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Christian Fendt

Publications and source records attributed to Christian Fendt.

At least 37 records · Page 2Linked to original sources
arXiv2021

The physics of the MHD disk-jet transition in binary systems: jetted spiral walls launched from disk spiral arms

We present a detailed physical analysis of the jet launching mechanism of a circum-stellar disk that is located in a binary system. Applying 3D resistive MHD simulations, we investigate the local and global properties of the system, such as the angular momentum transport and the accretion and ejection mass fluxes. In comparison to previous works, for the first time, we have considered the full magnetic torque, the presence of an outflow, thus the angular momentum transport by vertical motion, and the binary torque. We discuss its specific 3D structure, and how it is affected by tidal effects. We find that the spiral structure evolving in the disk is {\em launched into the outflow}. We propose to call this newly discovered structure a {\em jet spiral wall}. These spiral features follow the same time evolution, with the jet spiral somewhat lagging the disk spiral. We find that the vertical transport of angular momentum has a significant role in the total angular momentum budget also in a binary system. The same holds for the magnetic torque, however, the contribution from the $ϕ$-derivative of magnetic pressure and the $B_ϕB_r$ stresses are small. The gravity torque arising from the time-dependent 3D Roche potential becomes essential, as it constitutes the fundamental cause for all 3D effects appearing in our disk-jet system. Quantitatively, we find that the disk accretion rate in a binary system increases by $20\%$ compared to a disk around a single star. The disk wind mass flux increases by even 50\%.

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arXiv2021

General relativistic radiation transport: Implications for VLBI/EHT observations of AGN discs, winds and jets

In 2019, the Event Horizon Telescope Collaboration (EHTC) has published the first image of a supermassive black hole (SMBH) obtained via the Very Large Baseline Interferometry (VLBI) technique. In the future, it is expected that additional and more sensitive VLBI observations will be pursued for other nearby Active Galactic Nuclei (AGN), and it is therefore important to understand which possible features can be expected in such images. In this paper, we post-process General Relativistic Magneto-Hydrodynamical (GR-MHD) simulations which include resistivity, thus providing a self-consistent jet formation model, including resistive mass loading of a wind launched from a disc in Keplerian rotation. The ray-tracing is done using the General Relativistic Ray-Tracing code GRTRANS assuming synchrotron emission. We study the appearance of the black hole environment including the accretion disc, winds and jets under a large range of condition, varying black hole mass, accretion rate, spin, inclination angle, disc parameters and observed frequency. When we adopt M87-like parameters, we show that we can reproduce a ring-like feature (similar as observed by the EHT) for some of our simulations. The latter suggests that such Keplerian disc models thus could be consistent with the observed results. Depending on their masses, accretion rates, spin and the sensitivity of the observation, we note that other SMBHs may show additional features like winds and jets in the observations.

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arXiv2021

Jets, disc-winds and oscillations in general relativistic, magnetically driven flows around black hole

Relativistic jets and disc-winds are typically observed in BH-XRBs and AGNs. However, many physical details of jet launching and the driving of disc winds from the underlying accretion disc are still not fully understood. In this study, we further investigate the role of the magnetic field strength and structure in launching jets and disc winds. In particular, we explore the connection between jet, wind, and the accretion disc around the central black hole. We perform axisymmetric GRMHD simulations of the accretion-ejection system using adaptive mesh refinement. Essentially, our simulations are initiated with a thin accretion disc in equilibrium. An extensive parametric study by choosing different combinations of magnetic field strength and initial magnetic field inclination is also performed. Our study finds relativistic jets driven by the Blandford \& Znajek (BZ) mechanism and the disc-wind driven by the Blandford \& Payne (BP) mechanism. We also find that plasmoids are formed due to the reconnection events, and these plasmoids advect with disc-winds. As a result, the tension force due to the poloidal magnetic field is enhanced in the inner part of the accretion disc, resulting in disc truncation and oscillation. These oscillations result in flaring activities in the jet mass flow rates. We find simulation runs with a lower value of the plasma-$β$, and lower inclination angle parameters are more prone to the formation of plasmoids and subsequent inner disc oscillations. Our models provide a possible template to understand spectral state transition phenomena in BH-XRBs.

