Magnetohydrodynamic Outflows Research Articles

Three-dimensional GRMHD simulations of rapidly rotating stellar core collapse

ABSTRACT We present results from fully general relativistic (GR), three-dimensional (3D), neutrino-radiation magneto-hydrodynamic (MHD) simulations of stellar core collapse of a 20 M⊙ star with spectral neutrino transport. Our focus is to study the gravitational-wave (GW) signatures from the magnetorotationally (MR)-driven models. By parametrically changing the initial angular velocity and the strength of the magnetic fields in the core, we compute four models. Among our models, only those with cores having an initial magnetic field strength of 1012 G and rotation rates of 1 or 2 rad s−1 produce MHD jets. Seen from the direction perpendicular to the rotational axis, a characteristic waveform is obtained exhibiting a monotonic time increase in the wave amplitude. As previously identified, this stems from the propagating MHD outflows along the axis. We show that the GW amplitude from anisotropic neutrino emission becomes more than one order-of-magnitude bigger than that from the matter contribution, whereas seen from the rotational axis, both of the two components are in the same order-of-magnitudes. Due to the memory effect, the frequency of the neutrino GW from our full-fledged 3D-MHD models is in the range less than ∼10 Hz. Toward the future GW detection for a Galactic core-collapse supernova, if driven by the MR mechanism, the planned next-generation detector as DECIGO is urgently needed to catch the low-frequency signals.

Absorption lines from magnetically driven winds in X-ray binaries – II. High resolution observational signatures expected from future X-ray observatories

ABSTRACT In our self-similar, analytical, magnetohydrodynamic (MHD) accretion–ejection solution, the density at the base of the outflow is explicitly dependent on the disc accretion rate – a unique property of this class of solutions. We had earlier found that the ejection index $p \gt \sim 0.1 (\dot{M}_{\rm{acc}} \propto r^p)$ is a key MHD parameter that decides if the flow can cause absorption lines in the high resolution X-ray spectra of black hole binaries. Here, we choose three dense warm solutions with p = 0.1, 0.3, 0.45 and carefully develop a methodology to generate spectra which are convolved with the Athena and XRISM response functions to predict what they will observe seeing through such MHD outflows. In this paper two other external parameters were varied – extent of the disc, $\rm {r_o|_{\rm{max}}} = 10^5, \, 10^6 \, \, \rm {r_G}$, and the angle of the line of sight, i ∼ 10–25°. Resultant absorption lines (H and He-like Fe, Ca, Ar) change in strength and their profiles manifest varying degrees of asymmetry. We checked if (a) the lines and (ii) the line asymmetries are detected, in our suit of synthetic Athena and XRISM spectra. Our analysis shows that Athena should detect the lines and their asymmetries for a standard 100 ks observation of a 100 mCrab source – lines with equivalent width as low as a few eV should be detected if the 6–8 keV counts are larger than 104–105 even for the least favourable simulated cases.

Astrophysical magnetohydrodynamical outflows in the extragalactic binary system LMC X-1

In this work, at first we present a model of studying astrophysical flows of binary systems and microquasars based on the laws of relativistic magnetohydrodynamics. Then, by solving the time independent transfer equation, we estimate the primary and secondary particle distributions within the hadronic astrophysical jets as well as the emissivities of high energy neutrinos and γ-rays. One of our main goals is, by taking into consideration the various energy-losses of particles into the hadronic jets, to determine through the transport equation the respective particle distributions focusing on relativistic hadronic jets of binary systems. As a concrete example we examine the extragalactic binary system LMC X-1 located in the Large Magellanic Cloud, a satellite galaxy of our Milky Way Galaxy.

