2025 Simons Collaboration on Extreme Electrodynamics of Compact Sources Annual Meeting

The Simons Collaboration on Extreme Electrodynamics of Compact Sources, SCEECS, held its second annual meeting at the Simons Foundation in New York on February 27–28, 2025. There were 112 invited participants and several zoom participants from around the world, including all eight Principal Investigators, eight out of nine Co-Investigators (with the ninth being on maternity leave), and three out of seven Advisory Board members. Researchers from the fusion plasma, light source, astrophysics, and cosmology communities were also heavily engaged. The meeting was structured around five one hour quite pedagogic talks presented by Co-Is on Thursday followed by three presentations by colleagues from outside the collaboration on Friday. Although progress on theoretical problems was emphasized, the rapidly evolving observational and experimental context featured heavily in these discussions. Excellent posters were showcased and the education and outreach program were represented. The meeting at the Foundation was followed by a 1.5-day satellite session, where collaboration members and guests discussed black holes and neutron stars in panels and short talks.

Cosmologically-distant fast radio bursts, FRB, with sub-microsecond temporal structure and radio brightness temperatures as high as 1040 K were heavily discussed (Beloborodov, Gralla, Yuan). There is much circumstantial evidence that many FRB are associated with magnetars —neutron stars with magnetic fields as high as 100 GT, well above the Schwinger field. There has been considerable progress in developing competing paradigms for the evolution of immense magnetic flares caused either by the sudden development of magnetic instability at the stellar surface or following internal starquakes. According to one school, the radio emission originates near the surface, in the inner magnetosphere, and escapes along evacuated channels avoiding well-studied plasma absorption processes. The alternative view is that coherent radio waves are emitted in the outer magnetosphere, in an outflowing wind. There are two main types of wave: one propagating along the background magnetic field, like an Alfven mode; the other propagating more like a vacuum electromagnetic wave. The electrodynamics has been explored in a “force-free” electrodynamic limit where the inertia of the plasma is ignored, using a relativistic MHD formalism, where the plasma is approximated as a fluid and using a kinetic “Particle in Cell” description where individual electrons, positrons, and ions are followed. The ways in which these waves can evolve and interconvert is now much better understood, and this has led to much more prescriptive expectations which should be confronted by imminent observations.

The behavior of plasma in a magnetar magnetosphere introduces many body processes that go well beyond traditional quantum electrodynamics, in a similar fashion to nuclear and solid state physics expanding atomic physics. Although this regime is not directly accessible, future particle accelerator experiments, involving intense and energetic electrons interacting with coherent emission from X-ray lasers, promise to create conditions that exhibit many of the crucial features of magnetospheres, including exceeding an effective Schwinger field (Fiuza). This is of special interest to the collaboration because the kinetic codes that have been developed for astrophysics can be adapted for application to laboratory experiments that create instabilities, shocks, and reconnection and confronted with increasingly sophisticated diagnostic measurements.

There has been much progress in understanding the nuclear and condensed matter physics of neutron stars which as well as underlying the prime movers of FRBs is essential for understanding regular radio pulsars and accreting stars observed as X-ray and gamma ray sources (Caplan). Recent molecular dynamic simulations have been used to propose a quite radical view of the crusts which admits shear strains as large as ~ 0.1. The emerging description of dislocations, defects, and plastic flow is quite different from terrestrial metallurgical experience. The implications for observed quasiperiodic signals as well as transport properties are immediate, and so these results were heavily scrutinized. Simulations with greater dynamic range are eagerly awaited. The interaction with quantized vortex lines and the evolution of superconducting magnetic field, as presciently described fifty years ago, is also being actively revisited. This, in turn, has major implications for understanding how large an interior magnetic field can form in a nascent neutron star and how it might evolve over the lifetime of as magnetar.

