2024 Simons Collaboration on Extreme Electrodynamics of Compact Sources Annual Meeting

The first annual meeting of the Simons Collaboration on Extreme Electrodynamics of Compact Sources was held at the Simons Foundation in New York February 29 through March 1, with 91 in person participants and about ten zoom attendees. In addition to the eight speakers — the director, the deputy director and all six principal investigators — collaboration co-investigators and collaborators, fellows and students were present and heavily involved in the meeting.

The meeting began with a scientific overview by director Roger Blandford (Stanford). This talk set the stage for the meeting by explaining how observations of neutron stars and black holes were taking electrodynamics — both classical and quantum — into new territory. Magnetic field strength far above the Schwinger field are being seen from magnetars and potential differences as high as 100 ZV are inferred around massive black holes and in magnetars. Although there is no evidence, yet, that new basic physics is needed, new theoretical approaches are required to confront fresh observational discoveries. The collaboration is encouraging alternative approaches as the fastest way to down-select theoretical models and three examples of such debates — involving the magnetosphere around the black hole in M87, the evolution of electromagnetic field in electric zones and the acceleration of ultra-high-energy cosmic rays — were described.

Deputy director Sasha Philippov (Maryland) then followed with a summary of the collaboration’s ambitious plans to develop and use various computational approaches. He considered three levels of approximation — force-free electrodynamics, magnetohydrodynamics and kinetics, which are each appropriate for different classes of problem and where there are impressive increases in capability, especially through incorporating general relativity, quantum electrodynamics and radiative effects. Recently developed (largely by members of the collaboration) open-source toolkits were advertised. Their capabilities were exhibited through their recent applications to magnetospheres of pulsars and black holes, as well as magnetic explosions in magnetars. Special emphasis was given to modeling of pulsed TeV emission from Vela pulsar, discovered in 2023. The talk concluded with a preview of future computational challenges involving magnetospheres of magnetars, coronae of accretion disks and radiative magnetized turbulence.

The next talk, given by PI Yuri Levin (Columbia), described recent research on the physics and evolution of the magnetic field inside neutron stars which should be at least in the peta-Gauss range and could be much larger. These fields exert a major influence on the structure, thermal and rotational properties of the stars, which may be made manifest by observations of large irregularities and timing noise in the neutron star rotation frequency and possible surface wave modes. Particular emphasis was given to describing the different ways that quantized vortex lines and magnetic field might interact beneath the surfaces. This was linked to recent developments in theoretical nuclear physics. The magnetic field in the crust, and perhaps in the core, undergoes Hall drift and this opens up the possibility of giant “Hall waves.” This, in turn, may account for the large starquakes that have been associated with fast radio bursts. Recent ideas connecting Hall waves with the timing noise correlated with pulse profile changes were emphasized. The physics of the neutron star crusts could also be made manifest as periodic gravitational wave signals. The talk set a perfect stage for subsequent extremely fruitful discussions (including in satellite meetings) with members of the nuclear physics community invited to participate in the meeting.

The interior magnetic field of a neutron star underlies the structure and properties of its surrounding magnetosphere and this was the subject of PI Chris Thompson’s (Toronto) presentation. He highlighted the evidence for the extraordinary large field strengths found in magnetars and discussed their large influence on the propagation of X-rays and g-rays and the presence of extreme birefringence. He described the various mechanisms that have been proposed to account for magnetic outbursts associated with soft gamma repeaters. Much of the talk concerned the electrodynamics of magnetospheres around quiescent magnetars, highlighting the two competing theoretical mechanisms for sustaining electric current through electron-positron pair production. Polarization effects are very important and now observable through NASA’s Imaging X-ray Polarimetry Explorer (IXPE). These considerations provide a context for interpreting post-outburst radio emission from known galactic magnetars. They also pertain to the explanation of cosmologically distant fast radio bursts, which have become a very active topic of theoretical and observational research and are widely argued to be associated with magnetars.

PI Anatoly Spitkovsky (Princeton) followed this talk with a summary of recent progress in understanding how regular pulsars — neutron stars with surface magnetic field strength in the GG to TG range — “shine.” Although pulsars were discovered through their radio emission, and most of their detailed observations arise in this band, they are most luminous as gamma ray sources. Interpreting the periodic emission in gamma rays, originating from just beyond the light cylinder, has been the key to relating the observations to the electrodynamical features of an oblique, rotating magnetic dipole as described by force-free and kinetic simulations. These calculations highlight the central role of reconnecting current sheet in powering the observed high-energy emission. Current sheet reconnection is seen as a central process. By contrast, the radio emission appears to originate from much closer to the stellar surface, and plausible mechanisms, involving radiation of electromagnetic waves by time-dependent inhomogeneous electron-positron pair cascades, have recently been developed.

