Linear Acceleration Emission Research Articles

Linear acceleration emission of pulsar relativistic streaming instability and interacting plasma bunches

Context. Linear acceleration emission is one of the mechanisms that might explain intense coherent emissions of radio pulsars. This mechanism is not well understood, however, because the effects of collective plasma response and nonlinear plasma evolution on the resulting emission power must be taken into account. In addition, details of the radio emission properties of this mechanism are unknown, which limits the observational verification of the emission model. Aims. By including collective and nonlinear plasma effects, we calculate radio emission power properties by the linear acceleration emission mechanism that occurs via the antenna principle for two instabilities in neutron star magnetospheres: (1) the relativistic streaming instability, and (2) interactions of plasma bunches. Methods. We used 1D electrostatic relativistic particle-in-cell simulations to evolve the instabilities self-consistently. From the simulations, the power properties of coherent emission were obtained by novel postprocessing of electric currents. Results. We found that the total radio power by plasma bunch interactions exceeds the power of the streaming instability by eight orders of magnitude. The wave power generated by a plasma bunch interaction can be as large as 2.6 × 1016 W. The number of bunch interactions that are required to explain the typical pulsar power, 1018 − 1022 W, depends on how the coherent emissions of bunches are added up together. Although ∼4 × (101 − 105) simultaneously emitting bunches are necessary for an incoherent addition of their radiation power, ≳6 − 600 bunches can explain the total pulsar power if they add up coherently. The radio spectrum of the plasma bunch is characterized by a flatter profile for low frequencies and by a power-law index up to ≈ − 1.6 ± 0.2 for high frequencies. The plasma bunches simultaneously radiate in a wide range of frequencies, fulfilling no specific relation between emission frequency and height in the magnetosphere. The power of the streaming instability is more narrowband than that of the interacting bunches, with a high-frequency cutoff. In both instabilities, the angular width of the radiation decreases with increasing frequency. In addition, the wave power evolution depends on the pulsar rotation angle, causing microsecond fluctuations in the intensity because it oscillates between positive and negative wave interference as a function of the emission angle.

LINEAR ACCELERATION EMISSION IN PULSAR MAGNETOSPHERES

(abbrev.) Linear acceleration emission occurs when a charged particle is accelerated parallel to its velocity. We evaluate the spectral and angular distribution of this radiation for several special cases, including constant acceleration (hyperbolic motion) of finite duration. Based on these results, we find the following general properties of the emission from an electron in a linear accelerator that can be characterized by an electric field E acting over a distance L: (i) the spectrum extends to a cut-off photon energy ~ LE^2 MeV, where E is in units of the Schwinger critical field and L in units of the Compton wavelength of the electron. (ii) the total energy emitted by a particle traversing the accelerator is in agreement with the standard Larmor formula (iii) the low frequency spectrum is flat for hyperbolic trajectories, but in general depends on the details of the accelerator. We also show that linear acceleration emission complements curvature radiation in the strongly magnetized pair formation regions in pulsar magnetospheres. It dominates when the length L of the accelerator is less than the formation length of curvature photons, which is given by the ratio of the radius of curvature of the magnetic field lines to the Lorentz factor of the particle. In standard static models of pair creating regions linear acceleration emission is negligible, but it is important in more realistic dynamical models in which the accelerating field fluctuates on a short length-scale.

LINEAR ACCELERATION EMISSION. II. POWER SPECTRUM

The theory of linear acceleration emission (LAE) is developed for a large amplitude electrostatic wave in which all particles become highly relativistic in much less than a wave period. An Airy-integral approximation is shown to apply near the phases where the electric field passes through zero and the Lorentz factors of all particles have their maxima. The emissivity is derived for an individual particle and is integrated over frequency and solid angle to find the power radiated per particle. The result is different from that implied by the generalized Larmor formula which, we argue, is not valid in this case. We also discuss a mathematical inconsistency that arises when one evaluates the power spectrum by integrating the emissivity over solid angle. The correct power spectrum increases as the 4/3rd power of the frequency at low frequencies, and falls off exponentially above a characteristic frequency. We discuss application of LAE to the emission of high-frequency photons in an oscillating model for pulsars. We conclude that it cannot account for gamma-ray emission, but can play a role in secondary pair creation.

LINEAR ACCELERATION EMISSION. I. MOTION IN A LARGE-AMPLITUDE ELECTROSTATIC WAVE

We consider the motion of a charge in a large amplitude electrostatic wave with a triangular wave form relevant to an oscillating model of a pulsar magnetosphere. The (one-dimensional) orbit of a particle in such a wave is found exactly in terms of Weierstrass functions. The result is used to discuss linear acceleration emission (at both low and high frequencies) in an oscillating model for pulsars. An explicit expression for the emissivity is derived in an accompanying paper (Melrose & Luo 2009), and used here to derive an expression for the absorption coefficient at low frequencies. We show that absorption can be negative, corresponding to maser emission. For the large amplitude required to trigger pair creation in an oscillating model, the rate of the maser growth is too small to be effective. Effective growth requires smaller amplitudes, such that the maximum Lorentz factor gained by acceleration in the wave is $\lapprox10$.

Production of Pairs by Linear Acceleration Emission

AbstractThe possibility is considered that core emission in pulsars results from secondary e± pairs generated through linear acceleration emission (LAE), rather than the conventional curvature emission (CE). It is found that this is possible in principle, but only under conditions that appear not to be satisfied in practice.

