K-band polarimetry of an Sgr A* flare with a clear sub-flare structure ⋆
The supermassive black hole at the Galactic center, Sgr A*, shows frequent radiation outbursts, often called ’flares’. I n the near-infrared some of these flares were reported as showing i ntrinsic quasi-periodicities. The flux peaks associated wi th the quasi-periodic behavior were found to be highly polarized.  The aim of this work is to present new evidence to support previous findings of the properties of the polarized radiation fr om Sgr A* and to again provide strong support for the quasi-periodicity of∼18± 3 min reported earlier.  Observations were carried out at the European Southern Observatory’s Very Large Telescope on Paranal, Chile. We used th e NAOS/CONICA adaptive optics/near-infrared camera instrument. By fitting the polarimetr ic lightcurves with a hot-spot model, we addressed the question of whether the data are consistent with this model. To fit the observed data we used a general relativistic ray -tracing code in combination with a simple hot-spot/ring model.  We report on new polarization measurements of a K-band flare f rom the supermassive black hole at the Galactic center. The data provide very strong support for a quasi-periodicity of 15.5± 2 min. The mean polarization of the flare is consistent with th e direction of the electric field vector that was reported in previous obser vations. The data can be modeled successfully with a combined blob/ring model. The inclination i of the blob orbit must be i> 20 ◦ on a 3σ level, and the dimensionless spin parameter of the black hole is derived to be a⋆> 0.5.

title: Magnetic layers and neutral points near a rotating black hole
abstract: Magnetic layers are narrow regions where the field direction changes sharply. They often occur in the association with neutral points of the magnetic field. We show that an organized field can produce these structures near a rotating black hole (BH), and we identify them as potential sites of magnetic reconnection. To that end, we study the field lines affected by the frame-dragging effect, twisting the magnetic structure and changing the position of neutral points. We consider oblique fields in vacuum. We also include the possibility of translational motion of the black hole which may be relevant when the black hole is ejected from the system. The model settings apply to the innermost regions around black holes with the ergosphere dominated by a super-equipartition magnetic field and loaded with a negligible gas content.

title: Stellar capture by an accretion disc
abstract: The long-term evolution of a stellar orbit captured by a massive galactic centre via successive interactions with an accretion disc has been examined. An analytical solution describing the evolution of the stellar orbital parameters during the initial stage of the capture has been found. Our results are applicable to thin Keplerian discs with an arbitrary radial distribution of density and a rather general prescription for the star–disc interaction. Temporal evolution is given in the form of quadrature which can be carried out numerically.

title: Off-equatorial orbits in strong gravitational fields near compact objects
abstract: Near a black hole or an ultracompact star, the motion of particles is governed by a strong gravitational field. Electrically charged particles also feel the electromagnetic force arising due to currents inside the star or plasma circling around. We study the possibility that the interplay between gravitational and electromagnetic actions may allow for the stable, energetically bound off-equatorial motion of charged particles. This would represent the well-known generalized Stormer's 'halo orbits', which have been discussed in connection with the motion of dust grains in planetary magnetospheres. We demonstrate that such orbits exist and can be astrophysically relevant when a compact star or a black hole is endowed with a dipole-type magnetic field. In the case of the Kerr–Newman solution, numerical analysis shows that the mutually connected gravitational and electromagnetic fields do not allow the existence of stable halo orbits above the outer horizon of black holes. Such orbits are either hidden under the inner black-hole horizon, or they require the presence of a naked singularity.

title: Prospects of 3D mapping of the Galactic Centre clouds with X-ray polarimetry
abstract: Despite past panchromatic observations of the innermost part of the Milky Way, the overall structure of the Galactic Centre (GC) remains enigmatic in terms of geometry. In this paper, we aim to show how polarimetry can probe the three-dimensional position of the molecular material in the central ~100 pc of the GC. We investigate a model where the central supermassive black hole Sgr A* is radiatively coupled to a fragmented circumnuclear disc (CND), an elliptical twisted ring representative of the central molecular zone (CMZ), and the two main, bright molecular clouds Sgr B2 and Sgr C. 8 -- 35 keV integrated polarization mapping reveals that Sgr B2 and Sgr C, situated at the two sides of the CMZ, present the highest polarization degrees (66.5 and 47.8 per cent respectively), both associated with a polarization position angle psi = 90deg (normal to the scattering plane). The CND shows a lower polarization degree, 1.0 per cent with psi = -20.5deg, tracing the inclination of the CND with respect to the Galactic plane. The CMZ polarization is spatially variable. We also consider a range of spatial locations for Sgr A* and the reprocessing media, and investigate how the modeled three-dimensional geometry influences the resulting GC polarization. The two reflection nebulae are found to always produce high polarization degrees (>> 10 per cent). We show that a 500 ks observation with a broadband polarimeter could constrain the location and the morphology of the scattering material with respect to the emitting source, revealing the past activity of Sgr A*.