Turning to black holes, much of the discussion was stimulated by our evolving understanding of the region around the black holes in M87 and the galactic center as observed by the Event Horizon Telescope (Anantua). Polarization, and higher frequency observations together with additional baselines and the prospect of space-based telescopes are driving this line of study. Despite what has been learned there are still very different paradigms in play, for example if the positive charges in the emitting regions are mostly positrons or protons, that can describe the emission of the observed millimeter waves. The new observations promise to break these degeneracies.

One feature of sources like M87 is the development of strong (though still collisionless) MHD turbulent zones where relativistic electrons can be accelerated and emit synchrotron radiation as well as scatter X-rays and gamma-rays. This has been a very active area of investigation over the past few years (Boldyrev). The large-scale drivers can be shear flares and current sheets, as with the relativistic jets that emanate from black holes or differential rotation as happens in accretion disk coronae. Often, the environments are magnetically dominated. The wave spectra and 3D particle distribution functions that result are quite sensitive to these drivers and the resulting observed radiation can be used to understand the mechanisms at work.

Extreme electromagnetic processes, powered by compact sources are the most commonly invoked explanations for the observation of the cosmic rays with energies up to ~300 EeV or ~50J (Globus). Recent observations point to a heavy composition with rigidity up to ~ 5-10 EV and relatively local sources that exhibit a dipole distribution on the sky. Relativistic jets associated with active galactic nuclei, gamma ray bursts of many types may be responsible and new electromagnetic acceleration mechanisms are being described. Alternatively, a hierarchy of shock fronts from supernova explosions, through galactic winds and culminating in cluster accretion shocks may be at work. Future progress requires better understanding of particle interactions in atmospheric showers at center of mass energies beyond the reach of current accelerators. An exciting, and much discussed, recent development, relevant to understanding the source of the highest energy cosmic rays, is the report of a ~ 200 PeV muon neutrino.

SCEECS thanks the Simons Foundation for its ongoing support, great hospitality, efficient organization, and strong engagement in the exciting scientific questions that fascinate us all.

Richard Anantua
University of Texas, San Antonio

Hidden Figures – Towards EHT-Scale Signatures of Positrons and Axions

General relativistic magnetohydrodynamic simulations evolve realistic plasma inflows and outflows in jet/accretion flow/black hole (JAB) systems and, coupled with postprocessors with general relativistic ray tracers, enable us to test radiating particle energetics and composition against horizon-scale data for active galactic nuclei (AGN) such as M87. Using the “Observing” JAB Simulations methodology, we include positrons into ray-traced models of SANE and MAD HARM simulations of M87 with turbulent heating to find distinguishable Faraday-effect-mediated positron signatures. We then lay the groundwork for applying the “Observing” JAB Simulations methodology to detecting signatures of superradiant axion production around supermassive black holes given astrophysical confounds and a novel application of primordial black holes accreting positronium in KORAL simulations in the first few seconds of the universe under magnetorotational instability to grow into a substantial fraction of dark matter in the present day.
 

Andrei Beloborodov
Columbia University

Fast Radio Bursts and Extreme Electromagnetism

Fast radio bursts (FRBs) are the strongest observed electromagnetic waves in the universe. They are likely generated by magnetized neutron stars and carry information about unusual and extreme phenomena. This talk will describe attempts to understand the bizarre physics of ultrastrong electromagnetic waves, their vulnerability, their impact on a surrounding medium, and a possible mechanism of their generation. In addition to FRBs from isolated neutron stars, similar electromagnetic phenomena can be triggered in tight binary systems and accompany gravitational waves from their mergers.
 

Stanislav Boldyrev
University of Wisconsin, Madison

Alfvénic Turbulence and Particle Acceleration in a Relativistic Magnetically Dominated Plasma

The properties of strong Alfvénic turbulence in a magnetized relativistic collisionless plasma are examined. This type of turbulence is common in astrophysical plasmas and plays a significant role in energizing plasma particles. At smaller scales, it creates intermittent current structures and can be affected by tearing instability. In magnetically dominated environments, Alfvénic turbulence acts as an effective mechanism for the nonthermal acceleration of particles. The relative strength of turbulent magnetic fluctuations, compared to the guiding magnetic field, impacts the energy spectrum of the accelerated particles. Furthermore, it influences the distribution of the pitch angles of the particles, which may affect the radiative signatures of astrophysical objects.
 