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arXiv2021

A ring accelerator? Unusual jet dynamics in the IceCube candidate PKS 1502+106

On 2019/07/30.86853 UT, IceCube detected a high-energy astrophysical neutrino candidate. The Flat Spectrum Radio Quasar PKS 1502+106 is located within the 50 percent uncertainty region of the event. Our analysis of 15 GHz Very Long Baseline Array (VLBA) and astrometric 8 GHz VLBA data, in a time span prior and after the IceCube event, reveals evidence for a radio ring structure which develops with time. Several arc-structures evolve perpendicular to the jet ridge line. We find evidence for precession of a curved jet based on kinematic modelling and a periodicity analysis. An outflowing broad line region (BLR) based on the C IV line emission (Sloan Digital Sky Survey, SDSS) is found. We attribute the atypical ring to an interaction of the precessing jet with the outflowing material. We discuss our findings in the context of a spine-sheath scenario where the ring reveals the sheath and its interaction with the surroundings (narrow line region, NLR, clouds). We find that the radio emission is correlated with the $γ$-ray emission, with radio lagging the $γ$-rays. Based on the $γ$-ray variability timescale, we constrain the $γ$-ray emission zone to the BLR (30-200 $r_{\rm g}$) and within the jet launching region. We discuss that the outflowing BLR provides the external radiation field for $γ$-ray production via external Compton scattering. The neutrino is most likely produced by proton-proton interaction in the blazar zone (beyond the BLR), enabled by episodic encounters of the jet with dense clouds, i.e. some molecular cloud in the NLR.

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arXiv2021

Relativistic outflows from a GRMHD mean-field disk dynamo

We present simulations of thin accretion disks around black holes investigating a mean-field disk dynamo in our resistive GRMHD code (Vourellis et al. 2019) that is able to produce a large scale magnetic flux. We consider a weak seed field in an initially thin disk, a background (turbulent) magnetic diffusivity and the dynamo action. A standard quenching mechanism is applied to mitigate the otherwise exponential increase of the magnetic field. Comparison simulations of an initial Fishbone-Moncrief torus suggest that reconnection may provide another quenching mechanism. The dynamo-generated magnetic flux expands from the disk interior into the disk corona, becomes advected by disk accretion, and fills the axial region of the domain. The dynamo leads to an initially rapid increase in magnetic energy and flux, while for later evolutionary stages the growth stabilizes. Accretion towards the black hole depends strongly on the magnetic field structure that develops. The radial field component supports extraction of angular momentum and thus accretion. It also sets the conditions for launching a disk wind, initially from inner disk area. When a strong field has engulfed the disk, strong winds are launched that are predominantly driven by the pressure gradient of the toroidal field. For rotating black holes we identify a Poynting flux-dominated jet, driven by the Blandford-Znajek mechanism. This axial Poynting flux is advected from the disk and therefore accumulates at the expense of the flux carried by the disk wind, that is itself regenerated by the disk dynamo.

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arXiv2020

MHD accretion-ejection: jets launched by a non-isotropic accretion disk dynamo. II. A dynamo tensor defined by the disk Coriolis number

Astrophysical jets are launched from strongly magnetized systems that host an accretion disk surrounding a central object. Here we address the question how to generate the accretion disk magnetization and field structure required for jet launching. We continue our work from Paper I (Mattia & Fendt 2020a), considering a non-scalar accretion disk mean-field $α^2Ω$-dynamo in the context of large scale disk-jet simulations. We now investigate a disk dynamo that follows analytical solutions of mean-field dynamo theory, essentially based only on a single parameter, the Coriolis number. We thereby confirm the anisotropy of the dynamo tensor acting in accretion disks, allowing to relate both the resistivity and mean-field dynamo to the disk turbulence. Our new model recovers previous simulations applying a purely radial initial field, while allowing for a more stable evolution for seed fields with a vertical component. We also present correlations between the strength of the disk dynamo coefficients and the dynamical parameters of the jet that is launched, and discuss their implication for observed jet quantities.

