The interests of most of our members center on understanding physical processes, many of which share commonalities between neutron stars and black holes. Indeed, most members contributed to several of the six original topics. Our program is now organized around four cross-cutting physical themes with four thematic challenges.

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• T1: Radiative processes. How do neutron stars generate coherent radio waves in pulsars and FRBs?

Extreme sources can be observed across the entire electromagnetic spectrum, as well as via gravitational waves, neutrinos, and cosmic rays. Novel mechanisms producing γ-rays and neutrinos are actively explored. There is strong current interest in coherent radio emission from FRBs and pulsars, as well as in understanding how this emission escapes from its source. As described above, we have made significant progress in identifying new coherent emission mechanisms and testing often competing paradigms using first-principles numerical simulations. Specific examples include emission powered by magnetic reconnection, driven two-stream instabilities, and magnetized shock waves. We will continue to explore these mechanisms with increasing realism, including fully three-dimensional simulations and the incorporation of relevant radiative physics. We also aim to systematize nonlinear propagation effects and understand their observational consequences. The overarching goal is to develop these models to the point where competing scenarios can be decisively discriminated through observations.

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• T2: Magnetized flows. What is the global flow of mass, energy, angular momentum and magnetic flux around compact objects?

Neutron stars and black holes are surrounded by extensive regions through the global flows of mass, energy, angular momentum and electrical current, involving disks, jets, winds and magnetospheres. A central emphasis of the collaboration continues to be the development of global models of these flows. Magnetic field strength and spatial coherence play a central role in shaping the dynamics of accretion disks and any resulting outflows (winds and jets). The collaboration will focus on exploring the amplification through instabilities and dynamo processes and transport of magnetic fields in accretion disks of different geometries through both local and global simulations. We will also continue to investigate the role of kinetic effects in shaping the large-scale dynamics of accretion and magnetospheric flows, building on our successes with direct kinetic simulations and the incorporation of the new resistivity prescription into fluid formulations.

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• T3: Extreme magnetic field. How does magnetic evolution within the neutron star connect to outbursts above its surface?

As the association of FRB with magnetars has become stronger, there is now much more attention to the properties and evolution of super-Schwinger magnetic field within the cores, crusts and magnetospheres of young neutron stars. A key research direction is understanding the macroscopic dynamics of neutron star interiors and their implications for observable phenomena, including pulsar glitches and magnetar outbursts. These studies are enabled by novel computational tools developed by the collaboration that couple micro- and macro-scale dynamics. Synergy with magnetospheric simulations will enable complete, testable models of magnetar outbursts and provide improved constraints on the neutron star equation of state from X-ray observations of pulsars. The research also has potential implications for exploring axion physics in neutron star environments.

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• T4: Particle acceleration. How are particles accelerated to high enough energies to account for observed PeV γ-rays and neutrinos and ∼ 300 EeV cosmic rays?

In collisionless relativistic plasmas, dissipation commonly leads to particle acceleration rather than heat. Many new approaches to particle acceleration from suprathermal electrons to UHECRs are being investigated involving reconnection, shocks, shear flows, turbulence, direct acceleration, and their non-trivial interplay. These have immediate application to new observations of AGN, XRB, PWN, and beyond. One of the goals will be to connect continuously improving models of particle acceleration at shocks with observations of interacting wind binaries, large-scale jets, and PeV sources. Another key objective is to integrate our understanding of particle acceleration under conditions relevant to jet sheaths with the interpretation of spatially resolved images of synchrotron emission from astrophysical jets. Transport of energetic particles is an equally important challenge; we will build on our recent developments in studying the excitation of beam–plasma instabilities and particle confinement in magnetized turbulence, and address outstanding puzzles related to pulsar TeV halos and X-ray filaments.