arXiv subjects

James M. Stone

Publications and source records attributed to James M. Stone.

At least 37 records · Page 2Linked to original sources

arXiv2025

Tunable megawatt-scale sub-20 fs visible pulses from a fiber laser source

Ultrafast laser pulses that are both tunable in wavelength and very short in duration are essential tools in fields ranging from biomedical imaging to ultrafast spectroscopy. While resonant dispersive-wave emission in gas-filled hollow-core fibers is a powerful technique for generating such pulses, it has traditionally required complex and expensive pump laser systems. In this work, we present a more compact and accessible alternative that combines gain-managed nonlinear amplification with resonant dispersive-wave emission. Our system produces sub-20 femtosecond pulses tunable from 400 nm to beyond 700 nm, with energies up to 39 nJ and peak powers exceeding 2 MW, operating at a 4.8 MHz repetition rate. This compact and efficient laser source opens new avenues for deploying resonant dispersive-wave-based technologies for broader scientific and industrial applications.

physics.optics↗

arXiv2025

Mahakala: a Python-based Modular Ray-tracing and Radiative Transfer Algorithm for Curved Space-times

We introduce Mahakala, a Python-based, modular, radiative ray-tracing code for curved space-times. We employ Google's JAX framework for accelerated automatic differentiation, which can efficiently compute Christoffel symbols directly from the metric, allowing the user to easily and quickly simulate photon trajectories through non-Kerr spacetimes. JAX also enables Mahakala to run in parallel on both CPUs and GPUs. Mahakala natively uses the Cartesian Kerr-Schild coordinate system, which avoids numerical issues caused by the pole in spherical coordinate systems. We demonstrate Mahakala's capabilities by simulating 1.3 mm wavelength images (the wavelength of Event Horizon Telescope observations) of general relativistic magnetohydrodynamic simulations of low-accretion rate supermassive black holes. The modular nature of Mahakala allows us to quantitatively explore how different regions of the flow influence different image features. We show that most of the emission seen in 1.3 mm images originates close to the black hole and peaks near the photon orbit. We also quantify the relative contribution of the disk, forward jet, and counter jet to 1.3 mm images.

astro-ph.HE↗

arXiv2025

Global Simulations of Gravitational Instability in Protostellar Disks with Full Radiation Transport II. Locality of Gravitoturbulence, Clumpy Spirals, and Implications for Observable Substructure

Spiral perturbations in a gravitationally unstable accretion disk regulate disk evolution through angular-momentum transport and heating and provide an observational signature of gravitational instability (GI). We use global 3D simulations to systematically characterize and understand these spiral perturbations. The spiral perturbations and the resulting transport are overall insensitive to the cooling type, with the exception that radiative cooling, especially in the optically thick regime, reduces the amplitude of temperature perturbations. Spiral perturbations are localized around corotation, allowing transport to be approximated by a local $α$ viscosity to zeroth order in aspect ratio ($H/R$), but only after averaging over multiple orbits in time and/or multiple scale heights in space. Meanwhile, large-amplitude perturbations from strong gravitoturbulence can cause $\mathcal O(α^{1/2})$ deviation in the cooling rate of the disk. We develop empirical prescriptions for the angular-momentum transport, heating, and cooling in a gravitoturbulent disk that capture the deviation from a viscous, unperturbed disk to first order in $H/R$ and $α^{1/2}$. The spiral perturbations in saturated gravitoturbulence are clumpy, with dense clumps forming through the nonlinear coupling between multiple modes at different $m$. Observationally, the clumpy gravitoturbulence produced by saturated GI can be mistaken with observational noise or embedded companions, especially under finite resolution. Meanwhile, grand-design spirals with $m$-fold symmetry may be uncommon among disks in saturated gravitoturbulence, and we speculate that they may instead be a signature of recently triggered or decaying GI.

astro-ph.EP↗

arXiv2024

Resonance-free deep ultraviolet to near infrared supercontinuum generation in a hollow-core antiresonant fibre

