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

Abstract

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.