Constraining X-Ray Coronal Size with Transverse Motion of AGN Ultra-fast Outflows

Abstract One of the canonical physical properties of ultra-fast outflows (UFOs) seen in a diverse population of active galactic nuclei is its seemingly very broad width (i.e., Δv ≳ 10,000 km s−1), a feature often required for X-ray spectral modeling. While unclear to date, this condition is occasionally interpreted and justified as internal turbulence within the UFOs for simplicity. In this work, we exploit a transverse motion of a three-dimensional accretion disk-wind, an essential feature of nonradial outflow morphology unique to magnetohydrodynamic outflows. We argue that at least part of the observed line width of UFOs may reflect the degree of transverse velocity gradient due to Doppler broadening around a putative compact X-ray corona in the proximity of a black hole. In this scenario, line broadening is sensitive to the geometrical size of the corona, R c . We calculate the broadening factor as a function of coronal radius R c and velocity smearing factor f sm at a given plasma position. We demonstrate, as a case study of the quasar, PDS 456, that the spectral analysis favors a compact coronal size of R c /R g ≲ 10 where R g is gravitational radius. Such a compact corona is long speculated from both an X-ray reverberation study and the lamppost model for disk emission also consistent with microlensing results. Combination of such a transverse broadening around a small corona can be a direct probe of a substantial rotational motion perhaps posing a serious challenge to radiation-driven wind viewpoint.

Magnetic field amplification via protostellar disc dynamos

We numerically investigate the generation of a magnetic field in a protostellar disc via an $\alpha \Omega$-dynamo and the resulting magnetohydrodynamic (MHD) driven outflows. We find that for small values of the dimensionless dynamo parameter $\alpha_d$ the poloidal field grows exponentially at a rate $\sigma \propto \Omega_K \sqrt{\alpha_d}$, before saturating to a value $\propto \sqrt{\alpha_d}$. The dynamo excites dipole and octupole modes, but quadrupole modes are suppressed, because of the symmetries of the seed field. Initial seed fields too weak to launch MHD outflows are found to grow sufficiently to launch winds with observationally relevant mass fluxes of order $10^{-9} M_{\odot}/\rm{yr}$ for T Tauri stars. This suggests $\alpha \Omega$-dynamos may be responsible for generating magnetic fields strong enough to launch observed outflows.

HOT ELECTROMAGNETIC OUTFLOWS. I. ACCELERATION AND SPECTRA

The theory of cold, relativistic, magnetohydrodynamic outflows is generalized by the inclusion of an intense radiation source. In some contexts, such the breakout of a gamma-ray burst jet from a star, the outflow is heated to a high temperature at a large optical depth. Eventually it becomes transparent and is pushed to a higher Lorentz factor by a combination of the Lorentz force and radiation pressure. We obtain its profile, both inside and outside the fast magnetosonic critical point, when the poloidal magnetic field is radial and monopolar. Most of the energy flux is carried by the radiation field and the toroidal magnetic field that is wound up close to the rapidly rotating engine. Although the entrained matter carries little energy, it couples the radiation field to the magnetic field. Then the fast critical point is pushed inward from infinity and, above a critical radiation intensity, the outflow is accelerated mainly by radiation pressure. We identify a distinct observational signature of this hybrid outflow: a hardening of the radiation spectrum above the peak of the seed photon distribution, driven by bulk Compton scattering. The non-thermal spectrum -- obtained by a Monte Carlo method -- is most extended when the Lorentz force dominates the acceleration, and the seed photon beam is wider than the Lorentz cone of the MHD fluid. This effect is a generic feature of hot, magnetized outflows interacting with slower relativistic material. It may explain why some GRB spectra appear to peak at photon energies above the original Amati et al. scaling. A companion paper addresses the case of jet breakout, where diverging magnetic flux surfaces yield strong MHD acceleration over a wider range of Lorentz factor.