Supercontinuum generation in the ultraviolet spectral region is challenging in solid-core optical fibres due to solarization and photodarkening. Antiresonant hollow-core fibres have overcome this limitation and have been shown to guide ultraviolet light at sufficient intensity for ultraviolet spectral broadening through nonlinear optics in the filling gas. However, their ultraviolet guidance is usually limited by discontinuities caused by the presence of high-loss resonance bands. In this paper, we report on resonance-free supercontinuum generation spanning from the deep ultraviolet to the near infrared achieved through modulation instability in an argon-filled antiresonant hollow-core fibre. The fibre was directly fabricated using the stack-and-draw method with a wall thickness of approximately 90 nm, enabling continuous spectral coverage from the deep ultraviolet to the near infrared. We also report numerical simulations to investigate the supercontinuum bandwidth and the factors limiting it, finding that the overall dispersion landscape, and associated group-velocity matching of cross-phase modulation interactions, is the dominant constraint on spectral extension.

physics.optics↗

arXiv2024

Performance-Portable Binary Neutron Star Mergers with AthenaK

We introduce an extension to the AthenaK code for general-relativistic magnetohydrodynamics (GRMHD) in dynamical spacetimes using a 3+1 conservative Eulerian formulation. Like the fixed-spacetime GRMHD solver, we use standard finite-volume methods to evolve the fluid and a constrained transport scheme to preserve the divergence-free constraint for the magnetic field. We also utilize a first-order flux correction (FOFC) scheme to reduce the need for an artificial atmosphere and optionally enforce a maximum principle to improve robustness. We demonstrate the accuracy of AthenaK using a set of standard tests in flat and curved spacetimes. Using a SANE accretion disk around a Kerr black hole, we compare the new solver to the existing solver for stationary spacetimes using the so-called "HARM-like" formulation. We find that both formulations converge to similar results. We also include the first published binary neutron star (BNS) mergers performed on graphical processing units (GPUs). Thanks to the FOFC scheme, our BNS mergers maintain a relative error of $\mathcal{O}(10^{-11})$ or better in baryon mass conservation up to collapse. Finally, we perform scaling tests of AthenaK on OLCF Frontier, where we show excellent weak scaling of $\geq 80\%$ efficiency up to 32768 GPUs and $74\%$ up to 65536 GPUs for a GRMHD problem in dynamical spacetimes with six levels of mesh refinement. AthenaK achieves an order-of-magnitude speedup using GPUs compared to CPUs, demonstrating that it is suitable for performing numerical relativity problems on modern exascale resources.

astro-ph.HE↗

arXiv2024

Evolution of Supernova Remnants in a Cloudy Multiphase Interstellar Medium

We investigate the evolution of supernova remnants (SNRs) in a two-phase cloudy medium by performing a series of high-resolution (up to $Δx\approx0.01\,\mathrm{pc}$), 3D hydrodynamical simulations including radiative cooling and thermal conduction. We aim to reach a resolution that directly captures the shock-cloud interactions for the majority of the clouds initialized by the saturation of thermal instability. In comparison to the SNR in a uniform medium with the volume filling warm medium, the SNR expands similarly (following $\propto t^{2/5}$) but sweeps up more mass as the cold clouds contribute before shocks in the warm medium become radiative. However, the SNR in a cloudy medium continuously loses energy after shocks toward the cold clouds cool, resulting in less hot gas mass, thermal energy, and terminal momentum. Thermal conduction has little effect on the dynamics of the SNR but smooths the morphology and modifies the internal structure by increasing the density of hot gas by a factor of $\sim 3-5$. The simulation results are not fully consistent with many previous 1D models describing the SNR in a cloudy medium including a mass loading term. By direct measurement in the simulations, we find that, apart from the mass source, the energy sink is also important with a spatially flat cooling rate $\dot{e}\propto t^{-11/5}$. As an illustration, we show an example 1D model including both mass source and energy sink terms (in addition to the radiative cooling in the volume filling component) that better describes the structure of the simulated SNR.

astro-ph.GA↗

arXiv2024

Global Simulations of Gravitational Instability in Protostellar Disks with Full Radiation Transport. I. Stochastic Fragmentation with Optical-depth-dependent Rate and Universal Fragment Mass

