Abrupt acceleration of a ‘cold’ ultrarelativistic wind from the Crab pulsar

Abstract

Pulsars are thought to eject electron-positron winds that energize the surrounding environment, with the formation of a pulsar wind nebula. The pulsar wind originates close to the light cylinder, the surface at which the pulsar co-rotation velocity equals the speed of light, and carries away much of the rotational energy lost by the pulsar. Initially the wind is dominated by electromagnetic energy (Poynting flux) but later this is converted to the kinetic energy of bulk motion. It is unclear exactly where this takes place and to what speed the wind is accelerated. Although some preferred models imply a gradual acceleration over the entire distance from the magnetosphere to the point at which the wind terminates, a rapid acceleration close to the light cylinder cannot be excluded. Here we report that the recent observations of pulsed, very high-energy γ-ray emission from the Crab pulsar are explained by the presence of a cold (in the sense of the low energy of the electrons in the frame of the moving plasma) ultrarelativistic wind dominated by kinetic energy. The conversion of the Poynting flux to kinetic energy should take place abruptly in the narrow cylindrical zone of radius between 20 and 50 light-cylinder radii centred on the axis of rotation of the pulsar, and should accelerate the wind to a Lorentz factor of (0.5-1.0) × 10(6). Although the ultrarelativistic nature of the wind does support the general model of pulsars, the requirement of the very high acceleration of the wind in a narrow zone not far from the light cylinder challenges current models.

Simulations of stellar/pulsar-wind interaction along one full orbit

“…This is smaller than the conventional value Γ ∼ 10 4 −10 6 (see Khangulyan et al 2012;Aharonian et al 2012, and references therein), but high enough to reproduce the wind velocity contrast in the bow-like shock region, where instabilities trigger, and to capture important relativistic effects. Similarities and differences between the Γ-value adopted here, a more realistic one, and the non-relativistic case, are discussed in Sect.…”

Phase-resolved energy spectra of the Crab pulsar in the range of 50–400 GeV measured with the MAGIC telescopes

“…Other possible Ansätze to explain the VHE emission include the production of inverse Compton radiation in the unshocked pulsar wind outside the light cylinder by pulsed photons (Aharonian et al 2012;Aharonian & Bogovalov 2003), a striped pulsar wind (Pétri 2011), or the annular gap model presented in Du et al (2012). The two crucial spectral features to establish to test these models are the expected spectral upward-kink in the transition region between the curvature and the hard component, and the detection or exclusion of a terminal cutoff at a few hundred GeV.…”

Constraining Lorentz Invariance Violation Using the Crab Pulsar Emission Observed up to TeV Energies by MAGIC

“…Generally, the origin of VHE pulsar emission up to TeV energies is still debated in the literature (Aharonian et al 2012;Du et al 2012;Hirotani 2013), albeit it is out of the question that inverse Compton scattering must be at play in some way or another (Hirotani 2001;Aleksić et al 2011;Lyutikov et al 2012;Bogovalov 2014). Energy-dependent time drifts of the pulse can then follow those of the illuminating electron population, or the illuminated seed UV and X-ray photons, although this is less plausibly because of the involved Klein-Nishina scattering regime.…”