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arXiv2020

MHD accretion-ejection: jets launched by a non-isotropic accretion disk dynamo. I. Validation and application of selected dynamo tensorial components

Astrophysical jets are launched from strongly magnetized systems that host an accretion disk surrounding a central object. The origin of the jet launching magnetic field is one of the open questions for modeling the accretion-ejection process. Here we address the question how to generate the accretion disk magnetization and field structure required for jet launching. Applying the PLUTO code, we present the first resistive MHD simulations of jet launching including a non-scalar accretion disk mean-field $α^2Ω$-dynamo in the context of large scale disk-jet simulations. Essentially, we find the $α_ϕ$-dynamo component determining the amplification of the poloidal magnetic field, which is strictly related to the disk magnetization (and, as a consequence, to the jet speed, mass and collimation), while the $α_R$ and $α_θ$-dynamo components trigger the formation of multiple, anti-aligned magnetic loops in the disk, with strong consequences on the stability and dynamics of the disk-jet system. In particular, such loops trigger the formation of dynamo inefficient zones, which are characterized by a weak magnetic field, and therefore a lower value of the magnetic diffusivity. The jet mass, speed and collimation are strongly affected by the formation of the dynamo inefficient zones. Moreover, the $θ$-component of the $α$-dynamo plays a key role when interacting with a non-radial component of the seed magnetic field. We also present correlations between the strength of the disk toy dynamo coefficients and the dynamical parameters of the jet that is launched.

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arXiv2020

Birth of convective low-mass to high-mass second Larson cores

Stars form as an end product of the gravitational collapse of cold, dense gas in magnetized molecular clouds. This multi-scale scenario occurs via the formation of two quasi-hydrostatic cores and involves complex physical processes, which require a robust, self-consistent numerical treatment. The aim of this study is to understand the formation and evolution of the second Larson core and the dependence of its properties on the initial cloud core mass. We used the PLUTO code to perform high resolution, 1D and 2D RHD collapse simulations. We include self-gravity and use a grey FLD approximation for the radiative transfer. Additionally, we use for the gas EOS density- and temperature-dependent thermodynamic quantities to account for the effects such as dissociation, ionisation, and molecular vibrations and rotations. Properties of the second core are investigated using 1D studies spanning a wide range of initial cloud core masses from 0.5 to 100 $M_{\odot}$. Furthermore, we expand to 2D collapse simulations for a few cases of 1, 5, 10, and 20 $M_{\odot}$. We follow the evolution of the second core for $\geq$ 100 years after its formation, for each of these non-rotating cases. Our results indicate a dependence of several second core properties on the initial cloud core mass. For the first time, due to an unprecedented resolution, our 2D non-rotating collapse studies indicate that convection is generated in the outer layers of the second core, which is formed due to the gravitational collapse of a 1 $M_{\odot}$ cloud core. Additionally, we find large-scale oscillations of the second accretion shock front triggered by the standing accretion shock instability, which has not been seen before in early evolutionary stages of stars. We predict that the physics within the second core would not be significantly influenced by the effects of magnetic fields or an initial cloud rotation.

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arXiv2020

Global axisymmetric simulations of photoevaporation and magnetically driven protoplanetary disk winds