Matt Caplan
Illinois State University

Diffusion and Vortices in Strongly Coupled Plasmas

The mechanical properties of neutron star crust ultimately influence astrophysical emissions. Elastic moduli and transport coefficients in the solid are therefore essential microphysics input for modeling and understanding observations. Diffusion coefficients in strongly coupled plasmas are especially important for characterizing crustal elasticity but are challenging to calculate. Using molecular dynamics simulations, we have characterized diffusion in Yukawa crystals near melting, and in addition, we will show how vortex pinning and unpinning in the crust may ultimately involve diffusive processes requiring new coupled codes.
 

Frederico Fiúza
Instituto Superior Técnico, Lisbon

Extreme Plasma Physics: From Astrophysics to the Laboratory

Extraordinary discoveries related to neutron stars and black holes are opening new scientific frontiers that challenge our current knowledge of plasmas under extreme conditions and demand multi-disciplinary approaches. Recent advances in high-intensity lasers and particle beams are now enabling the generation of relevant high-energy-density and strong-field conditions in the laboratory. This is creating unique opportunities to probe the basic processes that control plasma dynamics and radiation emission in these extreme environments and benchmark theoretical and numerical models. Frederico Fiúza will discuss some of the main challenges, recent progress and exciting opportunities in this field.
 

Noémie Globus
IA-UNAM, Stanford University

Magnetic Keys to a Cosmic Puzzle

What are the sources of ultra-high-energy cosmic rays (UHECR), the most energetic particles we observe? The field of UHECR has been greatly advanced in the Auger and Telescope Array era, but the answer to this question is still unknown. Noémie Globus will discuss two magnetic “keys” to solving this cosmic puzzle. The first “key” is the role magnetic fields play in the acceleration processes, from the ordered magnetic fields around neutron stars and black holes to the turbulent magnetic fields at astrophysical shocks. The second “key” is the role magnetic fields play in cosmic-ray propagation in our galaxy and in the extragalactic medium. Globus will show how solving these keys will possibly unlock the door to our understanding of future UHECR observations, which may help us to identify the sources.
 

Sam Gralla
University of Arizona

Strong-Field Limits of Plasma Dynamics

Many of the most spectacular astrophysical phenomena are connected with plasma in strong magnetic fields. In this extreme field limit, all conceivable descriptions of the plasma reduce to the universal, simpler theory of force-free electrodynamics. However, the underlying plasma microphysics is essential for astrophysical modeling, since it impacts the boundary conditions for the force-free solution and determine the small corrections that influence observables. Sam Gralla will give a general overview of the strong-field limit and discuss two general approaches (top-down and bottom-up) for incorporating the underlying microphysics. Gralla will then describe a staggered perturbation approach to modeling systems in the strong-field limit, which are implemented using a two-fluid description. This approach has the potential to provide global magnetosphere models without the need for global particle-in-cell simulations
 

Yajie Yuan
Washington University, St. Louis

Nonlinear Plasma Wave Dynamics in Magnetar Magnetospheres

Magnetars are a special type of neutron star with the most intense magnetic fields known — reaching up to 100 gigatesla. They are usually young and active, powering a range of energetic transient events, from X-ray bursts to giant flares in soft gamma-rays, and possibly even fast radio bursts. One proposed trigger behind some of the transients is crustal activities (like starquakes) that launch plasma waves into the magnetosphere, most notably the Alfvén waves — a transverse wave that propagates along the magnetic field. The waves undergo several interesting nonlinear processes in the magnetosphere, potentially powering the bright, multiwavelength signals we observe. In this talk, Yajie Yuan will outline our systematic study of these nonlinear processes and show how a deeper understanding of plasma-wave dynamics can shed light on the extreme electrodynamics that govern magnetars and their remarkable emissions.