Pulsar Wind Nebulae

Dear Colleagues,

This Special Issue is devoted to the physics and astrophysics of pulsar wind nebulae (PWNe), a class of astrophysical sources that in recent years has gained much relevance in the landscape of high-energy astrophysics. The prototype of these bright non-thermal sources, powered by the highly relativistic wind emanating from highly spinning, strongly magnetized neutron stars, is the Crab Nebula, which is also the only unambiguously identified PeVatron in our Galaxy, namely the only source in which we have direct evidence of the presence of particles accelerated up to the highest energy at which we think protons are accelerated in our galaxy. Recent gamma-ray data from LHAASO hint at an additional emission component appearing in the Crab Nebula spectrum at PeV energies. This might be the first evidence of PeV ions in a pulsar wind. PWNe are leptonic PeVatrons, but are they also sources of PeV hadrons? This question is of particular relevance in a time when a combination of observations and theory casts doubts about the ability of supernova remnant shocks, so far considered the most likely sources of cosmic rays (CRs) at all energies, to reach the PeV energy range.

A related question concerns the mechanism by which particles are accelerated so efficiently in PWNe. These sources are extraordinary accelerators, converting the pulsar wind bulk energy into accelerated particles with efficiency well in excess of 10% and with an acceleration rate close to the theoretical maximum. All this is believed to occur at the magnetized relativistic shock that terminates the pulsar wind, the most hostile environment for the acceleration mechanisms usually invoked in astrophysics. How are particles accelerated so efficiently in PWNe?

Another theoretical and observational challenge has been opened up by the recent detection of the so-called TeV haloes, regions of gamma-ray emission surrounding evolved PWNe, where we are possibly observing a large suppression of particle diffusion. How are these regions produced? How widespread are they? Do they affect the spectrum of electrons and positrons released by PWNe in the interstellar medium? Do they affect, more generally, CR propagation in the Galaxy? These questions are particularly relevant for CR physics and gamma-ray observations and, in order to answer them, it is urgent to better understand particle escape from PWNe. Important information in this respect is expected to come from X-ray observations.

In the next few years, new and upcoming facilities to observe the sky at the highest energies are promising to provide us with the needed data to finally answer the century-old question of the origin of CRs of all species and of all energies, but a better understanding of PWN physics seems to be needed to conclude the quest successfully.

The aim of this Special Issue is to collect current knowledge from theory and observations about all aspects of the physics of PWNe, so as to make the point of where we stand and what kind of efforts are needed to answer the above questions.

Dr. Elena Amato
Prof. Dr. Felix Aharonian
Guest Editors

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• pulsar wind nebulae
• supernova remnants
• relativistic plasmas
• MHD
• particle acceleration
• particle transport
• non-thermal radiation
• cosmic rays
• gamma-rays