Photoevaporation and magnetically driven winds are two independent mechanisms to remove mass from protoplanetary disks. In addition to accretion, the effect of these two principles acting concurrently could be significant and the transition between those two has not been extensively studied and quantified in the literature yet. In order to contribute to the understanding of disk winds, we present the phenomena emerging in the framework of two-dimensional axisymmetric, non-ideal magnetohydrodynamic simulations including EUV-/ X-ray driven photoevaporation. Of particular interest are the examination of the transition region between photoevaporation and magnetically driven wind, the possibility of emerging magneto-centrifugal wind effects, as well as the morphology of the wind itself depending on the strength of the magnetic field. We use the PLUTO code in a 2.5D axisymmetric configuration with additional treatment of EUV-/ X-ray heating and dynamic ohmic diffusion based on a semi-analytical chemical model. We identify the transition between both outflow types to occur for values of the initial plasma beta $β\geq 10^7$, while magnetically driven winds generally outperform photoevaporation for stronger fields. In our simulations we observe irregular and asymmetric outflows for stronger magnetic fields. In the weak field regime the photoevaporation rates are slightly lowered by perturbations of the gas density in the inner regions of the disk. Overall, our results predict a wind with a lever arm smaller than 1.5, consistent with a hot magneto-thermal wind. Stronger accretion flows are present for values of $β< 10^7$.

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arXiv2019

Approaching the black hole by numerical simulations

Black holes represent extreme conditions of physical laws. Being predicted about a century ago, they are now accepted as astrophysical reality by most of the scientific community. Only recently more direct evidence of their existence has been found - the detection of gravitational waves from black hole mergers and of the shadow of a supermassive black hole in the center of a galaxy. Astrophysical black holes are typically embedded in an active environment which is affected by the strong gravity. When the environmental material emits radiation, this radiation may carry imprints of the black hole that is hosting the radiation source. In order to understand the physical processes that take place in the close neighbourhood of astrophysical black holes, numerical methods and simulations play an essential role. This is simply because the dynamical evolution and the radiative interaction are far too complex in order to allow for an analytic solution of the physical equations. A huge progress has been made over the last decade(s) in the numerical code development, as well as in the computer power that is needed to run these codes. This review tries to summarize the basic questions and methods that are involved in the undertaking of investigating the astrophysics of black holes by numerical means. It is intended for a non-expert audience interested in an overview over this broad field. The review comes along without equations and thus without a detailed expert discussion of the underlying physical processes or numerical specifics. Instead, it intends to illustrate the richness of the field and to motivate for further reading. The review puts some emphasis on magnetohydrodynamic simulations, but also touches radiation transfer and merger simulations, in particular pointing out differences in these approaches.

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arXiv2019

GR-MHD disk winds and jets from black holes and resistive accretion disks

We perform GR-MHD simulations of outflow launching from thin accretion disks. As in the non-relativistic case, resistivity is essential for the mass loading of the disk wind. We implemented resistivity in the ideal GR-MHD code HARM3D, extending previous works (Qian et al. 2017, 2018) for larger physical grids, higher spatial resolution, and longer simulation time. We consider an initially thin, resistive disk orbiting the black hole, threaded by a large-scale magnetic flux. As the system evolves, outflows are launched from the black hole magnetosphere and the disk surface. We mainly focus on disk outflows, investigating their MHD structure and energy output in comparison with the Poynting-dominated black hole jet. The disk wind encloses two components -- a fast component dominated by the toroidal magnetic field and a slower component dominated by the poloidal field. The disk wind transitions from sub to super-Alfvénic speed, reaching velocities $\simeq 0.1c$. We provide parameter studies varying spin parameter and resistivity level, and measure the respective mass and energy fluxes. A higher spin strengthens the $B_ϕ$-dominated disk wind along the inner jet. We disentangle a critical resistivity level that leads to a maximum matter and energy output for both, resulting from the interplay between re-connection and diffusion, which in combination govern the magnetic flux and the mass loading. For counter-rotating black holes the outflow structure shows a magnetic field reversal. We estimate the opacity of the inner-most accretion stream and the outflow structure around it. This stream may be critically opaque for a lensed signal, while the axial jet funnel remains optically thin.

