Abstract
Magnetic reconnection---a fundamental plasma physics process, where magnetic field lines of opposite polarity annihilate---is invoked in astrophysical plasmas as a powerful mechanism of nonthermal particle acceleration, able to explain fast-evolving, bright high-energy flares. Near black holes and neutron stars, reconnection occurs in the ``relativistic'' regime, in which the mean magnetic energy per particle exceeds the rest mass energy. This review reports recent advances in our understanding of the kinetic physics of relativistic reconnection: (1) Kinetic simulations have elucidated the physics of plasma heating and nonthermal particle acceleration in relativistic reconnection; (2) The physics of radiative relativistic reconnection, with its self-consistent interplay between photons and reconnection-accelerated particles---a peculiarity of luminous, high-energy astrophysical sources---is the new frontier of research; (3) Relativistic reconnection plays a key role in global models of high-energy sources, both in terms of global-scale layers, as well as of reconnection sites generated as a byproduct of local magnetohydrodynamic instabilities. We summarize themes of active investigation and future directions, emphasizing the role of upcoming observational capabilities, laboratory experiments, and new computational tools.
*We are grateful to NSF, NASA, the Department of Energy, and the Simons Foundation for funding support. We are extremely grateful to Hayk Hakobyan, Benoit Cerutti, Louise Willingale and Hantao Ji for contributing to this review: Hayk Hakobyan was instrumental in writing Section 6.2.1 and gave extensive feedback on Figure 7 and Table 1; Benoit Cerutti led the drafting of Section 6.2.3; Louise Willingale and Hantao Ji provided important contributions to Section 8.3. We thank the editor, Eliot Quataert, for insightful comments and suggestions on our manuscript and Luca Comisso, Jens Mahlmann, Emanuele Sobacchi, Navin Sridhar and Hao Zhang for comments and help with figures. We are also grateful to several collaborators and colleagues for many inspiring discussions on relativistic reconnection over the years.
