Semi-relativistic Plasmas Research Articles

Electron and Proton Energization in 3D Reconnecting Current Sheets in Semirelativistic Plasma with Guide Magnetic Field

Using 3D particle-in-cell simulation, we characterize energy conversion, as a function of guide magnetic field, in a thin current sheet in semirelativistic plasma, with relativistic electrons and subrelativistic protons. There, magnetic reconnection, the drift-kink instability (DKI), and the flux-rope kink instability all compete and interact in their nonlinear stages to convert magnetic energy to plasma energy. We compare fully 3D simulations with 2D in two different planes to isolate reconnection and DKI effects. In zero guide field, these processes yield distinct energy conversion signatures: ions gain more energy than electrons in 2Dxy (reconnection), while the opposite is true in 2Dyz (DKI), and the 3D result falls in between. The flux-rope instability, which occurs only in 3D, allows more magnetic energy to be released than in 2D, but the rate of energy conversion in 3D tends to be lower. Increasing the guide magnetic field strongly suppresses DKI, and in all cases slows and reduces the overall amount of energy conversion; it also favors electron energization through a process by which energy is first stored in the motional electric field of flux ropes before energizing particles. Understanding the evolution of the energy partition thus provides insight into the role of various plasma processes, and is important for modeling radiation from astrophysical sources such as accreting black holes and their jets.

Synchrotron Cutoff in Ultraluminous X-Ray Sources

The origin of spectral curvature at energies E ≃ 10 keV in ultraluminous X-ray sources (ULXs) is not well understood. In this paper, we propose a novel mechanism based on synchrotron radiation to explain this cutoff. We show that relativistic plasma can give rise to the observed spectral curvature for neutron star magnetic fields due to the variation in the latitude of synchrotron radiation. We analyze the NuSTAR data of two bright pulsar ULXs, NGC 5907 ULX1 and NGC 7793 P13, and provide estimates of the physical parameters of these sources. We fit the data for synchrotron emission at various latitudes and show that the spectral cutoff in these cases can be explained for a large range of acceptable physical parameters, e.g., a semirelativistic plasma with γ ≃ 20 for high latitudes or a highly relativistic plasma (γ ≃ 105) for emission close to the electron’s orbital plane in a typical magnetic field of B ≃ 1012 G. We also discuss how such an emission mechanism can be distinguished from other proposed models. A corollary to our study is that most ULXs might be neutron stars as they display such a spectral cutoff.

Exact solutions for nonlinear propagation of slow ion acoustic monotonic double layers and a solitary hole in a semirelativistic plasma

A small-amplitude slow ion acoustic monotonic double layer in an unmagnetized plasma consisting of relativistic drifting cold electrons and nonrelativistic drifting thermal ions is investigated. By using the reductive perturbation method, Schamel–Korteweg–de Vries (SKdV) and Schamel equations are derived. We used the linearization transformation to obtained the solutions of the SKdV and Schamel equations. The method is based upon a linearization principle that can be applied on nonlinearities which have a polynomial form. We illustrate the potential of the method by finding solutions of the SKdV and Schamel equations. Furthermore, we show that the monotonic double-layer solution is a nonlinear extension of the slow ion acoustic solitary hole having a negative trapping parameter in a semi relativistic plasma.

Nonlinear Propagation of Modified Electron Acoustic Monotonic Double Layers in a Semi-Relativistic Plasma

A small amplitude, modified electron acoustic monotonic double layer in an unmagnetized plasma consisting of relativistic drifting cold electrons and non-relativistic drifting thermal ions is investigated. By using the reductive perturbation method, Korteweg–de Vries (K–dV) like equations are derived. Solving the K–dV like equations of modified electron acoustic waves, we find that simple expressions are obtained for the amplitude and the inverse scale length in a semi-relativistic plasma. We show that all expressions are influenced by relativistic effects. Furthermore, we show that the monotonic double layer solution is a nonlinear extension of the modified electron acoustic solitary hole having negative trapping parameter in a semi-relativistic plasma.

Nonlinear propagation of small amplitude modified electron acoustic solitary waves in semirelativistic plasmas

Analytical solutions for the small amplitude modified electron acoustic solitary waves in an unmagnetized plasma consisting of thermal ions, thermal electrons and relativistic drifting cold electrons are presented. The influences of electron and ion temperature, electron densities and relativistic effects on the propagation of such waves are examined. It is found that the amplitude of the soliton associated with nonisothermal electron is affected much by the relativistic effects in comparison to the amplitude of the soliton associated with isothermal electrons.