Polarization signatures of strong gravity in AGN accretion discs

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

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: 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: 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: 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: Optimization of high-current ion beam acceleration and charge compensation in two cusps of induction linac
abstract: Results of the numerical simulation of the hollow high-current ion beam (HHCIB) dynamics in two magnet-isolated accelerating gaps separated by the drift gap are presented. The previous study has shown that the good charge and current compensations of the ion beam by the specially injected electron beam occur in the accelerating gaps of the induction linac. However in the drift gap the high positive electric potential due to the positive space charge of HHCIB was obtained because the essential difference between the electron and ion drift velocities exists under this compensation method. This disadvantage impairing the brightness of the ion beam can be considerably reduced by the additional injection of the thermal electrons into the drift region. In present report the some cases of the cold electron injection into drift gap are considered. The more optimal regime for the effective charge and current compensations of HHCIB without loss in the stability of ion beam was found.

title: Numerical modelling of time-space behavior of high-current relativistic electron beam in plasma waveguide
abstract: The physical processes in a magnetized plasma waveguide, in which a high-current relativistic electron beam (Sudan parameter S>1) is injected, are investigated by the developed 2.5-D relativistic electromagnetic code. The computer study has shown that the charge and current compensations of the high-current beam are considerably different from those of the low-current beam. In addition the self-consistent electromagnetic fields generated by a beam at the initial state radically alter both the linear and the nonlinear instability stages.<<ETX>>

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