Masanori Nakamura

Stability Properties of Magnetic Tower Jets

Stability properties of magnetic tower jets propagating in a gravitationally stratified background have been examined by performing three-dimensional magnetohydrodynamic simulations. The current-carrying, Poynting flux-dominated magnetic tower jet, which possesses a highly wound helical magnetic field, is subject to the current-driven instability (CDI). We find that, under general physical conditions including small perturbations in the initial background profiles, the propagating magnetic tower jets develop the nonaxisymmetric, m = 1 kink mode of the CDI. The kink mode grows on the local Alfven crossing timescale. In addition, two types of kink modes appear in the system. At the central region where external thermal pressure confinement is strong, only the internal kink mode is excited and will grow. A large distance away from the central region where the external thermal pressure becomes low, the external kink mode is observed. As a result, the exterior of magnetic tower jets will be deformed into a large-scale wiggled structure. We also discuss extensively the different physical processes that contribute to the overall stability properties of the magnetic tower jets. Specifically, when the jet propagates in an initially unperturbed background, we find that it can survive the kink mode beyond the point predicted by the well-known Kruskal-Shafranov (K-S) criterion. The stabilization in this case comes mainly from the dynamical relaxation of magnetic twists during the propagation of magnetic towers; the magnetic pitch is reduced, and the corresponding K-S critical wavelength becomes longer as the tower jet proceeds. Furthermore, we show that the pressure-driven and Kelvin-Helmholtz instabilities do not occur in the magnetic tower jets. This strongly suggests that the CDI is the primary reason for the wiggling structures in jets.

STRUCTURE OF MAGNETIC TOWER JETS IN STRATIFIED ATMOSPHERES

Based on a new approach on modeling the magnetically dominated outflows from AGNs (Li et al. 2006), we study the propagation of magnetic tower jets in gravitationally stratified atmospheres (such as a galaxy cluster environment) in large scales (> tens of kpc) by performing three-dimensional magnetohydrodynamic (MHD) simulations. We present the detailed analysis of the MHD waves, the cylindrical radial force balance, and the collimation of magnetic tower jets. As magnetic energy is injected into a small central volume over a finite amount of time, the magnetic fields expand down the backgroun d density gradient, forming a collimated jet and an expanded “lobe” due to the gradually decreasing background density and pressure. Both the jet and lobes are magnetically dominated. In addition, the injection and expansion produce a hydrodynamic shock wave that is moving ahead of and enclosing the magnetic tower jet. This shock can eventually break the hydrostatic equilibrium in the ambient medium and cause a global gravitational contraction. This contraction produces a strong compression at the head of the magnetic tower front and helps to collimate radially to produce a slendershaped jet. At the outer edge of the jet, the magnetic pressur e is balanced by the background (modified) gas pressure, without any significant contribution from the hoo p stress. On the other hand, along the central axis of the jet, hoop stress is the dominant force in shaping the cent ral collimation of the poloidal current. The system, which possesses a highly wound helical magnetic configurati on, never quite reaches a force-free equilibrium state though the evolution becomes much slower at late stages. The simulations were performed without any initial perturbations so the overall structures of the jet r emain mostly axisymmetric. Subject headings:magnetic fields — galaxies: active — galaxies: jets — methods : numerical — magnetohydrodynamics (MHD)

Multifrequency Polarimetry of the NRAO 140 Jet: Possible Detection of a Helical Magnetic Field and Constraints on Its Pitch Angle

We present results from multifrequency polarimetry of NRAO 140 using the Very Long Baseline Array. These observations allow us to reveal the distributions of both the polarization position angle and the Faraday rotation measure (RM). These distributions are powerful tools to discern the projected and line-of-sight components of the magnetic field, respectively. We find a systematic gradient in the RM distribution, with its sign being opposite at either side of the jet with respect to the jet axis. The sign of the RM changes only with the direction of the magnetic field component along the line of sight, so this can be explained by the existence of helical magnetic components associated with the jet itself. We derive two constraints for the pitch angle of the helical magnetic field from the distributions of the RM and the projected magnetic field; the RM distribution indicates that the helical fields are tightly wound, while the distribution of the projected magnetic field suggests they are loosely wound around the jet axis. This inconsistency may be explained if the Faraday rotator is not cospatial with the emitting region. Our results may point toward a physical picture in which an ultrarelativistic jet (spine) with a loosely wound helical magnetic field is surrounded by a subrelativistic wind layer (sheath) with a tightly wound helical magnetic field.

A Numerical Model of Hercules A by Magnetic Tower: Jet/Lobe Transition, Wiggling, and the Magnetic Field Distribution

We apply magnetohydrodynamic (MHD) modeling to the radio galaxy Hercules A for investigating the jetdriven shock, jet/lobe transition, wiggling, and magnetic field distribution associated with this source. The model consists of magnetic tower jets in a galaxy cluster environment, which has been discussed in a series of our papers. The profile of underlying ambient gas plays an imp ortant role in jet-lobe morphology. The balance between the magnetic pressure generated by axial current and the ambient gas pressure can determine the lobe radius. The jet body is confined jointly by the external press ure and gravity inside the cluster core radius Rc, while outside Rc it expands radially to form fat lobes in a steeply decreasing ambient thermal pressure gradient. The current-carrying jets are responsible for ge nerating a strong, tightly wound helical magnetic field. This magnetic configuration will be unstable against t he current-driven kink mode and it visibly grows beyond Rc where a separation between the jet forward and return currents occurs. The reversed pinch profile of global magnetic field associated with the jet and lobes pro duces projected B-vector distributions aligned with the jet flow and the lobe edge. AGN-driven shock powered b y the expanding magnetic tower jet surrounds the jet/lobe structure and heats the ambient ICM. The lobes expand subsonically; no obvious hot spots are produced at the heads of lobes. Several key features in our MHD modeling may be qualitatively supported by the observations of Hercules A. Subject headings:galaxies:individual: Hercules A — galaxies: active — galaxies: jets — methods: numerical — MHD

Control and monitoring system for the Greenland telescope: computers, network and software

We describe the control and monitoring system for the Greenland Telescope (GLT). The GLT is a 12-m radio telescope aiming to carry out the sub-millimeter Very Long Baseline Interferometry (VLBI) observations and image the shadow of the super massive black hole in M87. In November 2017 construction has been finished and commissioning activity has been started. In April 2018 we participated in the VLBI observing campaign for the Event Horizon Telescope (EHT) collaboration. In this paper we present the entire GLT control/monitoring system in terms of computers, network and software.