arXiv subjects

Cora Prather

Publications and source records attributed to Cora Prather.

12 recordsLinked to original sources

arXiv2026

Bridging Scales: Multi-Zone GRMHD-Informed Anisotropic AGN Feedback in Galaxies

We present a galaxy-scale black hole (BH) accretion and feedback model incorporating results from multi-zone general relativistic magnetohydrodynamic (GRMHD) simulations spanning the event horizon to the Bondi radius. The model replaces a fixed suppression of the Bondi rate with a nuclear-environment-dependent factor, $κ^{\rm hot}_{\rm cho}$, set by the local ratio of Bondi to gravitational radius, and distributes outflow mass, kinetic energy, and thermal energy according to the angular structure measured in the large-dynamic-range GRMHD calculations. We apply it to idealized isolated simulations of an M87*-like massive elliptical and a Milky Way/Sgr A*-like galaxy, spanning over three orders of magnitude in BH mass without tuning accretion or feedback efficiencies to match observables. In the M87*-like system, the GRMHD-informed angle-dependent feedback model suppresses late-time stellar mass growth more effectively than the isotropic reference model, despite similar long-term BH accretion rates. Varying the uncertain mass loading of the collimated polar component has a modest impact on BH growth and star formation, indicating that feedback anisotropy matters more than its detailed mass loading. We find that $κ^{\rm hot}_{\rm cho}$ varies with nuclear environment and time, yielding BH growth histories markedly different from fixed-suppression models. For Sgr A*, this variable factor yields an accretion rate more consistent with Event Horizon Telescope (EHT) constraints than a fixed, suppressed Bondi model. These results show that nuclear-environment-dependent accretion and feedback angular structure affect BH-galaxy evolution, requiring subgrid models to include more than a universal Bondi suppression factor and a scalar feedback efficiency.

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arXiv2026

Image Morphology of Misaligned Magnetically Arrested Disks

Horizon scale images probe the plasma dynamics and spacetime in the vicinity of accreting black holes. Interpreting these images requires models that connect image morphology to the structure of the underlying accretion flow. Disk misalignment is expected in many systems, yet most horizon scale image analyses assume that the emitting flow is aligned with the black hole spin. We investigate whether tilted accretion flows produce image features distinguishable from axisymmetric flows and whether their effects can be absorbed by changes in aligned model parameters. Using general relativistic magnetohydrodynamic simulations of tilted magnetically arrested disks spanning black hole spins $a^*=-0.9375$ to $0.9375$ and initial disk tilts $0^\circ$ to $30^\circ$, we generate synthetic images and quantify their morphology through fits to an aligned, semi-analytic emission model. Using this approach, we show that high spin prograde models show little sensitivity to disk tilt, as the inner emitting flow aligns with the black hole spin. In contrast, retrograde tilted disks develop warped, non-axisymmetric inner structure that distorts the inner shadow and strongly alters image morphology. While the aligned semi-analytic model reproduces most prograde images well, retrograde tilted disk images are fit less accurately and yield biased inferred parameters, especially for spin. The image morphologies indicate that tilt affects horizon scale images only when misalignment survives into the near horizon emitting region, motivating image models that include disk tilt as an explicit degree of freedom when alignment is uncertain.

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arXiv2026

Ring Position Angles and Spin in M87* and Sgr A*

Event Horizon Telescope (EHT) images of black holes appear as rings with a brightness asymmetry. Here, we expand on our previous study of the asymmetry magnitude $a_1$ to study the position angle of the peak brightness asymmetry $\mathrm{PA}_1$ in general relativistic magnetohydrodynamic (GRMHD) models. For larger spin magnitudes ($a_{*}>0$ and $a_{*}\lesssim-0.5$), the mean $\mathrm{PA}_1$ falls within $1σ$ of the approaching limb of the black hole, regardless of viewing inclination, disk magnetization, or source. By comparing the $(a_1, \mathrm{PA}_1)$ distribution in M87* observations with models, we demonstrate that we can mildly disfavor low-magnitude spins and strongly disfavor all spin vectors that point toward Earth. The alignment of $\mathrm{PA}_1$ relative to the large-scale jet axis may suggest that M87*'s disk does not have a large tilt. By combining $\mathrm{PA}_1$ with the pattern speed measured in optimistic 2026 M87* video conditions, the EHT can constrain whether M87* is prograde or retrograde with $\sim 84\%$ accuracy. In Sgr A*, we show that a detection of $(a_1, \mathrm{PA}_1)$ could constrain the magnitude and direction of the galactic center spin vector. Finally, if future EHT expansions increase the sample of horizon-scale sources, a simple set of observables (ring diameter, asymmetry magnitude, and asymmetry angle) could enable robust constraints on black hole mass, spin, and inclination.