GRAVITATIONAL WAVE SIGNATURES OF MAGNETOHYDRODYNAMICALLY DRIVEN CORE-COLLAPSE SUPERNOVA EXPLOSIONS

By performing a series of two-dimensional, special relativistic magnetohydrodynamic (MHD) simulations, we study signatures of gravitational waves (GWs) in the magnetohydrodynamically-driven core-collapse supernovae. In order to extract the gravitational waveforms, we present a stress formula including contributions both from magnetic fields and special relativistic corrections. By changing the precollapse magnetic fields and initial angular momentum distributions parametrically, we compute twelve models. As for the microphysics, a realistic equation of state is employed and the neutrino cooling is taken into account via a multiflavor neutrino leakage scheme. With these computations, we find that the total GW amplitudes show a monotonic increase after bounce for models with a strong precollapse magnetic field ($10^{12}$G) also with a rapid rotation imposed. We show that this trend stems both from the kinetic contribution of MHD outflows with large radial velocities and also from the magnetic contribution dominated by the toroidal magnetic fields that predominantly trigger MHD explosions. For models with weaker initial magnetic fields, the total GW amplitudes after bounce stay almost zero, because the contribution from the magnetic fields cancels with the one from the hydrodynamic counterpart. These features can be clearly understood with a careful analysis on the explosion dynamics. We point out that the GW signals with the increasing trend, possibly visible to the next-generation detectors for a Galactic supernova, would be associated with MHD explosions with the explosion energies exceeding $10^{51}$ erg.

EMERGENCE OF PROTOPLANETARY DISKS AND SUCCESSIVE FORMATION OF GASEOUS PLANETS BY GRAVITATIONAL INSTABILITY

We use resistive magnetohydrodynamical simulations with the nested grid technique to study the formation of protoplanetary disks around protostars from molecular cloud cores that provide the realistic environments for planet formation. We find that gaseous planetary-mass objects are formed in the early evolutionary phase by gravitational instability in regions that are decoupled from the magnetic field and surrounded by the injection points of the magnetohydrodynamical outflows during the formation phase of protoplanetary disks. Magnetic decoupling enables massive disks to form and these are subject to gravitational instability, even at ~ 10 AU. The frequent formation of planetary-mass objects in the disk suggests the possibility of constructing a hybrid planet formation scenario, where the rocky planets form later under the influence of the giant planets in the protoplanetary disk.

LONG-TERM EVOLUTION OF SLOWLY ROTATING COLLAPSAR IN SPECIAL RELATIVISTIC MAGNETOHYDRODYNAMICS

We present our numerical results of two-dimensional magnetohydrodynamic (MHD) simulations of the collapse of rotating massive stars in light of the collapsar model of gamma-ray bursts (GRBs). Pushed by recent evolution calculations of GRB progenitors, we focus on lower angular momentum of the central core than those taken mostly in previous studies. By performing special relativistic simulations including both realistic equation of state and neutrino cooling, we follow a long-term evolution of the slowly rotating collapsars up to ~10 s, accompanied by the formation of jets and accretion disks. Our results show that for the GRB progenitors to function as collapsars, there is a critical initial angular momentum, below which matter is quickly swallowed to the central objects, no accretion disks and no MHD outflows are formed. When larger than the criteria, we find the launch of the MHD jets in the following two ways. For models with stronger initial fields, the pressure amplified inside the accretion disk can drive the MHD outflows, which makes the strong explosions like a magnetic tower. For models with weaker initial fields, the tower stalls first and the subsequent MHD outflows are produced by the twisting of the turbulent inflows of the accreting material from the equatorial to the polar regions. Regardless of the difference in the formation, the jets can attain only mildly relativistic speeds with the explosion energy less than 1049 erg. To obtain stronger neutrino energy depositions in the polar funnel regions heated from the accretion disk, we find that smaller initial angular momentum is favorable. This is because the gravitational compression makes the temperature of the disk higher. Due to high neutrino opacity inside the disk, we find that the luminosities of ν e and become almost comparable, which is advantageous for making the energy deposition rate larger. We discuss how the energy deposition can be as efficient as the magnetically driven processes for energetizing jets. Among the computed models, we suggest that the model with the initial angular momentum of j ~ 1.5j lso (j lso: the angular momentum of the last stable orbit) and with initial field strength of ~1010 G, provides a most plausible condition for making fireballs for GRBs, because such model is appropriate not only for producing the MHD outflows quickly by the towers, but also for obtaining the stronger neutrino heating in the evacuated polar funnel.