title: 2.5-Dimension Numerical Simulation Of Selfmodulation of Relativistic Electron Bunches in a Plasma.

title: Numerical simulation of high-current ion linear induction accelerator with additional electron beam injection

title: 2.5D numerical simulation of relativistic electron bunches (REB) interaction with underdense and overdense plasmas
abstract: The results of REB-plasma interaction numerical simulation by means of our relativistic particle EM code-are presented. Nonlinear evolution of a single REB injected into underdense and overdense plasmas is studied in detail for cylindrical geometry. It is found that in case of comparable bunch and plasma densities an instantaneous amplitude of excited longitudinal electric field substantially depends on density ratio, relativistic factor and space density distribution of REB. Transversal electric field can considerably exceed the longitudinal one, causing more severe restrictions to initial phase and radial position of particles to be accelerated.<<ETX>>

title: Focusing of relativistic electron bunches at the wake-field excitation in plasma
abstract: Analytical and numerical investigations of the trajectories of probing beam electrons in a two dimensional wake-field, generated in a plasma by a dense bunch of relativistic electrons with Gaussian longitudinal and transverse distributions of density have been carried out. Based on the calculations of probing beam deviations, the diagnostic instruments are developed for the parameters of experiments conducted at NSC KIPT. The diagnostic instruments include an electron gun generating a 10 keV electron beam with a current of 10 /spl mu/A and 2 mm in diameter, which passes through the chamber of interaction and falls on the collector of 10 mm diameter. The collector (screen) is placed in front of the first plate of a microchannel amplifier which consists of three microchannel plates (MCP) with sizes between 20 and 30 mm. The 3 kV voltage was applied to each plate. The total amplification of MCP amplifier is 10/sup 4/ to 10/sup 5/ depending on the number of particles incident on the first plate. As a result of probing beam deviations due to the excited wake-field, the electrons fall on the first plate of the amplifier and are registered by its anode located behind the third plate. The calculated probing beam deviations and the amplification attained with the MCP amplifier permit one to detect and investigate the electrical wake-fields excited by a sequence of relativistic bunches ( number of particles in a bunch is 2/spl middot/10/sup 9/, energy is 14 MeV) in a plasma of 10/sup 11/-10/sup 13/ cm/sup -3/ density. The intensity of the fields registered by this technique is no less than 2 kV/cm.

title: Kinetic simulation of fields excitation and particle acceleration by laser beat wave in non-homogeneous plasmas
abstract: Resonance excitation of longitudinal plasma electrostatic wave by double-frequency laser radiation is investigated numerically to study in detail conditions of particle beat wave acceleration. The computer simulation is based on the highly specialized code SUR, using splitting technique. Both the space uniform and slightly non-uniform cases are investigated. Maintaining of phase synchronism between accelerated particles and excited longitudinal wave is provided by a choice of density plasma profile.

title: 2,5-dimensional numerical simulation of propagation of the finite sequence of relativistic electron bunches (REB) in tenuous and dense plasmas
abstract: Particle simulation results on the wake-field excitation with the programmed finite sequence of REB in tenuous and dense plasmas are presented. The REB/background plasma configuration is described by full set of 2D3V relativistic Vlasov equations for each plasma species and nonlinear Maxwell equations for self-consistent electromagnetic field. The physical parameters in runs carried out are close to those used in laboratory experiment at Kharkov IPT (Ya. Fainberg et al., 1994). The simulation points to the fact that the reached value of electric field depends on charge distribution and choice of intervals between bunches in programmed sequence.