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arXiv2018

First Core Properties: From Low- to High-mass Star Formation

In this study, the main goal is to understand the molecular cloud core collapse through the stages of first and second hydrostatic core formation. We investigate the properties of Larson's first and second cores following the evolution of the molecular cloud core until formation of Larson's cores. We expand these collapse studies for the first time to span a wide range of initial cloud masses from 0.5 to 100 Msun. Understanding the complexity of the numerous physical processes involved in the very early stages of star formation requires detailed thermodynamical modeling in terms of radiation transport and phase transitions. For this we use a realistic gas equation of state via a density and temperature-dependent adiabatic index and mean molecular weight to model the phase transitions. We use a gray treatment of radiative transfer coupled with hydrodynamics to simulate Larson's collapse in spherical symmetry. We reveal a dependence of a variety of first core properties on the initial cloud mass. The first core radius and mass increase from the low-mass to the intermediate-mass regime and decrease from the intermediate-mass to the high-mass regime. The lifetime of first cores strongly decreases towards the intermediate- and high-mass regime. Our studies show the presence of a transition region in the intermediate-mass regime. Low-mass protostars tend to evolve through two distinct stages of formation which are related to the first and second hydrostatic cores. In contrast, in the high-mass star formation regime, the collapsing cloud cores rapidly evolve through the first collapse phase and essentially immediately form Larson's second cores.

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arXiv2018

Long-term simulation of MHD jet launching from an orbiting star-disk system

We present fully three-dimensional magnetohydrodynamic jet launching simulations of a jet source orbiting in a binary system. We consider a time-dependent binary gravitational potential, thus all tidal forces that are experienced in the non-inertial frame of the jet-launching primary. We investigate systems with different binary separation, different mass ratio, and different inclination between the disk plane and the orbital plane. The simulations run over a substantial fraction of the binary orbital period. All simulations show similar local and global non-axisymmetric effects such as local instabilities in the disk and the jet, or global features such as disk spiral arms and warps, or a global re-alignment of the inflow-outflow structure. The disk accretion rate is higher than for axisymmetric simulations, most probably due to the enhanced angular momentum transport by spiral waves. The disk outflow leaves the Roche lobe of the primary and becomes disturbed by tidal effects. While a disk-orbit inclination of $10\degr$ still allows for a persistent outflow, an inclination of $30\degr$ does not, suggesting a critical angle in between. For moderate inclination we find indication for jet precession such that the jet axis starts to follow a circular pattern with an opening cone of $\simeq 8\degr$. Simulations with different mass ratio indicate a change of time scales for the tidal forces to affect the disk-jet system. A large mass ratio (a massive secondary) leads to stronger spiral arms, a higher (average) accretion and a more pronounced jet-counter jet asymmetry.

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arXiv2018

Jet launching in resistive GR-MHD black hole - accretion disk systems

We investigate the launching mechanism of relativistic jets from black hole sources, in particular the strong winds from the surrounding accretion disk. Numerical investigations of the disk wind launching - the simulation of the accretion-ejection transition - have so far almost only been done for non-relativistic systems. From these simulations we know that resistivity, or magnetic diffusivity, plays an important role for the launching process. Here, we extend this treatment to general relativistic magnetohydrodynamics (GR-MHD) applying the resistive GR-MHD code rHARM. Our model setup considers a thin accretion disk threaded by a large-scale open magnetic field. We run a series of simulations with different Kerr parameter, field strength and diffusivity level. Indeed we find strong disk winds with, however, mildly relativistic speed, the latter most probably due to our limited computational domain. Further, we find that magnetic diffusivity lowers the efficiency of accretion and ejection, as it weakens the efficiency of the magnetic lever arm of the disk wind. As major driving force of the disk wind we disentangle the toroidal magnetic field pressure gradient, however,magneto-centrifugal driving may also contribute. Black hole rotation in our simulations suppresses the accretion rate due to an enhanced toroidal magnetic field pressure that seems to be induced by frame-dragging. Comparing the energy fluxes from the Blandford-Znajek-driven central spine and the surrounding disk wind, we find that the total electromagnetic energy flux is dominated by the total matter energy flux of the disk wind (by a factor 20). The kinetic energy flux of the matter outflow is comparatively small and comparable to the Blandford-Znajek electromagnetic energy flux.