Magnetic fields in Planetary Nebulae: paradigms and related MHD frontiers

AbstractMany, if not all, post AGB stellar systems swiftly transition from a spherical to a powerful aspherical pre-planetary nebula (pPNE) outflow phase before waning into a PNe. The pPNe outflows require engine rotational energy and a mechanism to extract this energy into collimated outflows. Just radiation and rotation are insufficient but a symbiosis between rotation, differential rotation and large scale magnetic fields remains promising. Present observational evidence for magnetic fields in evolved stars is suggestive of dynamically important magnetic fields, but both theory and observation are rife with research opportunity. I discuss how magnetohydrodynamic outflows might arise in pPNe and PNe and distinguish different between approaches that address shaping vs. those that address both launch and shaping. Scenarios involving dynamos in single stars, binary driven dynamos, or accretion engines cannot be ruled out. One appealing paradigm involves accretion onto the primary post-AGB white dwarf core from a low mass companion whose decaying accretion supply rate owers first the pPNe and then the lower luminosity PNe. Determining observational signatures of different MHD engines is a work in progress. Accretion disk theory and large scale dynamos pose many of their own fundamental challenges, some of which I discuss in a broader context.

Structure and nuclear composition of general relativistic, magnetohydrodynamic outflows from neutrino-cooled disks

We compute the structure and degree of neutronization of general relativistic magnetohydrodynamic (GRMHD) outflows originating from the inner region of neutrino-cooled disks. We consider both, outflows expelled from a hydrostatic disk corona and outflows driven by disk turbulence. We show that in outflows driven thermally from a static disk the electron fraction quickly evolves to its equilibrium value which is dominated by neutrino capture. Those outflows are generally proton rich and, under certain conditions, can be magnetically dominated. They may also provide sites for effective production of 56Ni. Centrifugally driven outflows and outflows driven by disk turbulence, on the other hand, can preserve the large in-disk neutron excess. Those outflows are, quite generally, subrelativistic by virtue of the large mass flux driven by the additional forces.

Two-flow magnetohydrodynamical jets around young stellar objects

We present the first-ever simulations of non-ideal magnetohydrodynamical (MHD) stellar winds coupled with disc-driven jets where the resistive and viscous accretion disc is self-consistently described. The transmagnetosonic, collimated MHD outflows are investigated numerically using the VAC code. Our simulations show that the inner outflow is accelerated from the central object hot corona thanks to both the thermal pressure and the Lorentz force. In our framework, the thermal acceleration is sustained by the heating produced by the dissipated magnetic energy due to the turbulence. Conversely, the outflow launched from the resistive accretion disc is mainly accelerated by the magneto-centrifugal force. We also show that when a dense inner stellar wind occurs, the resulting disc-driven jet have a different structure, namely a magnetic structure where poloidal magnetic field lines are more inclined because of the pressure caused by the stellar wind. This modification leads to both an enhanced mass ejection rate in the disc-driven jet and a larger radial extension which is in better agreement with the observations besides being more consistent.

Magnetorotational collapse of massive stellar cores to neutron stars: Simulations in full general relativity

