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Rich and massive clusters of galaxies at intermediate redshift are capable of magnifying and distorting the images of background galaxies. A comparison of different mass estimators among these clusters can provide useful information about the distribution and composition of cluster matter and their dynamical evolution. Using a hitherto largest sample of lensing clusters drawn from literature, we compare the gravitating masses of clusters derived from the strong/weak gravitational lensing phenomena, from the X-ray measurements based on the assumption of hydrostatic equilibrium, and from the conventional isothermal sphere model for the dark matter profile characterized by the velocity dispersion and core radius of galaxy distributions in clusters. While there is an excellent agreement between the weak lensing, X-ray and isothermal sphere model determined cluster masses, these methods are likely to underestimate the gravitating masses enclosed within the central cores of clusters by a factor of 2--4 as compared with the strong lensing results. Such a mass discrepancy has probably arisen from the inappropriate applications of the weak lensing technique and the hydrostatic equilibrium hypothesis to the central regions of clusters as well as an unreasonably large core radius for both luminous and dark matter profiles. Nevertheless, it is pointed out that these cluster mass estimators may be safely applied on scales greater than the core sizes. Namely, the overall clusters of galaxies at intermediate redshift can still be regarded as the dynamically relaxed systems, in which the velocity dispersion of galaxies and the temperature of X-ray emitting gas are good indicators of the underlying gravitational potentials of clusters., Comment: 16 pages with 7 PS figures, MNRAS in press

We make use of four galaxy catalogs based on four different semi-analytical models (SAMs) implemented in the Millennium simulation to study the environmental effects and the model dependence of galaxy merger rate. We begin the analyses by finding that galaxy merger rate in the SAMs has mild redshift evolution, consistent with results of previous works. To study the environmental dependence of galaxy merger rate, we adopt two estimators, the local overdensity (1+{\delta}n) defined as the surface density from the nth-nearest-neighbor (n = 6 is chosen in this study) and the host halo mass Mh. We find that galaxy merger rate Fmg shows strong dependence on the local overdensity (1+{\delta}n) and the dependence is similar at all redshifts. For the overdensity estimator, the merger rate Fmg is found about twenty times larger in the densest regions than in under-dense ones in two of the four models while it is roughly four times higher in the other two. In other words, the discrepancies of the merger rate difference between two extremes can differ by a factor of ~ five depending on the SAMs adopted. On the other hand for the halo mass estimator, Fmg does not monotonically increase with the host halo mass Mh, but peaks in the $M_h$ range between 10^12 and 10^13 h-1 M{\Theta}, which corresponds to group environments. High merger rate in high local density regions corresponds primarily to the high merge rate in group environments......

Quantum turbulence that exhibits vortex creation, annihilation and interactions is demonstrated as an exact solution of the time-dependent, free-particle Schr\"odinger equation evolved from a smooth random-phased initial condition. Relaxed quantum turbulence in 2D and 3D exhibits universal scaling in the steady-state energy spectrum as k-1 in small scales. Due to the lack of dissipation, no evidence of the Kolmogorov-type forward energy cascade in 3D or the inverse energy cascade in 2D is found, but the rotational and potential flow components do approach equi-partition in the scaling regime. In addition, the 3D vortex line-line correlation exhibits universal behaviour, scaled as \Deltar^-2, where \Deltar is the separation between any two vortex line elements, in fully developed turbulence. We also show that the quantum vortex is not frozen to the matter, nor is the vortex motion induced by other vortices via Biot-Savart's law. Thus, the quantum vortex is actually a nonlinear wave, propagating at a speed very different from a classical vortex., Comment: 9 pages, 14 figures

The effects of drag on an idealized relativistic MHD wind of radial geometry are studied. The astrophysical motivation is to understand the effects of radiation drag on the dynamics of a jet or wind passing through the intense radiation field of an accreting compact object. From a critical point analysis, it is found that a slow magnetosonic point can appear in a dragged flow even in the absence of gravitational force, as a result of a balance between the drag force and the combination of thermal pressure and centrifugal forces. As in the undragged case, the Alfven point does not impose any constraints on the flow. Although it is formally possible for a dragged flow to possess more than one fast magnetosonic point, it is shown that this is unlikely in practice. In the limit of a 'cold', centrifugally driven flow, it is shown that the fast magnetosonic point moves to infinite radius, just as in the drag-free case. For a given mass flux, the total energy output carried to infinity, and the final partition between the kinetic energy and the Poynting flux, are the same for the dragged and the drag-free flows. The main effects of radiation drag are to increase the amount of energy and angular momentum extracted from the source and to redistribute the regions where acceleration occurs in the flow. This is accomplished through the storage and release of magnetic energy, as a result of additional winding and compression of the field caused by the action of the drag. For a relativistic wind, the dissipated energy can exceed the final kinetic energy of the flow and may be comparable to the total flow energy (which is dominated by Poynting flux). The energy lost to radiation drag will appear as a Doppler-boosted beam of scattered radiation, which could dominate the background radiation if the flow is well-collimated.

