Modeling the X-ray fractional variability spectrum of Active Galactic Nuclei using multiple flares
Using Monte-Carlo simulations of X-ray flare distributions across the accretion disk of active galactic nuclei (AGN), we obtain modeling results for the energy-dependent fractional variability amplitude. Referring to previous results of this model, we illustrate the relation between the shape of the point-to-point fractional variability spectrum, F_pp, and the time-integrated spectral energy distribution, F_E. The results confirm that the spectral shape and variability of the iron Kalpha line are dominated by the flares closest to the disk center.

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

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: 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: 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: Plasma wake-field acceleration of charged particles by selfmodulated long relativistic electron bunch
abstract: High-amplitude plasma wake-waves are excited by high-density relativistic electron bunches (REB) moving in a plasma. The wake- fields can be used to accelerate charged particles, to serve as electrostatic wigglers in free-electron plasma lasers (FEL), and also can find many other applications.

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: 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.