George Machabeli

On the nature of pulsar radio emission

A theory of pulsar radio emission generation, in which the observed waves are produced directly by maser-type plasma instabilities operating at the anomalous cyclotron--Cherenkov resonance ο k∥ v∥+ οBγres=0 and the Cherenkov drift resonance ο - k∥ v∥- k⊥ ud =0, is capable of explaining the main observational characteristics of pulsar radio emission. The instabilities are due to the interaction of the fast particles from the primary beam and the tail of the distribution with the normal modes of a strongly magnetized one-dimensional electron--positron plasma. The waves emitted at these resonances are vacuum-like, electromagnetic waves that may leave the magnetosphere directly. In this model, the cyclotron--Cherenkov instability is responsible for the core-emission pattern and the Cherenkov drift instability produces conal emission. The conditions for the development of the cyclotron--Cherenkov instability are satisfied for both typical and millisecond pulsars provided that the streaming energy of the bulk plasma is not very high γp≈ 10. In a typical pulsar the cyclotron--Cherenkov and Cherenkov drift resonances occur in the outer parts of the magnetosphere at rres, ≈ 109cm. This theory can account for various aspects of pulsar phenomenology, including the morphology of the pulses, their polarization properties and their spectral behaviour. We propose several observational tests for the theory. The most prominent prediction is the high altitudes of the emission region and the linear polarization of conal emission in the plane orthogonal to the local osculating plane of the magnetic field.

THEORY OF NP 0532 PULSAR RADIATION AND THE NATURE OF THE ACTIVITY OF THE CRAB NEBULA

Plasma processes in the magnetospheres of pulsars are considered. A self-consistent radiation model of the pulsar NP 0532 and the Crab Nebula is presented. It is based on the consideration of cyclotron instability in a relativistic electron-positron plasma in a strong magnetic field. This instability onsets near the light cylinder of pulsar NP 0532. The pitch-angle and energy distribution of particles, caused by cyclotron instability development is found. It is shown that X- and γ-ray radiation of the pulsar is explained by the synchrotron radiation of beam particles. The scattering of excited oscillations on plasma particles is responsible for nonzero pitch-angles of the latter. As a result, synchrotron radiation originates which is within optical frequencies. The synchrotron luminosity evaluated coincides with that observed for NP 0532 in that spectral region. Cyclotron waves are pumped to lower frequencies due to nonlinear scattering by plasma particles and leave the pulsar magnetosphere as observable radio waves.The spectrum of ultra-relativistic electron-positron pairs ejected from the NP 0532 pulsar magnetosphere into the Crab Nebula is calculated. The expected spectrum of the synchrotron radiation of these particles in the Nebula agrees well with its observed spectrum at optical and X-ray frequencies.

Cherenkov-Curvature Radiation and Pulsar Radio Emission Generation

Electromagnetic processes associated with a charged particle moving in a strong circular magnetic field are considered in cylindrical coordinates. We investigate the relation between the vacuum curvature emission and Cherenkov emission and argue that, for the superluminal motion of a particle in the inhomogeneous magnetic field in a dielectric, the combined effects of magnetic field inhomogeneity and the presence of a medium give rise to the synergetic Cherenkov-curvature emission process. We find the conditions under which the operator relations between electric field and electric displacement in cylindrical coordinates may be approximated by algebraic relations. For nonresonant electromagnetic waves, the interaction with particles streaming along the curved magnetic field may be described in the WKB approximation. For resonant waves interacting with superluminal particles we use a plane-wave approximation to compute the local dielectric tensor of a plasma in a weakly inhomogeneous magnetic field. We find in this approximation the polarization of normal modes in the plasma, Cherenkov-curvature and Cherenkov-drift emissivities and growth rates.

Parametric mechanism of the rotation energy pumping by a relativistic plasma

An investigation of the kinematics of a plasma stream rotating in the pulsar magnetosphere is presented. On the basis of an exact set of equations describing the behavior of the plasma stream, the increment of the instability is obtained, and the possible relevance of this approach for the understanding of the pulsar rotation energy pumping mechanism is discussed.