A fully relativistic analysis has been performed of the asymptotic structure of stationary axisymmetric hydromagnetic winds. If a flow fills the region containing the rotation axis, then the flux surfaces in the flow must collimate to a set of current-carrying cylindrical surface extending to infinite transverse radius, collimate to a set of cylindrical surfaces extending to a finite radius, or collimate to a current-free paraboloidal field configuration which fills up the entire space. If an asymptotically cylindrical flow carries a finite current at radii well beyond the light cylinder, then the Lorentz factor of the terminal flow speed on a given flux surface is proportional to the total current enclosed within this flux surface. If a flow is of type II paraboloidal, then its asymptotic energy flux is carried entirely by the gas motion rather than the electromagnetic fields.

A nearly stationary pulsar wind is shown to be always collimating along the direction of rotation. It follows that the conventional picture of the equatorial pulsar wind can occur only in a highly time-dependent manner, which, if true, may account for the observed time variation within a single pulse. Furthermore, a mechanism, via excitation of magnetosolitons, is proposed for explaining the continuous generation of energetic electrons, which yield the observed radio emission in the Type II supernova remnants. Fully relativistic analyses will be presented to substantiate these claims.

Motivated by problems concerning the storage and subsequent escape of the solar magnetic field, a study has been made of how a magnetic layer embedded in a convectively stable atmosphere evolves due to axisymmetric instabilities driven by magnetic buoyancy. The initial equilibrium consists of a toroidal field sheared by a weaker poloidal component. The linear stability problem is investigated for both ideal and resistive MHD, and the nonlinear evolution is followed by numerical integration of the equations of motion. In all cases, the instability is greatly affected by the distribution and strength of the poloidal field. In particular, both the horizontal and vertical scales of the motions are controlled by the location of the surface on which the poloidal field vanishes: the resonant surface. In the nonlinear regime, a resonant surface close to the interface between the magnetized and field-free fluid leads to the localization of the instability, so that only a fraction of the magnetic region is disrupted by the motions. By contrast, a deeply seated resonant surface leads to the complete disruption of the layer and to the formation of large, helical magnetic fragments whose identity is preserved for the entire simulation.

A self-similar model for the flow evolution in a turbulent mixing layer is presented. The model is dependent on the interaction between vortex pairing, unstable collective excitation, and turbulent diffusion. An integrable nonlinear equation governing the flow expansion is derived from the model.

A new type of relativistic magnetosonic soliton, which is electrically charged with a gigavolt potential, is found to exist in a magnetized electron-positron-proton plasma. Relativistic collisionless shocks resulting from such solitons can carry an even larger electric potential at the shock front. GeV electrons and positrons in some active astrophsyical sources may be produced due to acceleration by these electric fields.

A novel mechanism, by which the intense flux of neutrinos in a supernova explosion can effectively transfer energy and momentum into the stellar matter outside the neutrinosphere, is proposed. The energy and momentum rate can be greater by orders of magnitude than that conventionally accepted. Roughly, the new rate scales with the first power of the Fermi constant G F , instead of the second power resulting from collision between individual neutrinos and fermions. Through scattering of neutrinos and sound waves, it increases the scattering cross section by a factor G F -1 , which may shed new light on the problem of the stalled supernova explosion

With a two-field Lagrangian of the Hamilton-Jacobi equation, a coupling between the two fields can be introduced to yield an appropriate quantum Lagrangian. The quantum effect is nothing more than a quantum pressure force acting on the otherwise classical pressureless fluid when one of the fields is properly renormalized. Inclusion of the electromagnetic coupling under the present formulation shows that the collisionless quantum fluid picture remains valid. Similar, but not identical, to the classical collisionless fluids, the flow circulation and magnetic flux can remain locked together even for a spatially and temporally varying magnetic field. Extensions beyond the present scope of quantum formulation are also discussed.