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arXiv2016

rHARM: Accretion and Ejection in Resistive GR-MHD

Turbulent magnetic diffusivity plays an important role for accretion disks and the launching of disk winds. We have implemented magnetic diffusivity, respective resistivity in the general relativistic MHD code HARM. This paper describes the theoretical background of our implementation, its numerical realization, our numerical tests and preliminary applications. The test simulations of the new code rHARM are compared with an analytic solution of the diffusion equation and a classical shock tube problem. We have further investigated the evolution of the magneto-rotational instability (MRI) in tori around black holes for a range of magnetic diffusivities. We find indication for a critical magnetic diffusivity (for our setup) beyond which no MRI develops in the linear regime and for which accretion of torus material to the black hole is delayed. Preliminary simulations of magnetically diffusive thin accretion disks around Schwarzschild black holes that are threaded by a large-scale poloidal magnetic field show the launching of disk winds with mass fluxes of about 50% of the accretion rate. The disk magnetic diffusivity allows for efficient disk accretion that replenishes the mass reservoir of the inner disk area and thus allows for long-term simulations of wind launching for more than 5000 time units.

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arXiv2016

An extensive numerical survey of the correlation between outflow dynamics and accretion disk magnetization

We investigate the accretion-ejection process of jets from magnetized accretion disks. We apply a novel approach to the jet-launching problem in order to obtain correlations between the physical properties of the jet and the underlying disk. We extend and confirm the previous works of \citet{2009MNRAS.400..820T} and \citet{2010A&A...512A..82M} by scanning a large parameter range for the disk magnetization, $μ_{\rm D} = 10^{-3.5} ... 10^{-0.7}$. We disentangle the disk magnetization at the foot point of the outflow as the main parameter that governs the properties of the outflow. We show how the four jet integrals known from steady-state MHD are correlated to the disk magnetization at the jet foot point. This agrees with the usual findings of the steady-state theory, however, here we obtain these correlations from time-dependent simulations that include the dynamical evolution of the disk in the treatment. In particular, we obtain robust correlations between the local disk magnetization and (i)the outflow velocity, (ii) the jet mass loading, (iii) jet angular momentum, and (iv) the local mass accretion rate. Essentially we find that strongly magnetized disks launch more energetic and faster jets, and, due to a larger Alfvén lever arm, these jets extract more angular momentum from the underlying disk. These kind of disk-jet systems have, however, a smaller mass loading parameter and a lower mass ejection-to-accretion ratio. The jets are launched at the disk surface where the magnetization is $μ(r,z) \simeq 0.1$. The magnetization rapidly increases vertically providing the energy reservoir for subsequent jet acceleration. We find indication for a critical disk magnetization $μ_{\rm D} \simeq 0.01$ that separates the regimes of magnetocentrifugally-driven and magnetic pressure-driven jets.

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arXiv2015

Wobbling and precessing jets from warped disks in binary systems

We present results of the first ever three-dimensional (3D) magnetohydrodynamic (MHD) simulations of the accretion-ejection structure. We investigate the 3D evolution of jets launched symmetrically from single stars but also jets from warped disks in binary systems. We have applied various model setups and tested them by simulating a stable and bipolar symmetric 3D structure from a single star-disk-jet system. Our reference simulation maintains a good axial symmetry and also a bipolar symmetry for more than 600 rotations of the inner disk confirming the quality of our model setup. We have then implemented a 3D gravitational potential (Roche potential) due to a companion star and run a variety of simulations with different binary separations and mass ratios. These simulations show typical 3D deviations from axial symmetry, such as jet inclination outside the Roche lobe or spiral arms forming in the accretion disk. In order to find indication for precession effects, we have also run an exemplary parameter setup, essentially governed by a small binary separation of only $\simeq{200}$ inner disk radii. This simulation shows strong indication that we observe the onset of a jet precession caused by the wobbling of the jet launching disk. We estimate the opening angle of the precession cone defined by the lateral motion of the the jet axis of about 4 degree after about 5000 dynamical time steps.

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