On Optical Synchrotron Emission of Radio Pulsars

It is well known that the distribution function of relativistic particles is one-dimensional at the pulsar surface. Any transverse momentum will be lost via synchrotron emission during a very short time (≤10-20 s). However, the cyclotron instability causes an appearance of transverse momenta of relativistic particles in the outer parts of pulsar magnetospheres. We formulate the equation controlling such processes, and obtain the solution of this equation. It gives the distribution of pitch angles ψ for particles emitting the synchrotron radiation. We show that this radiation is generated near the light cylinder. The mean value of the pitch angle corresponds to optical frequencies in this region. For example, ~ 10-6 for the radio pulsar PSR B0656+14 (in the observers frame), and the frequency of the spectral peak is equal to 3 × 1014 Hz. The flux observed in the optical range requires for this pulsar a density of radiating electrons ~109 cm-3. Some other models of synchrotron emission are discussed. We show that optical emission will be observed in pulsars with small angles β between the rotation and magnetic axes. Pulsars with shorter periods must have harder spectra.

Escaping Radio Emission from Pulsars: Possible Role of Velocity Shear

It is demonstrated that the velocity shear, intrinsic to the e+e- plasma present in the pulsar magnetosphere, can efficiently convert the nonescaping longitudinal Langmuir waves (produced by some kind of a beam or stream instability) into propagating (escaping) electromagnetic waves. It is suggested that this shear-induced transformation may be the basic mechanism needed for the eventual generation of the observed pulsar radio emission.

Ultra High Energy Electrons Powered by Pulsar Rotation

A new mechanism of particle acceleration, driven by the rotational slow down of the Crab pulsar, is explored. The rotation, through the time dependent centrifugal force, can efficiently excite unstable Langmuir waves in the electron-positron (hereafter e±) plasma of the star magnetosphere. These waves, then, Landau damp on electrons accelerating them in the process. The net transfer of energy is optimal when the wave growth and the Landau damping times are comparable and are both very short compared to the star rotation time. We show, by detailed calculations, that these are precisely the conditions for the parameters of the Crab pulsar. This highly efficient route for energy transfer allows the electrons in the primary beam to be catapulted to multiple TeV (~ 100 TeV) and even PeV energy domain. It is expected that the proposed mechanism may, unravel the puzzle of the origin of ultra high energy cosmic ray electrons.

Dynamical feedback of the curvature drift instability on its saturation process

Aims. We investigate the reconstruction of pulsar magnetospheres close to the light cylinder surface to study the curvature drift instability (CDI) responsible for the twisting of magnetic field lines in the mentioned zone. The influence of plasma dynamics on the saturation process of the CDI is studied. Methods. On the basis of the Euler, continuity, and induction equations, we derive the increment of the CDI and analyze parametrically excited drift modes. The dynamics of the reconstruction of the pulsar magnetosphere is studied analytically. Results. We show that there is a possibility of a parametrically excited rotational-energy pumping-process in the drift modes. It is indicated by the generation of a toroidal component of the magnetic field that transforms the field lines into such a configuration, in which plasma particles do not experience any forces. At this stage, the instability process saturates and the further amplification of the toroidal component to the magnetic field lines is suspended.

On the kinematics of a corotating relativistic plasma stream in the perpendicular rotator model of a pulsar magnetosphere

An investigation of the kinematics of a rotating relativistic plasma stream in the perpendicular rotator model of the pulsar magnetosphere is presented. It is assumed that the plasma (ejected from the pulsar) moves along the pulsar magnetic field lines and also corotates with them. The field lines are considered to be radial straight lines, located in the plane which is perpendicular to the pulsar rotation axis. The necessity of taking particle inertia into account is discussed. It is argued that the “massless” (“force-free”) approximation cannot be used for the description of this problem. The frame selection is discussed and it is shown that it is convenient to discuss the problem in the noninertial frame of ZAMOs (Zero Angular Momentum Observers). The equation of motion and the exact set of equations describing the behaviour of a relativistic plasma stream in the pulsar magnetosphere is obtained. The possible relevance of this investigation for the understanding of the formation process of a pulsar magnetosphere is discussed.

ON THE VERY HIGH ENERGY SPECTRUM OF THE CRAB PULSAR

In the present paper, we construct a self-consistent theory interpreting the observations from the MAGIC Cherenkov Telescope of the very high energy (VHE) pulsed emission from the Crab pulsar. In particular, on the basis of Vlasovs kinetic equation, we study the process of quasi-linear diffusion (QLD) developed by means of the cyclotron instability. This mechanism provides simultaneous generation of low (radio) and VHE (0.01-25 GeV) emission on light cylinder scales in one location of the pulsar magnetosphere. A different approach to the synchrotron emission is considered, giving the spectral index of the VHE emission ({beta} = 2) and the exponential cutoff energy (23 GeV) in good agreement with the observational data.