MeV Emission from Pulsar Wind Nebulae: Understanding Extreme Particle Acceleration in Highly Relativistic Outflows

The Earth is constantly bombarded from outer space by energetic particles (e.g., electrons e −, positrons e +, protons, nuclei; (1)). Where and how these “cosmic rays” are produced is poorly understood, with various particle types and energies likely originating from different sources. Particularly mysterious is the source of high-energy e ± produced in our Galaxy, especially those responsible for both the high fraction of e + in the GeV cosmic ray lepton spectrum (e.g., (2)) and the e ± and observed excess of microwaves (e.g., (3)) and γ-rays (e.g., (4)) detected towards the Galactic center and bulge. While these particles could be evidence for exotic forms of dark matter (e.g., (5)), they might also be produced by “normal” astrophysical sources such as pulsars (e.g., (6; 7)) – the strongly magnetized, rapidly rotating neutron stars whose rotational energy powers an ultra-relativistic outflow (commonly referred to as a “pulsar wind”) whose interaction with the surrounding medium creates a pulsar wind nebula (PWN; see (8) for a recent review). While the detection of TeV emission from numerous PWNe (e.g., (9; 10)) strongly suggest they contain e ± with PeV or higher energies, how and to what energies these particles are produced is unknown, let alone their dependence on the properties of the pulsar, pulsar wind, and surrounding medium. A major reason for this uncertainty is the lack of information concerning their MeV properties, since the synchrotron emission from the highest energy e ± peaks in this waveband. Only by combining the MeV spectrum of PWNe measured by proposed missions (e.g., AMEGO (11), LOX) with that obtained at lower (primarily radio and X-ray) and higher (TeV) photon energies by current and hopefully future (e.g., SKA, ngVLA, ATHENA, AXIS, LYNX, CTA) facilities is it possible to measure the full spectrum of e ± in these sources. The resultant insights into the underlying acceleration mechanism would significantly impact many areas of astrophysics – from indirect searches for dark matter (as described above) to the origin of cosmic rays to the physics of relativistic outflows observed from active galactic nuclei (AGN), γ-ray bursts (GRBs), and some gravitational wave (GW) events (e.g., (12))