Three-dimensional Radiation Magnetohydrodynamic Simulations Research Articles

Radiation MHD Simulations of Soft X-Ray Emitting Regions in Changing Look AGN

Strong soft X-ray emission called soft X-ray excess is often observed in luminous active galactic nuclei (AGN). It has been suggested that the soft X-rays are emitted from a warm (T = 106 ∼ 107 K) region that is optically thick for the Thomson scattering (warm Comptonization region). Motivated by the recent observations that soft X-ray excess appears in changing look AGN (CLAGN) during the state transition from a dim state without broad emission lines to a bright state with broad emission lines, we performed global three-dimensional radiation magnetohydrodynamic simulations, assuming that the mass accretion rate increases and becomes around 10% of the Eddington accretion rate. The simulation successfully reproduces a warm, Thomson-thick region outside the hot radiatively inefficient accretion flow near the black hole. The warm region is formed by efficient radiative cooling due to inverse Compton scattering. The calculated luminosity 0.01−0.08 L Edd is consistent with the luminosity of CLAGN. We also found that the warm Comptonization region is well described by the steady model of magnetized disks supported by azimuthal magnetic fields. When the antiparallel azimuthal magnetic fields supporting the radiatively cooled region reconnect around the equatorial plane of the disk, the temperature of the region becomes higher by releasing the magnetic energy transported to the region.

Acoustic-gravity wave propagation characteristics in three-dimensional radiation hydrodynamic simulations of the solar atmosphere.

There has been tremendous progress in the degree of realism of three-dimensional radiation magneto-hydrodynamic simulations of the solar atmosphere in the past decades. Four of the most frequently used numerical codes are Bifrost, CO5BOLD, MANCHA3D and MURaM. Here we test and compare the wave propagation characteristics in model runs from these four codes by measuring the dispersion relation of acoustic-gravity waves at various heights. We find considerable differences between the various models. The height dependence of wave power, in particular of high-frequency waves, varies by up to two orders of magnitude between the models, and the phase difference spectra of several models show unexpected features, including ±180° phase jumps. This article is part of the Theo Murphy meeting issue 'High-resolution wave dynamics in the lower solar atmosphere'.

Radiation Magnetohydrodynamic Simulations of Soft X-ray Emitting Regions in Active Galactic Nuclei

AbstractWe present the results of global three-dimensional radiation magnetohydrodynamic simulations of the formation of soft X-ray emitting regions in active galactic nuclei by applying a radiation magnetohydrodynamic code based on the M1-closure scheme. The effect of Compton cooling is taken into account. When the surface density of the accretion flow exceeds the upper limit of the radiatively inefficient accretion flow (RIAF), the optically thin, hot accretion flow near the black hole co-exists with the soft X-ray emitting, warm (T = 106 – 107 K) Comptonized region around r = 20 – 40rs, where rs is the Schwarzschild radius. Numerical results indicate that when the accretion rate approaches the Eddington accretion rate, the warm Comptonized region stays in optically thin for effective optical depth, Thomson thick, and radiation pressure dominant state. This region is found to oscillate between a geometrically thin, cool state and a geometrically thick state inflated by radiation pressure. The time variability of the accretion flow is consistent with that of the narrow-line Seyfert 1 galaxies.

Global Radiation Magnetohydrodynamic Simulations of sub-Eddington Accretion Disks around Supermassive Black Holes

Abstract We conduct global three-dimensional radiation magnetohydrodynamic simulations of the inner regions of accretion flows around a 5 × 108 M ⊙ black hole, with mass accretion rates reaching 7% and 20% of the Eddington value. We choose initial field topologies that result in an inner disk supported by magnetic pressure, with surface density significantly smaller than the values predicted by the standard thin-disk model as well as a much larger disk scale height. The disks do not show any sign of thermal instability over many thermal timescales. More than half of the accretion is driven by radiation viscosity in the optically thin coronal region for the case of the lower accretion rate, while accretion in the optically thick part of the disk is driven by the Maxwell and Reynolds stresses from turbulence caused by magnetorotational instability. Optically thin plasma with gas temperatures ≳108 K is generated only in the inner ≈10 gravitational radii in both simulations, and is more compact in the case of the higher accretion rate. Such plasma does not form at larger radii because the surface density increases outward with radius, causing less dissipation outside the photosphere. In contrast to standard thin-disk models, the surface density in our simulations increases with increasing mass accretion rate at each radius. This causes a relatively weaker hot plasma component for the simulation with a higher accretion rate. We suggest that these results may provide a physical mechanism for understanding some of the observed properties of coronae and spectra of active galactic nuclei.