The second day was begun by PI Amir Levinson (Tel Aviv), who discussed black hole electrodynamics. He introduced the subject describing the rich phenomenology associated with stellar and massive black holes and the challenges that this presents to theory and simulations. These have been brought into focus by observations of gamma ray bursts, active galactic nuclei, tidal disruption events and galactic superluminal sources. Many of these phenomena are ultimately powered by black hole rotation which is tapped by magnetic flux threading the event horizon. However, the consequences are multiscale and interface strongly with the orbiting accretion disks, their X-ray emitting active coronae and outflows and especially their relativistic jets, which are over-represented in the observations when they point towards us and are Doppler-boosted. Recent work using fluid and kinetic simulations describing the interplay of inflowing plasma and active electromagnetic field in the magnetosphere surrounding the black hole was emphasized. In addition, the problems of pair creation and gas entrainment, which connect strongly to the observations were discussed.

PI Ellen Zweibel (Wisconsin) presented a general introduction to the various types of magnetic dynamo processes thought to be present in our cosmic sources and which underlie many of the investigations that interface more directly with the observations. The distributions range from pictorial and topological, involving elementary operations styled “stretch,” “twist,” “fold” and “reconnection,” and are driven by buoyancy, differential rotation and electromagnetic stress associated with a mean magnetic field. The interpretation in terms of magnetic helicity was highlighted. More detailed kinetic investigations, especially involving effects of reconnection and plasma pressure anisotropy, were also discussed. The extension of classical dynamo theory to new regimes involving relativistic flows and nuclear matter, as expected in mergers of neutron stars, are an exciting frontier. Recent work describing a dynamo appearing in the process of Kelvin-Helmholtz instability was highlighted. A major challenge to progress is to connect local descriptions with global outcomes which can be imaged in disks, jets and pulsar wind nebulae.

The final talk by PI Tsvi Piran (Hebrew University) emphasized the enormous, upcoming opportunities for putting theoretical investigations of neutron star and black hole electrodynamics to observational test and discriminating between them. The observations are both direct, as in the impressive Event Horizon Telescope images (and movies to come) of the black hole in M87, and indirect, for example in the burgeoning ample of detailed observations of fast radio bursts, with all of this research being conducted in the public eye. Some of the sources are transient, which requires rapid follow-up protocols to be implemented throughout the electromagnetic, gravitational wave and neutrino windows, as exemplified by gamma ray bursts, where the original burst lasts for seconds, while the afterglows can persist for years. Some sources are recurrent, like quasars, and require survey approaches. Many of the new and upcoming facilities will open up novel diagnostic approaches with outcomes that range from the assured to the highly optimistic. SCEECS investigators have a large role to play in utilizing this opportunity to the full.

Overall, this, our first annual meeting, was seen as a great success by collaboration members and our guests. From the formal talks, through the questions that followed, to the breaktime conversations, to the more detailed reports, much from junior investigators, in the three satellite meetings, it was clear that there is high enthusiasm for this “rapidly moving field.” There were well-attended posters, including a novel outreach utility, and these provided further opportunities for interaction. Overall, there was a breaking down of barriers between subdisciplines, revealing new bonds and a mutual desire to strengthen connections between the Simons Collaboration on Extreme Electrodynamics of Compact Sources and the larger research community. It was especially gratifying to see the connection between the nuclear physics and light source communities strengthening. We thank the Simons Foundation for the opportunity it has given us all.

Roger Blandford
Stanford University

Scientific Overview
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The development of classical electromagnetism and quantum electrodynamics are highlights of nineteenth and twentieth century physics, respectively. Recent, remarkable discoveries, involving neutron stars and black holes, are taking electrodynamics into unfamiliar and “extreme” territory, requiring new theoretical approaches. Examples include 100 GT magnetic fields surrounding neutron stars (and interior fields perhaps a hundred times greater), the production of radio waves with effective temperatures of 10^40 K, gravitational wave sources with powers as high as 10^49 W, the emission of neutrinos and gamma rays with energies in the PeV range and the acceleration of individual cosmic rays to ~ZeV energy (~100 J), perhaps involving EMFs as large as 10^23 V, generated by spinning, black holes.

The Simons Collaboration on Extreme Electrodynamics of Compact Sources was formed to address these challenges using theoretical, computational, observational and experimental approaches and to help choose between many different approaches to these problems. Recent developments in this rapidly growing field will be summarized as an introduction to the talks which will follow.
 

Sasha Philippov
University of Maryland

Computational Perspective
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Over the last decade, numerical simulations emerged as the primary tool to understand the behavior of electromagnetic fields and relativistic plasmas around neutron stars and black holes. At the fluid level, general relativistic magnetohydrodynamics (GRMHD) and force-free electrodynamics (GRFFE) codes provide a global view of these systems. At the kinetic level, particle-in-cell codes have been augmented to include the propagation of high-energy photons emitted by relativistic particles, as well as the creation and annihilation of electron-positron pairs. In this talk, Sasha Philippov will highlight successes in applying these techniques to understanding the radiation of pulsars across the electromagnetic spectrum and the formation of jets and flaring mechanisms of black holes. Philippov will finish by describing future steps in extending these techniques to magnetospheres of magnetars, developing continuum methods for studying relativistic plasmas, and plans for studying general-relativistic plasmas using non-ideal multi-fluid simulations.
 