We study magnetohydrodynamic (MHD) effects arising in the collapse of magnetized, rotating, massive stellar cores to proto-neutron stars (PNSs). We perform axisymmetric numerical simulations in full general relativity with a hybrid equation of state. The formation and early evolution of a PNS are followed with a grid of $2500\ifmmode\times\else\texttimes\fi{}2500$ zones, which provides better resolution than in previous (Newtonian) studies. We confirm that significant differential rotation results even when the rotation of the progenitor is initially uniform. Consequently, the magnetic field is amplified both by magnetic winding and the magnetorotational instability (MRI). Even if the magnetic energy ${E}_{\mathrm{EM}}$ is much smaller than the rotational kinetic energy ${T}_{\mathrm{rot}}$ at the time of PNS formation, the ratio ${E}_{\mathrm{EM}}/{T}_{\mathrm{rot}}$ increases to 0.1\char21{}0.2 by the magnetic winding. Following PNS formation, MHD outflows lead to losses of rest mass, energy, and angular momentum from the system. The earliest outflow is produced primarily by the increasing magnetic stress caused by magnetic winding. The MRI amplifies the poloidal field and increases the magnetic stress, causing further angular momentum transport and helping to drive the outflow. After the magnetic field saturates, a nearly stationary, collimated magnetic field forms near the rotation axis and a Blandford-Payne\char21{}type outflow develops along the field lines. These outflows remove angular momentum from the PNS at a rate given by $\stackrel{\ifmmode \dot{}\else \textperiodcentered \fi{}}{J}\ensuremath{\sim}\ensuremath{\eta}{E}_{\mathrm{EM}}{C}_{B}$, where $\ensuremath{\eta}$ is a constant of order $\ensuremath{\sim}0.1$ and ${C}_{B}$ is a typical ratio of poloidal to toroidal field strength. As a result, the rotation period quickly increases for a strongly magnetized PNS until the degree of differential rotation decreases. Our simulations suggest that rapidly rotating, magnetized PNSs may not give rise to rapidly rotating neutron stars.

Two-component magnetohydrodynamical outflows around young stellar objects

We present the first-ever simulations of non-ideal magnetohydrodynamical (MHD) stellar magnetospheric winds coupled with disc-driven jets where the resistive and viscous accretion disc is self-consistently described. These innovative MHD simulations are devoted to the study of the interplay between a stellar wind (having different ejection mass rates) and an MHD disc-driven jet embedding the stellar wind. The transmagnetosonic, collimated MHD outflows are investigated numerically using the VAC code. We first investigate the various angular momentum transports occurring in the magneto-viscous accretion disc. We then analyze the modifications induced by the interaction between the two components of the outflow. Our simulations show that the inner outflow is accelerated from the central object's hot corona thanks to both the thermal pressure and the Lorentz force. In our framework, the thermal acceleration is sustained by the heating produced by the dissipated magnetic energy due to the turbulence. Conversely, the outflow launched from the resistive accretion disc is mainly accelerated by the magneto-centrifugal force.}{The simulations show that the MHD disc-driven outflow extracts angular momentum more efficiently than do viscous effects in near-equipartition, thin-magnetized discs where turbulence is fully developed. We also show that, when a dense inner stellar wind occurs, the resulting disc-driven jet has a different structure, namely a magnetic structure where poloidal magnetic field lines are more inclined because of the pressure caused by the stellar wind. This modification leads to both an enhanced mass-ejection rate in the disc-driven jet and a larger radial extension that is in better agreement with the observations, besides being more consistent.

Bihelical magnetic relaxation and large scale magnetic field growth

A unified, three-scale system of equations accommodating nonlinear velocity driven helical dynamos, as well as time-dependent relaxation of magnetically dominated unihelical or bihelical systems is derived and solved herein. When opposite magnetic helicities of equal magnitude are injected on the intermediate and small scales, the large scale magnetic helicity grows kinematically (independent of the magnetic Reynolds number) to equal that on the intermediate scale. For both free and driven relaxation large scale fields are rapidly produced. Subsequently, a dissipation-limited dynamo, driven by growth of small scale kinetic helicity, further amplifies the large scale field. The results are important for astrophysical coronae fed with bihelical structures by dynamos in their host rotators. The large scale for the rotator corresponds to the intermediate scale for the corona. That bihelical magnetic relaxation can produce global scale fields may help to explain the formation of astrophysical coronal holes and magnetohydrodynamic outflows.

