Conference

Magnetars produce the brightest detected outbursts in the X-ray and radio bands, offering unique opportunities to probe extreme plasma physics and exotic quantum electrodynamic processes. Magnetospheric reconnection is a suspected mechanism for generating bursts and giant flares. However, modeling the interplay between small-scale reconnection processes and the global structure of magnetar magnetospheres remains a major theoretical and computational challenge. This is due to the multi-scale nature of the problem: resolving localized reconnection while capturing the global dynamics of the star in a single numerical simulation is computationally prohibitive. Global models have primarily used force-free schemes which are unable to capture dissipation, recent advances have made global ideal magnetohydrodynamic models viable but will still lack the explicit resistivity needed to carefully study the reconnection physics. This motivates the use of local resistive relativistic magnetohydrodynamic simulations which probe the details of reconnection in the magnetospheric regime. These local simulations allow for the quantification of magnetic energy dissipation and the role it plays in powering emission due to magnetic reconnection inaccessible to force-free models. Furthermore, the use of ideal global models motivates the careful quantification of numerical dissipation in ideal schemes. We use 1D and 2D tests to quantify the nature of magnetic dissipation in resistive and ideal, relativistic magnetohydrodynamic and force-free-electrodynamic schemes. Our tests, which are agnostic to the form of the effective numerical dissipation operator in ideal schemes, probe both Ohmic dissipation and magnetic reconnection of current structures in the highly magnetized strong guide field regime. These tests characterize exactly how well schemes commonly used to model magnetic dissipation in magnetar and pulsar magnetospheres perform compared to resistive relativistic magnetohydrodynamics. We find Ohmic dissipation in both ideal magnetohydrodynamic and force-free schemes to be subdiffuse, while producing an analogue to the Sweet-Parker regime at low resolutions and an asymptotic reconnection rate at high resolutions. The resistive force-free scheme we test is found to exactly reproduce the Ohmic diffusion and the Sweet-Parker regime, but differs from the full resistive relativistic magnetohydrodynamic result in the asymptotic regime.

Plasmoid-dominated magnetic reconnection is known to convert magnetic energy into heat and kinetic energy and is thought to be closely related to high-energy emission features originating near compact objects. We present preliminary results of high-resolution special-relativistic resistive magnetohydrodynamic simulations of reconnecting 3D current sheets starting from a Harris equilibrium. We focussed on identifying and quantifying flux rope structures and how the properties of produced magnetosonic waves (potentially generating winds around compact objects) rely on the underlying plasma description. We show that while the initial stage does not differ substantially from 2D results, a secondary turbulent reconnection phase can only be studied in 3D.

Observations of luminous black holes in X-ray binaries and Seyfert galaxies show power-law emission, thought to originate from photons that inverse Compton scatter off a hot electron cloud. If the coronal electrons are heated by magnetic dissipation, i.e. reconnection or turbulence, then one might expect to observe direct synchrotron emission in the radio/mm from these electrons. However, because timing studies constrain the X-ray emission to be within ~10 rg of the central black hole, the direct synchrotron emission from this compact volume would be strongly self-absorbed until much further away from BH. In this talk, I will question the de facto definition of the corona as a compact, X-ray-emitting region and shift instead to a paradigm where the corona encompasses multiple layers with distinct spectral components. Motivated by highly-magnetized winds found in GRMHD simulations, I will present a model for such an extended, outflowing corona. I will discuss this model in the context of radio-quiet AGN, where recent observations have demonstrated the presence of compact mm emission.

Tilted magnetically arrested disks (MADs) around black holes—where strong magnetic fields regulate accretion and jets—exhibit striking alignment behavior dictated by black hole spin direction. Using 3D general-relativistic magnetohydrodynamic (GRMHD) simulations of tilted MADs, we find that prograde disks align via a two-stage process: an initial rapid alignment phase, ending at the flux saturation timescale, followed by a slower, spin-independent phase. In contrast, retrograde MADs remain persistently misaligned, with their inner disks precessing four times faster than weakly magnetized systems—a potential explanation for high-frequency quasi-periodic oscillations (QPOs). By analyzing magnetic and hydrodynamic torques within ideal GRMHD, we show that alignment in prograde disks is dominated by electromagnetic stresses from the magnetosphere. However, the same magnetic forces— which always act to align the disk with the black hole spin—are significantly weaker in retrograde disks, allowing opposing hydrodynamic torques to dominate. These results suggest that jets alone may not be sufficient to align MAD disks, instead highlighting the magnetosphere’s crucial role in mediating spin-disk coupling.

What powers the hard, non-thermal X-rays from accreting compact objects has been a longstanding mystery. In my talk, I will address the underlying question of what energizes the particles of the Comptonizing “corona” against the strong inverse Compton (IC) cooling losses with first-principle particle-in-cell simulations of magnetic reconnection subject to IC cooling in magnetically dominated electron-positron plasmas, and in mildly-magnetized electron-ion plasmas. I will also show---using results of global resistive GRMHD simulations of accreting black holes---that the black hole jet sheath is a site of efficient electromagnetic dissipation through processes such as magnetic reconnection and turbulence. The distribution of bulk motions of the radially outflowing plasma along the jet sheath also resembles a Maxwellian distribution with an effective bulk temperature of a few 100 keV, and this could be a candidate for the Comptonizing corona.

A new version of the Athena++ AMR MHD code implemented using the Kokkos library for performance-portability will be described. The code is being used for a variety of problems related to accretion onto (and merger of) compact objects. This talk will focus on results from general relativistic radiation MHD models of accretion in luminous systems such as X-ray binaries and AGN. The models allow observationally important quantities such as the radiative efficiency, variability, and kinetic feedback in winds to be measured over a range of luminosities, from sub-Eddington to highly super-Eddington. A new “cyclic-zoom” approach for modeling such flows over very long time scales will also be briefly introduced.

An alternative interpretation of the EHT observations of M87, involving a strongly magnetized ergomagnetosphere, that expels essentially of the gas supplied at large radius as a jet-collimating MHD wind is described. In this case, the emitting regions are approximated as rapidly evolving flares or sparks. Six generic consequences, that can be sought in existing data, are described.

In this talk I will review my work on relational observables in perturbative quantum gravity and put it in context of quantizing coordinates in gravity. I will then discuss more recent work on quantum reference frames and give some outlook on how these two strands could fit together.

Causal set theory is an approach to quantum gravity where the underlying spacetime is a locally finite poset. Glaser gave a formula for the causal set theory analogue of the d'Alembertian in general dimension (growing out of previous work of Sorkin, Benincasa and Dowker, and Dowker and Glaser). The formula contains rational coefficients. Who can resist trying to find something that these coefficients count -- not me! -- so I will tell you about such a something.