Radio emission from polar caps in pulsars

Radiation from a charge accelerated along its path or Linear Acceleration Emission (LAE) involves a number of subtleties (Pauli 1921; Ginzburg 1970, 1989). Potential interest of the mechanism for astrophysics has been pointed out by Wagoner (1969). Melrose (1978) and Rowe (1995) have studied amplified LAE from time-varying electric fields for radio pulsars. In contrast with the latter work our calculations are for static electric field structures or double layers (DLs) as are thought to occur in magnetospheres of neutron stars. In ordinary stellar atmospheres a LAE maser can operate in non-relativistic DLs (Kuijpers 1990) at a frequencyω≈kDLυ≈ 2π/ttr, and a wave vectorwithkDL= 2π/L(Lis the DL length,υis the particle speed, andttris the transit time of the DL by the particle). The emission process can be considered as scattering of the electrostatic electric field on fast electrons into electromagnetic radiation satisfying the resonance condition:, when the frequency of the radiated mode in the frame of the emitting electron equals the Doppler shifted frequency of the electric field of the DL (DL wave frequencyωDL≈ 0). For relativistic DLs, as are applicable to pulsar magnetospheres, the emission is expected to be beamed under an angleθ≈γ−1and the frequency of emission boosted (ω≈kDLυ(1 −υcosθ/c)−1≈γ2kDLυ).

Linear acceleration emission in a strong electric field

We consider linear acceleration emission by particles accelerated in a self-consistent longitudinal wave of superluminal phase speed and calculate the expected optical depth. An application to radio pulsar emission is envisaged.

Radiation from electrostatic double layers in laboratory plasmas

Experiments on the radiation from strong electrostatic double layers are performed in a double plasma machine. The spectra show that double-layer related emission exists next to global plasma phenomena. The author finds evidence for a scattering process at low frequencies, a linear acceleration emission mechanism at frequencies below the ambient plasma frequency and beam-plasma interaction at the anode side of the double layer at frequencies around 300 MHz. The brightness temperature of the emitted radiation is approximately two to three orders of magnitude higher than the temperature of the plasma.

Linear Acceleration Emission: A Detailed Analysis

AbstractAn exact expression is given for the orbit of a charged particle accelerated by a non-propagating electrostatic wave. The corresponding current is obtained, for untrapped particles, as an expansion in Bessel functions, and a Lorentz transform allows us to treat the case of a propagating wave. An appropriate form for the absorption coefficient is derived, and an interesting angular and frequency dependence is revealed when the propagating wave is superluminal. Finally, the possible application to pulsars, particularly in explaining the multicomponent pulse profiles, is discussed briefly.

Coherent Radio-Emission Mechanisms for Pulsars

AbstractCoherent emission mechanisms may be classified as (i) maser mechanisms, attributed to negative absorption by resonant particles in a resistive instability, (ii) a reactive or hydrodynamic instability, or (iii) to emission by bunches. Known coherent emission mechanisms in radio astronomy are plasma emission in solar radio bursts, maser emission in OH and other molecular line sources, electron-cyclotron maser emission from the planets, and pulsar emission. Pulsar radio emission is the brightest of all known coherent emission, and its brightness temperature is close to the maximum conceivable in terms of energy efficiency. Three possible pulsar radio emission mechanisms warrant serious consideration in polar cap models; here these are called coherent curvature emission, relativistic plasma emission, and free electron maser emission, respectively.1.Coherent curvature emission is attributed to emission by bunches. There is a fundamental weakness in existing theoretical treatments which do not allow for any velocity dispersion of the particles. There is no satisfactory mechanism for the formation of the required bunches, and were such bunches to form they would quickly lose their ability to emit coherently due to the curvature of the field lines.2.Relativistic plasma emission is a multistage emission process involving the generation of plasma turbulence and the partial conversion of this turbulence into escaping radiation. In pulsars the dispersion characteristics of the relativistic electron-positron plasma determines the form of the turbulence, which may be in either longitudinal waves or Alfvèn-like waves. Various instabilities have been suggested to produce turbulence, and a streaming instability is one possibility. Alternatively, in a detailed model proposed by Beskin et al. (1988) the instability depends intrinsically on the curvature of the field lines, and in a theory discussed by Kazbegi et al. (1988), a cyclotron instability generates the turbulence relatively far from the neutron star.3.Free electron maser emission or linear acceleration emission requires an oscillating electric field, postulated to be due to a large amplitude electrostatic wave. A recent analysis of this mechanism (Rowe 1992) shows that it allows emission in two different regimes that provide a possible basis for the interpretation of core and conal emission in pulsars. Effective maser emission seems to require Lorentz factors smaller than other constraints allow.Other suggested theories for the emission mechanism include one that arises from a loophole in the proof that curvature absorption cannot be negative, and another that involves a closed “electrosphere” in which the radio emission is attributed to emission by bunches formed as a result of pair production due to a primary charge accelerated towards the star by its Coulomb field.

A Linear Acceleration Emission Mechanism

AbstractA theory of linear acceleration emission, with possible application to pulsars, is developed. The equation of motion for a relativistic charged particle accelerated in the field of a longitudinal wave is solved to first order in the electric potential and used to derive the first order current. The power emitted and the absorption coefficient are calculated and the conditions under which masing can occur are discussed.

Amplified linear acceleration emission applied to pulsars

The theory of amplified linear acceleration emission (ALAE) due to relativistic electrons accelerated by a parallel electric field is developed and discussed as a possible radio emission mechanism for pulsars. The emisssion from a singel particle is primarily curvature emission at ..omega.. or approx. =100) and (b) large-amplitude electric oscillations on a time scale of less than about a microsecond, which should lead to microsecond fine-structure in the observed emission.« less