbits which could be approximated by narrow annuli.

BLACK HOLES: OBSERVATIONAL PROPERTIES

In the article we review observational features of black holes where an influence of a gravitational field is dominant and we must use strong gravitational field approach for GR. Recent X-ray observations of microquasars and Seyfert galaxies reveal broad emission lines in their spectra, which can arise in the innermost parts of accretion disks. Simulations indicate that at low inclination angle the line is measured by a distant observer as characteristic two-peak profile. However, at high inclination angles (&gt; 85°) two additional peaks arise. This phenomenon was discovered by Matt et al. (1993) using the Schwarzschild black hole metric to analyze such an effect. They assumed that the effect is applicable to a Kerr metric far beyond the range of parameters that they exploited. We check and confirm their hypothesis about such a structure of the spectral line shape for the Kerr metric case. We discuss how analysis of the iron spectral line shapes could give an information about an upper limit of magnetic field near black hole horizon.

The iron K α-line diagnostics of the rotating black hole metric in Seyfert galaxies

Observations of Seyfert galaxies in X-ray region reveal the wide emissive lines in their spectra, which can arise in inner parts of accretion disks, where the effects of General Relativity (GR) must be taken into account. A spectrum of a solitary emission line of a hot spot in Kerr accretion disk is simulated depending on the radial coordinate r and the angular momentum a = J/ M of a black hole, under the assumption of equatorial circular motion of a hot spot. It is shown that the characteristic two-peak line profile with the sharp edges arises at a large distance (about r ≈ (3–10) r g). The inner regions emit the line, which is observed with one maximum and extremely wide red wing. We present results of simulations for the isothermal and Shakura–Sunayev disks.

Accretion Disk Oscillations and QPOs in Black Hole X-ray Binaries

The observed quasi-periodic oscillations (QPOs) in low-mass X-ray binaries have been suggested to be probably related to the characteristic oscillation modes in the inner part of a relativistic accretion disk. Here we study the non-damping oscillation modes in the disk by considering the local perturbations and viscosity effects. We find that the trapped oscillations are amplified significantly when the viscosity parameter increases and the trapped size varies when the radial wavenumber and Mach number change. Our results support that the hundred-hertz QPOs and their low fractional rms amplitudes found in some black hole X-ray binaries may be explained by the trapped non-damping oscillation modes in the inner part of an accretion disk around a rapidly rotating black hole.

The Shape of the Kα Line as the Evidence for the Black Hole Existence

Observations of Seyfert galaxies in X-ray region reveal the wide emissive lines in their spectra, which can arise in inner parts of accretion disks, where the effects of General Relativity (GR) must be taken into account. A spectrum of a solitary emission line of a hot spot in Kerr accretion disk is simulated, depending on the radial coordinate r and the angular momentum a = J/M of a black hole, under the assumption of equatorial circular motion of a hot spot. It is shown that the characteristic two-peak line profile with the sharp edges arises at a large distance, (about r ≈ (3 − 10) rg). The inner regions emit the line, which is observed with one maximum and extremely wide red wing. We present results of simulations for the isothermal and Shakura – Sunayev disks.

Magnetic fields in active galactic nuclei and microquasars

Observations of active galactic nuclei and microquasars by ASCA, RXTE, Chandra and XMM-Newton indicate the existence of wide X-ray emission lines of heavy ionized elements in their spectra. The emission can arise in the inner parts of accretion discs where the effects of general relativity must be counted; moreover, such effects can dominate. We describe a procedure to estimate an upper limit of the magnetic fields in the regions where X-ray photons are emitted. We simulate typical profiles of the iron K α line in the presence of a magnetic field and compare them with observational data. As an illustration, we find H < 10 1 0 -10 1 1 Gs for Seyfert galaxy MCG-6-30-15. Using the perspective facilities of measurement devices (e.g. Constellation-X mission) a better resolution of the blue peak structure of the iron K α line will allow us to find the value of the magnetic fields if the latter are high enough.

Trapped Disk Oscillations and Stable QPOs in MicroQuasars

The observed quasi-periodic oscillations (QPOs) in low-mass X-raybinaries have been suggested to be probably related to the characteristicoscillation modes in the inner part ofa relativistic accretion disk. In this paper we study the trapped feature of these oscillation modes by considering the radial perturbations and viscosity effects. We find that the trappedoscillations are amplified significantly when the viscosity parameterincreases and the trapped size varies when the radial wavenumber and Mach number change. Our results support that the stablehundred-hertz QPOs and their low fractional rms amplitudes found intwo Galactic microquasars may be explained by some trapped non-damping oscillation modes in the inner part of accretiondisks around rotating black holes.