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arXiv2026

Event Horizon Telescope Pattern Speeds in the Visibility Domain

The Event Horizon Telescope is preparing to produce time sequences of black hole images, or movies. In anticipation, we developed an autocorrelation technique to measure apparent rotational motion using the image-domain pattern speed $Ω_p$. Here, we extend this technique to the visibility domain and introduce the visibility amplitude pattern speed $Ω_{\mathrm{VA}}$. We show that in the Illinois v3 library of EHT source models, $Ω_{\mathrm{VA}}$ depends on the source inclination, black hole mass, black hole spin, accretion state (MAD or SANE), and baseline length, and then provide approximate fits for this dependence. We show that $Ω_{\mathrm{VA}}$ is particularly sensitive to baseline length for MAD (strongly magnetized) models, and that the slope of this dependence can be used to constrain black hole spin. As with $Ω_p$, models predict that $Ω_{\mathrm{VA}}$ is well below the Keplerian frequency in the emission region for all model parameters. This is consistent with the idea that $Ω_{\mathrm{VA}}$ measures an angular phase speed for waves propagating through the emission region. Finally, we identify the information that would be provided by space-based millimeter VLBI such as the proposed BHEX mission.

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arXiv2026

KHARMA: Flexible, Portable Performance for GRMHD

KHARMA (an acronym for "Kokkos-based High-Accuracy Relativistic Magnetohydrodynamics with Adaptive mesh refinement") is a new open-source code for conducting general-relativistic magnetohydrodynamic simulations in stationary spacetimes, primarily of accretion systems. It implements among other options the High-Accuracy Relativistic Magnetohydrodynamics (HARM) scheme, but is written from scratch in C++ with the Kokkos programming model in order to run efficiently on both CPUs and GPUs. In addition to being fast, KHARMA is written to be readable, modular, and extensible, separating functionality into "packages," representing, e.g., algorithmic components or physics extensions. Components of the core ideal GRMHD scheme can be swapped at runtime, and additional packages are included to simulate electron temperature evolution, viscous hydrodynamics, and for designing chained multi-scale "bridged" simulations. This chapter presents the computational environment and requirements for KHARMA, features and design which meet these requirements, and finally, validation and performance data.

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arXiv2026

Iharm3D: Vectorized General Relativistic Magnetohydrodynamics

Iharm3D is an open-source C code for simulating black hole accretion systems in arbitrary stationary spacetimes using ideal general-relativistic magnetohydrodynamics (GRMHD). It is an implementation of the HARM ("High Accuracy Relativistic Magnetohydrodynamics") algorithm outlined in Gammie et al. (2003) with updates as outlined in McKinney & Gammie (2004) and Noble et al. (2006). The code is most directly derived from Ryan et al. (2015) but with radiative transfer portions removed. HARM is a conservative finite-volume scheme for solving the equations of ideal GRMHD, a hyperbolic system of partial differential equations, on a logically Cartesian mesh in arbitrary coordinates.

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arXiv2026

Comparison of Polarized Radiative Transfer Codes used by the EHT Collaboration

Interpretation of resolved polarized images of black holes by the Event Horizon Telescope (EHT) requires predictions of the polarized emission observable by an Earth-based instrument for a particular model of the black hole accretion system. Such predictions are generated by general relativistic radiative transfer (GRRT) codes, which integrate the equations of polarized radiative transfer in curved spacetime. A selection of ray-tracing GRRT codes used within the EHT collaboration is evaluated for accuracy and consistency in producing a selection of test images, demonstrating that the various methods and implementations of radiative transfer calculations are highly consistent. When imaging an analytic accretion model, we find that all codes produce images similar within a pixel-wise normalized mean squared error (NMSE) of 0.012 in the worst case. When imaging a snapshot from a cell-based magnetohydrodynamic simulation, we find all test images to be similar within NMSEs of 0.02, 0.04, 0.04, and 0.12 in Stokes I, Q, U , and V respectively. We additionally find the values of several image metrics relevant to published EHT results to be in agreement to much better precision than measurement uncertainties.

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arXiv2026

Observational distinguishability of the Kerr and Kerr-Hayward metrics to EHT

Astrophysical black holes appear well-represented by the Kerr metric, whose maximal analytic extension contains a ringlike curvature singularity. We study a phenomenological regularized modification of Kerr, the modified Kerr-Hayward metric, which removes curvature blowup at the level of the stationary metric while preserving in detail many features of polarized black hole images now testable by the Event Horizon Telescope (EHT). To establish this, we produce new general relativistic magnetohydrodynamics (GRMHD) simulations of a magnetized plasma in a Kerr-Hayward spacetime, then we extend the EHT analysis framework to perform polarized radiative transfer in this spacetime. We detail our methodology for implementing this modified spacetime into an open-source pipeline. From fluid quantities such as the magnetic flux parameter and jet efficiency, to image quantities such as the polarization pattern and the photon ring structure, our results for the Kerr-Hayward metric appear functionally indistinguishable from the Kerr metric. Our study finds that under the assumptions of a fixed-background GRMHD and polarized radiative transfer pipeline, this regularized phenomenological modification to the Kerr metric can yield observables that are effectively indistinguishable in EHT measurements.