Simulation of the small-scale magnetism in main-sequence stellar atmospheres

Context.Observations of the Sun tell us that its granular and subgranular small-scale magnetism has significant consequences for global quantities such as the total solar irradiance or convective blueshift of spectral lines.Aims.In this paper, properties of the small-scale magnetism of four cool stellar atmospheres, including the Sun, are investigated, and in particular its effects on the radiative intensity and flux.Methods.We carried out three-dimensional radiation magnetohydrodynamic simulations with the CO5BOLD code in two different settings: with and without a magnetic field. These are thought to represent states of high and low small-scale magnetic activity of a stellar magnetic cycle.Results.We find that the presence of small-scale magnetism increases the bolometric intensity and flux in all investigated models. The surplus in radiative flux of the magnetic over the magnetic field-free atmosphere increases with increasing effective temperature,Teff, from 0.47% for spectral type K8V to 1.05% for the solar model, but decreases for higher effective temperatures than solar. The degree of evacuation of the magnetic flux concentrations monotonically increases withTeffas does their depression of the visible optical surface, that is the Wilson depression. Nevertheless, the strength of the field concentrations on this surface stays remarkably unchanged at ≈1560 G throughout the considered range of spectral types. With respect to the surrounding gas pressure, the field strength is close to (thermal) equipartition for the Sun and spectral type F5V but is clearly sub-equipartition for K2V and more so for K8V. The magnetic flux concentrations appear most conspicuous for model K2V owing to their high brightness contrast.Conclusions.For mean magnetic flux densities of approximately 50 G, we expect the small-scale magnetism of stars in the spectral range from F5V to K8V to produce a positive contribution to their bolometric luminosity. The modulation seems to be most effective for early G-type stars.

The Effects of Magnetic Fields on the Dynamics of Radiation Pressure–dominated Massive Star Envelopes

Abstract We use three-dimensional radiation magnetohydrodynamic simulations to study the effects of magnetic fields on the energy transport and structure of radiation pressure–dominated main sequence massive star envelopes at the region of the iron opacity peak. We focus on the regime where the local thermal timescale is shorter than the dynamical timescale, corresponding to inefficient convective energy transport. We begin with initially weak magnetic fields relative to the thermal pressure, from 100 to 1000 G in differing geometries. The unstable density inversion amplifies the magnetic field, increasing the magnetic energy density to values close to equipartition with the turbulent kinetic energy density. By providing pressure support, the magnetic field’s presence significantly increases the density fluctuations in the turbulent envelope, thereby enhancing the radiative energy transport by allowing photons to diffuse out through low-density regions. Magnetic buoyancy brings small-scale magnetic fields to the photosphere and increases the vertical energy transport, with the energy advection velocity proportional to the Alfvén velocity, although in all cases we study, photon diffusion still dominates the energy transport. The increased radiative and advective energy transport causes the stellar envelope to shrink by several scale heights. We also find larger turbulent velocity fluctuations compared with the purely hydrodynamic case, reaching at the stellar photosphere. The photosphere also shows vertical oscillations with similar averaged velocities and periods of a few hours. The increased turbulent velocity and oscillations will have strong impacts on the line broadening and periodic signals in massive stars.

The dynamics of the solar chromosphere: comparison of model predictions with millimeter-interferometer observations

We analyze the millimeter intensity spectrum expected from the dynamic model of Carlsson & Stein together with the interferometric observations of the quiet Sun obtained at a wavelength of 3.5 mm with the Berkeley-Illinois-Maryland Array. The observational data products (Fourier and wavelet spectra, brightness histograms) are compared with the corresponding products obtained for the Carlsson & Stein (CS) models. We estimate how the limited spatial resolution of the observations influences the comparison with the predictions of chromospheric dynamic models and discuss the limitations of a one-dimensional non-magnetic modeling approach. In addition, we test the effect of the integration time of the BIMA observations on the dynamic signatures. The dependence of the observed brightness variations on spatial resolution is studied by employing artificial image degradation and approximating the obtained dependence by power laws. We are able to establish a correspondence between the CS model predictions and the observational data under assumptions on the horizontal coherence length of the oscillations. The reconstructed brightness rms values indicate that, assuming the coherence length of oscillating elements to be of order of 1, the oscillation power in the observations recorded with 10 resolution agrees within a factor of 2 with the power predicted by the CS model. We argue that millimeter continuum observations promise to be an important diagnostic of chromospheric structure and dynamics. Based on the analysis carried out in this work, the appropriate wavelengths to look for dynamic signatures are in the range 0.8-5.0 mm. Further millimeter interferometric observations with longer sequences and higher spatial resolution are highly desirable along with the development of realistic three-dimensional radiation magnetohydrodynamic simulations.

Dynamic models of the sun from the convection zone to the chromosphere

AbstractThe chromosphere in internetwork regions of the quiet Sun was regarded as a static and homogeneous layer for a long time. Thanks to advances in observations and numerical modelling, the wave nature of these atmospheric regions received increasing attention during the last decade. Recent three-dimensional radiation magnetohydrodynamic simulations with CO5BOLD feature the chromosphere of internetwork regions as a dynamic and intermittent phenomenon. It is a direct product of interacting waves that form a mesh-like pattern of hot shock fronts and cool post-shock regions. The waves are excited self-consistently at the top of the convection zone. In the middle chromosphere above an average height of 1000 km, plasma beta gets larger than one and magnetic fields become more important. The model chromosphere exhibits a magnetic field that is much more homogeneous than in the layers below and evolves much faster. That includes fast propagating (MHD) waves. Further improvements of the simulations like time-dependent hydrogen ionisation are currently in progress. This class of models is capable of explaining apparently contradicting diagnostics such as carbon monoxide and UV emission at the same time.