Fragmentation in a gravitationally unstable accretion disk can be an important pathway for forming stellar/planetary companions. To characterize quantitatively the condition for and outcome of fragmentation under realistic thermodynamics, we perform global 3D simulations of gravitationally unstable disks at various cooling rates and cooling types, including the first global simulations of gravitational instability that employ full radiation transport. We find that fragmentation is a stochastic process, with the fragment generation rate per disk area $p_{\rm frag}$ showing an exponential dependence on the parameter $β\equivΩ_K t_{cool}$, where $Ω_K$ is the Keplerian rotation frequency and $t_{cool}$ is the average cooling timescale. Compared to a prescribed constant $β$, radiative cooling in the optically thin/thick regime makes $p_{\rm frag}$ decrease slower/faster in $β$; the critical $β$ corresponding to $\sim 1$ fragment per orbit is $\approx$3, 5, 2 for constant $β$, optically thin, and optically thick cooling, respectively. The distribution function of the initial fragment mass is remarkably insensitive to disk thermodynamics. Regardless of cooling rate and optical depth, the typical initial fragment mass is $m_{frag} \approx 40 M_{tot}h^3$, with $M_{tot}$ being the total (star+disk) mass and $h=H/R$ being the disk aspect ratio. Applying this result to typical Class 0/I protostellar disks, we find $m_{frag}\sim 20 M_J$, suggesting that fragmentation more likely forms brown dwarfs. Given the finite width of the $m_{frag}$ distribution, forming massive planets is also possible.

astro-ph.EP↗

arXiv2024

Magnetized Accretion onto and Feedback from Supermassive Black Holes in Elliptical Galaxies

We present three-dimensional magnetohydrodynamic (MHD) simulations of the fueling of supermassive black holes in elliptical galaxies from a turbulent cooling medium on galactic scales, taking M87* as a typical case. We find that the mass accretion rate is increased by a factor of $\sim 10$ compared with analogous hydrodynamic simulations. The scaling of $\dot{M} \sim r^{1/2}$ roughly holds from $\sim 10\,\mathrm{pc}$ to $\sim 10^{-3}\,\mathrm{pc}$ ($\sim 10\, r_\mathrm{g}$) with the accretion rate through the event horizon being $\sim 10^{-2}\, M_\odot\,\mathrm{yr^{-1}}$. The accretion flow on scales $\sim 0.03-3\,\mathrm{kpc}$ takes the form of magnetized filaments. Within $\sim 30\,\mathrm{pc}$, the cold gas circularizes, forming a highly magnetized ($β\sim 10^{-3}$) thick disk supported by a primarily toroidal magnetic field. The cold disk is truncated and transitions to a turbulent hot accretion flow at $\sim0.3\,\mathrm{pc}$ ($10^3\,r_\mathrm{g}$). There are strong outflows towards the poles driven by the magnetic field. The outflow energy flux increases with smaller accretor size, reaching $\sim 3\times10^{43}\,\mathrm{erg\,s^{-1}}$ for $r_\mathrm{in}=8\,r_\mathrm{g}$; this corresponds to a nearly constant energy feedback efficiency of $η\sim0.05-0.1$ independent of accretor size. The feedback energy is enough to balance the total cooling of the M87/Virgo hot halo out to $\sim 50$ kpc. The accreted magnetic flux at small radii is similar to that in magnetically arrested disk models, consistent with the formation of a powerful jet on horizon scales in M87. Our results motivate a subgrid model for accretion in lower-resolution simulations in which the hot gas accretion rate is suppressed relative to the Bondi rate by $\sim (10r_\mathrm{g}/r_\mathrm{B})^{1/2}$.

astro-ph.HE↗

arXiv2024

AthenaK: A Performance-Portable Version of the Athena++ AMR Framework

We describe AthenaK: a new implementation of the Athena++ block-based adaptive mesh refinement (AMR) framework using the Kokkos programming model. Finite volume methods for Newtonian, special relativistic (SR), and general relativistic (GR) hydrodynamics and magnetohydrodynamics (MHD), and GR-radiation hydrodynamics and MHD, as well as a module for evolving Lagrangian tracer or charged test particles (e.g., cosmic rays) are implemented using the framework. In two companion papers we describe (1) a new solver for the Einstein equations based on the Z4c formalism and (2) a GRMHD solver in dynamical spacetimes also implemented using the framework, enabling new applications in numerical relativity. By adopting Kokkos, the code can be run on virtually any hardware, including CPUs, GPUs from multiple vendors, and emerging ARM processors. AthenaK shows excellent performance and weak scaling, achieving over one billion cell updates per second for hydrodynamics in three-dimensions on a single NVIDIA Grace Hopper processor and with a typical parallel efficiency of 80% on 65536 AMD GPUs on the OLCF Frontier system. Such performance portability enables AthenaK to leverage modern exascale computing systems for challenging applications in astrophysical fluid dynamics, numerical relativity, and multimessenger astrophysics.

