Pulsar radio emission mechanisms: a critique

Abstract

We consider critically the three most widely favored pulsar radio emission mechanisms: coherent curvature emission (CCE), beam-driven relativistic plasma emission (RPE) and anomalous Doppler emission (ADE). We assume that the pulsar plasma is one dimensional (1D), streaming outward with a bulk Lorentz factor γs ≫ 〈γ〉 − 1 ≳ 1, where 〈γ〉 is the intrinsic spread in the rest frame of the plasma. We argue that the formation of beams in a multi-cloud model is ineffective in the intrinsically relativistic case for plausible parameters, because the overtaking takes too long. We argue that the default choice for the particle distribution in the rest frame is a Jüttner distribution and that relativistic streaming should be included by applying a Lorentz transformation to the rest-frame distribution, rather than the widely assumed relativistically streaming Gaussian distribution. We find that beam-driven wave growth is severely restricted by (a) the wave properties in pulsar plasma, (b) a separation condition between beam and background, and (c) the inhomogeneity of the plasma in the pulsar frame. The growth rate for the kinetic instability is much smaller and the bandwidth of the growing waves is much larger for a Jüttner distribution than for a relativistically streaming Gaussian distribution. No reactive instability occurs at all for a Jüttner distribution. We conclude that none of CCE, RPE and ADE in tenable as the generic pulsar radio emission mechanism for “plausible” assumptions about the pulsar plasma.

Radio Emission of Pulsars. II. Coherence Catalyzed by Cerenkov-Unstable Shear Alfvén Waves

“…This constraint is a familiar one in the case of Langmuir waves; it implies a tension between rapid growth and curvature emission by Langmuir solitons above the plasma cutoff (e.g. Lakoba et al 2018;Melrose et al 2021a). The growth of trapped Alfvén waves is subject to a similar constraint: starting with a conservative growth criterion, we find that the peak of the curvature spectrum is ∼ 10 −2 of the cutoff frequency.…”

“…Very quickly a simple physical picture predicated on charge clumping in an electromagnetic cascade was developed, giving a transparent explanation for the observed linear polarization swings (Radhakrishnan & Cooke 1969;Ruderman & Sutherland 1975). But over five decades, no consensus has emerged on how the required charged soliton structures would develop; theorists have vigorously explored a variety of alternative emission mechanisms (see Melrose et al 2021a for a recent critical review). None of the alternatives have yielded a comparably compelling explanation for the linear polarization behavior that is observed in many, but not all, pulsars (e.g.…”

Single-pulse studies of three millisecond pulsars

“…As alternative mechanisms, relativistic plasma emission, anomalous Doppler emission, linear acceleration emission, and maser emission have been proposed. However, none of these theoretical methods are capable of fully explaining the pulsar emission mechanism (Melrose 1995;Melrose et al 2021). Therefore, single-pulse studies, including singlepulse-polarimetry, of pulsars with a variety of properties are crucial to fully understand pulsar emission physics (Rankin 1986).…”

Repeating fast radio bursts from collapses of the crust of a strange star

“…Their high brightness temperature makes astrophysicists believe that the radio pulse originates from the so-called coherent emission like the radio pulsar does, 1 , 2 although details for such collective radio emissions are still unclear. 3 , 4 Since the first discovery, 5 rapid progress has been made in the FRB field. Along with the increasing FRB number, it was found that some of them are individual events, i.e., do not repeat within a monitor period, while some sources are obviously repeating.…”