AMiBA is a 3 mm wavelength astronomical interferometer consisting of 0.6 m to 1.2 m antennae. An artificial source is required for its calibration. We present an approach of using a tunable CW source to calibrate the phase and amplitude response. Performance and problems of the system are discussed.

A strong (~ few × 105 G) and slightly twisted magnetic field (of flux 1023 Mx) can be stored in a dynamo flux tube within the overshoot region provided that the tube has a slightly lower temperature than the background gas, which has a stratification profile γ(d ln ρ/d ln P) - 1 ≥ B2/8πP, where P, ρ, B and γ are the gas pressure, density, magnetic field strength, and adiabatic index. Pumped by the intense vortical downdraft, the outer layer of dynamo flux tube is proposed to possess opposite magnetic helicity from that in the tube interior, as a direct result of the conservation of magnetic helicity. When the field-line twist in the outer layer exceeds some threshold, we show that the helicity-reversal layer can be subject to MHD instabilities that trigger partial flux eruption. In the nonlinear stage, flux ropes of 1022 Mx with magnetic helicity of a definite sign are expected to be ejected into the convection zone, leaving magnetic helicity of the opposite sign to accumulate in the main flux tube. Such a helicity removal mechanism breaks the conservation of magnetic helicity within the dynamo flux tube and can circumvent the recently discovered difficulty (the so-called α quench) for the generation of large-scale fields. We argue that the dynamo α is likely to resume a value of the same order as that of the kinematic turbulent dynamo. We further suggest that the helical flux expulsion is an indispensable component for the solar dynamo to operate properly. A natural prediction of this scenario is that strong sunspots in the same hemisphere should possess magnetic helicity of the same sign, regardless of the solar cycles. Specifically, the helical field lines in strong sunspots should obey the left-hand rule in the northern hemisphere and the right-hand rule in the southern hemisphere.

Ideal MHD stability in the solar corona is studied in order to understand the issue of magnetic energy storage. It is shown that circular flux tubes confined by a potential field can store a large fraction of their energy in nonpotential form. Such tubes are stabilized by their circularity, by line tying, and by their external field. In contrast, interacting, noncircular tubes can be far less stable, despite line tying and an external field. If a substantial fraction of the magnetic energy in an active region is to be stably stored in nonpotential form, only a very simple magnetic geometry is probably required. 28 references.

Test particle simulation is performed to examine the mechanism previously proposed by Chiueh for particle acceleration by a nearly perpendicular shock where only downstream MHD turbulence is present. A nonnegligible fraction of low-energy particles is observed gaining energy by more than three orders of magnitude in a few hundred cyclotron periods. Criteria for effective acceleration given previously are examined and confirmed. The numerical results suggest that in the nonrelativistic energy range, a power-law particle energy spectrum, with the power index determined by the degree to which the criteria are satisfied, may form.

A model in which low-energy ions are accelerated by a nearly perpendicular MHD shock without upstream turbulence is presented. In the present scheme, vigorous particle cross-field scattering by downstream turbulence competes with the convection downstream, sending particles back to the shock front for acceleration. Some fraction of the low-energy ions are accelerated up to maximal energy prior to escaping upstream to possibly undergo diffusion shock acceleration. The model can explain particle injection of the diffusive shock acceleration along with ion acceleration during solar flares.

The equilibrium structure and magnetic-reconnection (MR) processes in current sheets produced by global MHD forces are investigated theoretically, considering the case (complementary to that studied by Parker, 1979 and 1983) where the magnetic field near the sheet lies on good flux surfaces, with substantial accumulation of magnetic stress. The derivation and application of the model, based on the approximate MHD equations developed by Strauss (1976) for tokamak plasmas, are presented in detail. Three main stages of MR are characterized: a linear sheet-tearing mode which grows more rapidly than the magnetic-tearing mode for diffuse profiles, acceleration of MR by small shear-flow-driven MHD fluctuations acting as an anomalous resistivity, and rapid MR in a broad turbulent layer. This mechanism is shown to provide the rapid current dissipation required by some models of solar flares and coronal heating. 48 references.

A plasma instability which may lead to the thermal coupling of ions and electrons in two-temperature accretion flows on a time scale shorter than the Coulomb coupling time has been found. The processes involved with the instability are described. It is shown that the instability has time to grow only in regions of large-amplitude, small-scale MHD turbulence.