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arXiv2026

Sensitivities of Black Hole Images from GRMHD Simulations

The advent of high-fidelity imaging of supermassive black holes calls for efficient and robust data-analysis methods. In this work, we use $\texttt{Jipole}$, a differentiable, $\texttt{ipole}$-based radiative transfer code, to enable gradient-based analyses of images generated from state-of-the-art general relativistic magnetohydrodynamic (GRMHD) simulations. We compute image sensitivities, i.e., pixel-wise derivatives of the intensity with respect to model parameters, which form the Jacobian of the forward model and define a local map from parameter space to image space. Using these sensitivities in a mock data analysis, we find that GRMHD-based images generate a structured error landscape for parameter fitting, with anisotropies and local minima, making parameter exploration nontrivial but still tractable when guided by gradient information. We characterize this landscape through the Jacobian and assess the feasibility of gradient-based recovery under idealized, blurred, and noisy conditions. Our results show that automatic differentiation-computed image gradients can guide parameter exploration effectively even in the presence of noise. These findings establish a basis for efficient, high-precision model--data comparisons in black hole imaging and motivate the integration of these sensitivities into advanced inference frameworks.

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arXiv2026

Supermassive Black Holes: modelling strongly and weakly magnetised misaligned accretion disks

In this paper, we carry out a numerical study of misaligned accretion disks around spinning supermassive black holes. Here, we conduct a parameter survey covering a range of initial disk misalignment angles ($\mathcal{T}_\mathrm{init}=15^\circ, 45^\circ, 75^\circ$) with either the Magnetically Arrested Disk (MAD) or Standard And Normal Evolution (SANE) magnetic field configurations, using the general-relativistic magnetohydrodynamic (GRMHD) code KHARMA. We find that models in the MAD state can align with the black hole up to $\sim10 \, r_g$, even in extremely misaligned models ($\mathcal{T}_\mathrm{init}=75^\circ$), which has not been seen before. Models without a dynamically important magnetic field remain misaligned up to the black hole, with a maximum disk tilt at $\sim10 \, r_g$, the peak's radial distance from the black hole increases with increasing disk misalignment. However, the maximum disk tilt does not have a linear relationship with the initial disk misalignment, and appears to have a maximum value of $\sim50^\circ$. We also show misaligned disk simulations produced in KHARMA are consistent with other GRMHD codes, for a variety of problems.

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arXiv2026

Polarization Signatures from GRMHD Simulations of Black Hole Accretion

This chapter tells the still-unfolding story of extracting polarization signatures from general relativistic magnetohydrodynamics simulations of accretion disks. In some sense, this effort is premature as there are still very few results of this kind. Much more abundant are phenomenological models. Nevertheless, we feel now is the time to rally the community to this cause. Since the focus of this book is on X-ray polarimetry, we focus exclusively on simulations of accretion onto compact objects. Most of the relevant work so far has been on black hole accretion disks, though neutron stars are also viable targets for X-ray polarimetry. The focus of our chapter is on how X-ray polarimetry coupled with accretion simulations might help us better understand properties of the disks, coronae, and jets that are the dominant components of accreting compact sources. We briefly illustrate the promise of this technique by demonstrating how it has already been used in the case of the Event Horizon Telescope (using radio polarimetry). We also speculate about where this field may be heading in the near future.

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arXiv2026

Observational Properties of Near-Maximally Spinning Supermassive Black Holes

Black holes described by the Kerr metric can have a theoretical maximum dimensionless spin parameter of $a_\bullet = 1$, but several effects may limit the maximum spin parameter in astrophysical systems. We perform general relativistic magnetohydrodynamics simulations of accretion flows around black holes with $a_\bullet = 0.9375$ and $a_\bullet = 0.998$, each corresponding to a proposed astrophysical limit in the literature. We then perform full polarized general relativistic ray-tracing to produce astrophysical movies of these simulations, as can be spatially resolved by the Event Horizon Telescope (EHT) and its extensions. Although many properties of black holes and accretion flows evolve rapidly as $a_\bullet \to 1$, we find that our $a_\bullet=0.9375$ and $a_\bullet=0.998$ simulations are remarkably similar, both in terms of their GRMHD fluid properties and their full-Stokes, time-variable images. This suggests that previous work using simulations with $a_\bullet \approx 0.9375$ may be representative of models with $a_\bullet \gtrsim 0.9375$ in most practical cases. Our calculations suggest that shape and size constraints on the photon ring, enabled by extensions of the EHT into space by missions such as the Black Hole Explorer (BHEX) may be the only practical way to distinguish between models with different spin parameters as $a\to 1$.

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