Launching of Jets and The Vertical Structure of Accretion Disk

The launching of magnetohydrodynamic outflows from accretion disks is considered. We formulate a model for the local vertical structure of a thin disk threaded by a poloidal magnetic field of dipolar symmetry. The model consists of an optically thick disk matched to an isothermal atmosphere. The disk is supposed to be turbulent and possesses an effective viscosity and an effective magnetic diffusivity. In the atmosphere, if the magnetic field lines are inclined sufficiently to the vertical, a magnetocentrifugal outflow is driven and passes through a slow magnetosonic point close to the surface. We determine how the rate of mass loss varies with the strength and inclination of the magnetic field. In particular, we find that for disks in which the mean poloidal field is sufficiently strong to stabilize the disk against the magnetorotational instability, the mass-loss rate decreases extremely rapidly with increasing field strength and is maximal at an inclination angle of 40°-50°. For turbulent disks with weaker mean fields, the mass-loss rate increases monotonically with increasing strength and inclination of the field, but the solution branch terminates before achieving excessive mass-loss rates. Our results suggest that efficient jet launching occurs for a limited range of field strengths and a limited range of inclination angles in excess of 30°. In addition, we determine the direction and rate of radial migration of the poloidal magnetic flux and discuss whether configurations suitable for jet launching can be maintained against dissipation.

Systematic construction of exact magnetohydrodynamic models for astrophysical winds and jets

By a systematic method we construct general classes of exact and self-consistent axisymmetric magnetohydrodynamic (MHD) solutions describing flows that originate in the near environment of a central gravitating astrophysical object. The unifying scheme contains two large groups of exact MHD outflow models: (I) meridionally self-similar models with spherical critical surfaces; and (II) radially self-similar models with conical critical surfaces. This classification includes known polytropic models, such as the classical Parker description of a stellar wind and the Blandford &38; Payne model of a disc wind; it also contains non-polytropic models, such as those of winds/jets in Sauty &38; Tsinganos, Lima, Tsinganos &38; Priest, and Trussoni, Tsinganos &38; Sauty. Besides the unification of all known cases under a common scheme, several new classes emerge and some are briefly analysed; they could be explored for a further understanding of the physical properties of MHD outflows from various magnetized and rotating astrophysical objects in stellar or galactic systems.

Formation of Stationary Magnetohydrodynamic Outflows from a Disk by Time‐dependent Simulations

We report the discovery of a family of stationary magnetohydrodynamic (MHD) outflows from a disk found by time-dependent, axisymmetric MHD simulations. The initial poloidal magnetic field threading the disk was taken to be a tapered monopole field, with the field strength on the disk decreasing outwardly to small values. At the disk surface, matter was given a small initial push with a speed less than the slow magnetosonic speed. In the simulations, the outflowing matter is observed to be accelerated by magnetic and centrifugal forces up to velocities in excess of the fast magnetosonic speed and in excess of the escape speed.

Three-dimensional magnetic field configurations in relativistic magnetohydrodynamic outflows

A class of steady 3D relativistic magnetohydrodynamic (RMHD) winds is constructed from a primary radial RMHD outflow of spherical symmetry. The energetic source for modifying the given radial wind is the Poynting flux into the outflowing plasma due to interaction of transverse flow and magnetic field at a reference sphere surrounding a central object. The energy flux associated with such three-dimensional transverse RMHD perturbations is conserved to the leading order of large radial distance r. Asymptotically, the radial magnetic field B0 varies as 1/r-squared while transverse field b approaches about 1/r such that b dominates B0 at large r. Furthermore, the nonlinear effect of transverse electric force due to charge separation acts against transverse magnetic force. Since magnetic field configurations and their strengths are crucial for a realistic modeling of synchrotron emissions, the class of three-dimensional RMHD winds described here provides an important basis for synchrotron calculations which can be used to probe the structures of extragalactic radio jets and supernova remnants. 28 refs.