astro-ph.IM↗

arXiv2024

Optical absorption spectrum reveals gaseous chlorine in anti-resonant hollow core fibres

We have observed unexpected spectral attenuation of ultraviolet light in freshly drawn hollow core optical fibres. When the fibre ends are left open to atmosphere, this loss feature dissipates over time. The loss matches the absorption spectrum of gaseous (molecular) chlorine and, given enough time, the transmission spectrum of the fibre recovers to that expected from the morphological structure of the fibre. Our measurements indicate an initial chlorine concentration of 0.45 $μ$ mol/cm$^{3}$ in the hollow core, equivalent to 1.1 mol% Cl$_{2}$ at atmospheric pressure.

physics.optics↗

arXiv2024

The three-dimensional structure of black hole accretion flows within the plunging region

We analyse, using new analytical models and numerical general relativistic magnetohydrodynamic simulations, the three-dimensional properties of accretion flows inside the plunging region of black hole spacetimes (i.e., at radii smaller than the innermost stable circular orbit). These simulations are of thick discs, with aspect ratios of order unity $h/r \sim 1$, and with a magnetic field geometry given by the standard low-magnetization "SANE" configuration. This work represents the first step in a wider analysis of this highly relativistic region. We show that analytical expressions derived in the "thin disc" limit describe the numerical results remarkably well, despite the large aspect ratio of the flow. We further demonstrate that accretion within this region is typically mediated via spiral arms, and that the geometric properties of these spiral structures can be understood with a simple analytical model. These results highlight how accretion within the plunging region is fundamentally two dimensional in character, which may have a number of observational implications. We derive a modified theoretical description of the pressure within the plunging region which accounts for turbulent heating and may be of use to black hole image modelling.

astro-ph.HE↗

arXiv2024

Tearing-mediated reconnection in magnetohydrodynamic poorly ionized plasmas. I. Onset and linear evolution

In high-Lundquist-number plasmas, reconnection proceeds via onset of tearing, followed by a nonlinear phase during which plasmoids continuously form, merge, and are ejected from the current sheet (CS). This process is understood in fully ionized, magnetohydrodynamic plasmas. However, many plasma environments, such as star-forming molecular clouds and the solar chromosphere, are poorly ionized. We use theory and computation to study tearing-mediated reconnection in such poorly ionized systems. In this paper, we focus on the onset and linear evolution of this process. In poorly ionized plasmas, magnetic nulls on scales below $v_{\rm A,n0}/ν_{\rm ni0}$, with $v_{\rm A,n0}$ the neutral Alfvén speed and $ν_{\rm ni0}$ the neutral-ion collision frequency, will self-sharpen via ambipolar diffusion. This sharpening occurs at an increasing rate, inhibiting the onset of reconnection. Once the CS becomes thin enough, however, ions decouple from neutrals and thinning of the CS slows, allowing tearing to onset in a time of order $ν_{\rm ni0}^{-1}$. We find that the wavelength and growth rate of the mode that first disrupts the forming sheet can be predicted from a poorly ionized tearing dispersion relation; as the plasma recombination rate increases and ionization fraction decreases, the growth rate becomes an increasing multiple of $ν_{ni0}$ and the wavelength becomes a decreasing fraction of $v_{\rm A,n0}/ν_{\rm ni0}$.

astro-ph.SR↗

arXiv2024

Guidance of ultraviolet light down to 190 nm in a hollow-core optical fibre

We report an anti-resonant hollow core fibre with ultraviolet transmission down to 190 nm, covering the entire UV-A, UV-B and much of the UV-C band. Guidance from 190 - 400 nm is achieved apart for a narrow high loss resonance band at 245 - 265 nm. The minimum attenuation is 0.13 dB/m at 235 nm and 0.16 dB/m at 325 nm. With an inscribed core diameter of ~ 12 $μ$m, the fibre's bend loss at 325 nm was 0.22 dB per turn for a bend radius of 3 cm at 325 nm.