Yuri Levin
Columbia University

Magnetized Neutron Stars
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The magnetic fields outside neutron stars are measured to range from ~kiloTesla to ~100 GigaTesla. The fields inside neutron stars could be as much as a hundred times larger than this. These magnetic fields exert a major influence on the structure, thermal evolution and dynamics, as well as act as a source and transducer of observed explosions. The physical description of neutron stars involves the application and generalization of condensed matter physics up to densities beyond that of nuclear matter. Recent progress and future challenges will be summarized.
 

Chris Thompson
CITA

Quantum Aspects of Magnetar Electrodynamics
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Magnetars are neutron stars whose magnetic fields reach petaGauss strengths, 10–100 times the Schwinger field. Detectable outside the Milky Way, the activity of magnetars has been closely monitored in nearby galactic sources, including the emission of fast radio bursts and giant gamma-ray flares. The underlying mechanism is therefore more directly constrained than in supernovae or in other types of gamma ray bursters. The super-Schwinger magnetic field outside the star supports intense electric currents and strongly modifies the most basic QED processes, introducing new sources of plasma and electromagnetic collisionality. More than one self-consistent plasma state has been identified theoretically; X-ray and infrared measurements of quiescent magnetars are consistent with an emitting electron-positron gas of transrelativistic energy and moderate optical depth. Direct imprints of super-Schwinger magnetic fields can be found in the hard X-ray spectrum of annihilating pairs and the splitting of X-rays during bright outbursts. Polarization measurements provide additional probes of magnetic birefringence. Finally, regulation of the electron density by QED effects can leave an imprint on the radio and gamma-ray spectrum of a large-amplitude electromagnetic pulse that escapes a magnetar.
 

Anatoly Spitkovsky
Princeton University

Radiation from Neutron Stars

Magnetospheric emission plays an important role in the observational appearance of neutron stars. Magnetospheric plasma is responsible for both the coherent and incoherent forms of pulsed nonthermal emission, plays a role in the thermal heating of the surface, and determines the multimessenger signals such as the leptonic and hadronic particle fluxes from neutron stars and electromagnetic counterparts of gravitational waves from mergers. Historically, the modeling of magnetospheric emission has been hampered by the lack of reliable magnetospheric solutions that incorporate plasma production. Such solutions, obtained with fully kinetic particle-in-cell techniques, have become possible in recent years. They highlight the importance of relativistic reconnection in the current sheet in the outer magnetosphere, which can produce both incoherent gamma-rays and coherent “giant pulse” radio emission. Time-dependent pair production near the surface shows signatures of the long-thought polar coherent radio emission. Particle acceleration in the current sheet beyond the light cylinder may also be responsible for multi-TeV pulsed emission from some pulsars. Anatoly Spitkovsky will review the status of current models of magnetospheric emission and highlight future steps and opportunities.
 

Amir Levinson
Tel Aviv University

Black Hole Electrodynamics
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The magnetospheres of spinning black holes are commonly thought to be the power source of disparate high-energy astrophysical phenomena, observed on stellar as well as galactic scales. Key processes include the generation of relativistic jets, episodic reconnection events and formation of magnetized coronae. The magnetic energy dissipated in the jet, magnetospheric current sheets and turbulent corona, is ultimately converted to the radio-to-gamma-ray (and conceivably neutrino) emission observed in these systems. The large separation of kinetic and MHD scales renders self-consistent calculations infeasible and introduces great challenges for modeling magnetospheric processes. Some key questions addressed by SCEECS are: What is the origin of the radio-loud/radio-quiet dichotomy? What is the primary dissipation mechanism of relativistic magnetized jets? How is the plasma needed to maintain the inner magnetosphere force-free injected? What is the nature of accretion disk coronae, and can they effectively accelerate protons and emit neutrinos? What is the underlying microphysics of accretion flows? Several key aspects of black hole electrodynamics will be discussed, and some recent progress in the study of plasma dynamics of black hole magnetospheres and jets will be described.
 

Ellen Zweibel
University of Wisconsin

Magnetized Accretion Disks
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When material falls into the gravitational potential well of a compact body — be it an isolated neutron star or black hole, a merger or a failed supernova explosion — it almost inevitably encounters an angular momentum barrier, settles into a disk and reaches the central source only when it has shed its angular momentum excess. Accretion torques are almost certainly turbulent or magnetic in nature, with the latter being responsible for the powerful collimated jets observed to emanate from many disks as well as for the turbulent torques. Because magnetic fields also control the heating and cooling of disk plasma, they are implicated in the formation of disk coronae and in producing the radiation spectrum of disk plasma. Understanding how strong, coherent, magnetic fields develop in the extreme environments characterizing black hole and neutron star disks is therefore critically important and will be the focus of this talk.
 

Tsvi Piran
Hebrew University of Jerusalem

Observational Future
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Much of the interest in extreme electrodynamics of compact sources has been driven by remarkable, recent discoveries. The near-term observational prospects for further discovery are bright and this presents an opportunity to make predictions on the basis of quite different theoretical models which will then be adopted or discarded. SCEECS members are engaged in this activity which will be described in the context of active galactic nuclei, fast radio bursts, gamma ray bursts, ultra high energy cosmic rays and other sources.