Talks

Readings:
The basics of gravitational wave theory, by Flanagan & Hughes, https://iopscience.iop.org/article/10.1088/1367-2630/7/1/204
Gravitational-Wave Tests of General Relativity with Ground-Based Detectors and Pulsar-Timing Arrays, by Yunes et al. https://arxiv.org/pdf/2408.05240
Tests of General Relativity with GW150914, LIGO Collaboration, https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116.221101
Tests of general relativity with the binary black hole signals from the LIGO-Virgo catalog GWTC-1, LIGO Collaboration, https://journals.aps.org/prd/abstract/10.1103/PhysRevD.100.104036
Probing the black hole metric: Black hole shadows and binary black-hole inspirals, by Psaltis et al., https://journals.aps.org/prd/abstract/10.1103/PhysRevD.103.104036
New horizons for fundamental physics with LISA, https://link.springer.com/article/10.1007/s41114-022-00036-9

For the first time in history, we have captured images of black holes, allowing us to study their event horizons—the very boundaries beyond which nothing escapes. This achievement, realized through the Event Horizon Telescope (EHT), was the result of two decades of innovation, collaboration, and technological advancements. By linking radio telescopes across the globe, we created an Earth-sized observatory capable of resolving the shadows cast by supermassive black holes. These images have confirmed Einstein’s predictions, tested gravity in extreme conditions, and provided unprecedented insights into black hole physics. But this is just the beginning. Future advancements in imaging techniques, additional telescopes, and space-based interferometry will sharpen our view and allow us to explore black holes across the universe. As we push the boundaries of observation, we will uncover even more mysteries about these enigmatic cosmic giants and their role in shaping galaxies.

With the ongoing transition toward exascale computing to tackle a range of open questions via numerical simulations, the development of GPU-optimized codes has become essential. In this talk, I will highlight the key features of AsterX, a novel open-source, modular, GPU-accelerated general relativistic magnetohydrodynamic (GRMHD) code for fully dynamical spacetimes in 3D Cartesian coordinates. Built for exascale applications, AsterX integrates with CarpetX, the new driver for the Einstein Toolkit, leveraging AMReX for block-structured adaptive mesh refinement (AMR). The code employs the flux-conservative Valencia formulation for GRMHD, and uses high-resolution shock capturing schemes to ensure accurate hydrodynamic modeling. Alongside discussions on the ongoing code development, I will also present the results of comprehensive 1D, 2D, and 3D GRMHD tests conducted on OLCF's Frontier supercomputer, highlighting AsterX's performance gains through subcycling in time and demonstrating its scaling efficiency across thousands of nodes.

In certain scenarios, the accreted angular momentum of plasma onto a black hole could be low; however, how the accretion dynamics depends on the angular momentum content of the plasma is still not fully understood. We present three-dimensional, general relativistic magnetohydrodynamic simulations of low angular momentum accretion flows around rapidly spinning black holes (with spin $a = +0.9$). The initial condition is a Fishbone-Moncrief (FM) torus threaded by a large amount of poloidal magnetic flux, where the angular velocity is a fraction $f$ of the standard value. For $f = 0$, the accretion flow becomes magnetically arrested and launches relativistic jets but only for a very short duration. After that, free-falling plasma breaks through the magnetic barrier, loading the jet with mass and destroying the jet-disk structure. Meanwhile, magnetic flux is lost via giant, asymmetrical magnetic bubbles that float away from the black hole. The accretion then exits the magnetically arrested state. For $f = 0.1$, the dimensionless magnetic flux threading the black hole oscillates quasi-periodically. The jet-disk structure shows concurrent revival and destruction while the gas efficiency at the event horizon changes accordingly. For $f \geq 0.3$, we find that the dynamical behavior of the system starts to approach that of a standard accreting FM torus. Our results thus suggest that the accreted angular momentum is an important parameter that governs the maintenance of a magnetically arrested flow and launching of relativistic jets around black holes.

It has been proposed recently that Inverse Compton scattering of soft photons by pairs acceleration in reconnecting current sheets that form during MAD states, can be the source of the TeV emission detected in M87. In this talk I’ll argue that synchrotron emission by ions accelerated in the current sheet is expected to be the dominant source of the GeV emission observed. The analysis is based on 3D, radiative PIC simulations of magnetic reconnection in pair-ion plasma, under conditions anticipated in M87 during MAD states.

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.