physics.optics↗

arXiv2023

Dissipation of AGN jets in a clumpy interstellar medium

Accreting supermassive black holes (SMBHs) frequently power jets that interact with the interstellar/circumgalactic medium (ISM/CGM), regulating star-formation in the galaxy. Highly supersonic jets launched by active galactic nuclei (AGN) power a cocoon that confines them and shocks the ambient medium. We build upon the models of narrow conical jets interacting with a smooth ambient medium, to include the effect of dense clouds that are an essential ingredient of a multiphase ISM. The key physical ingredient of this model is that the clouds along the supersonic jet-beam strongly decelerate the jet-head, but the subsonic cocoon easily moves around the clouds without much resistance. We propose scalings for important physical quantities -- cocoon pressure, head & cocoon speed, and jet radius. We obtain, for the first time, the analytic condition on clumpiness of the ambient medium for the jet to dissipate within the cocoon and verify it with numerical simulations of conical jets interacting with a uniform ISM with embedded spherical clouds. A jet is defined to be dissipated when the cocoon speed exceeds the speed of the jet-head. We compare our models to more sophisticated numerical simulations, direct observations of jet-ISM interaction (e.g., quasar J1316+1753), and discuss implications for the Fermi/eROSITA bubbles. Our work also motivates effective subgrid models for AGN jet feedback in a clumpy ISM unresolved by the present generation of cosmological galaxy formation simulations.

astro-ph.GA↗

arXiv2023

A Global 3-D Simulation of Magnetospheric Accretion: I. Magnetically Disrupted Disks and Surface Accretion

We present a 3-D ideal MHD simulation of magnetospheric accretion onto a non-rotating star. The accretion process unfolds with intricate 3-D structures driven by various mechanisms. First, the disc develops filaments at the magnetospheric truncation radius ($R_T$) due to magnetic interchange instability. These filaments penetrate deep into the magnetosphere, form multiple accretion columns, and eventually impact the star at $\sim$30$^o$ from the poles at nearly the free-fall speed. Over 50% (90%) of accretion occurs on just 5% (20%) of the stellar surface. Second, the disc region outside $R_T$ develops large-scale magnetically dominated bubbles, again due to magnetic interchange instability. These bubbles orbit at a sub-Keplerian speed, persisting for a few orbits while leading to asymmetric mass ejection. The disc outflow is overall weak because of mostly closed field lines. Third, magnetically-supported surface accretion regions appear above the disc, resembling a magnetized disc threaded by net vertical fields, a departure from traditional magnetospheric accretion models. Stellar fields are efficiently transported into the disc region due to above instabilities, contrasting with the ``X-wind'' model. The accretion rate onto the star remains relatively steady with a 23% standard deviation. The periodogram reveals variability occurring at around 0.2 times the Keplerian frequency at $R_T$, linked to the large-scale magnetic bubbles. The ratio of the spin-up torque to $\dot{M}(GM_*R_T)^{1/2}$ is around 0.8. Finally, after scaling the simulation, we investigate planet migration in the inner protoplanetary disc. The disc driven migration is slow in the MHD turbulent disc beyond $R_T$, while aerodynamic drag plays a significant role in migration within $R_T$.

astro-ph.SR↗

arXiv2023

Damping of MHD Turbulence in A Partially Ionized Medium

The coupling state between ions and neutrals in the interstellar medium plays a key role in the dynamics of magnetohydrodynamic (MHD) turbulence, but is challenging to study numerically. In this work, we investigate the damping of MHD turbulence in a partially ionized medium using 3D two-fluid (ions+neutrals) simulations generated with the AthenaK code. Specifically, we examine the velocity, density, and magnetic field statistics of the two-fluid MHD turbulence in different regimes of neutral-ion coupling. Our results demonstrate that when ions and neutrals are strongly coupled, the velocity statistics resemble those of single-fluid MHD turbulence. Both the velocity structures and kinetic energy spectra of ions and neutrals are similar, while their density structures can be significantly different. With an excess of small-scale sharp density fluctuations in ions, the density spectrum in ions is shallower than that of neutrals. When ions and neutrals are weakly coupled, the turbulence in ions is more severely damped due to the ion-neutral collisional friction than that in neutrals, resulting in a steep kinetic energy spectrum and density spectrum in ions compared to the Kolmogorov spectrum. We also find that the magnetic energy spectrum basically follows the shape of the kinetic energy spectrum of ions, irrespective of the coupling regime. In addition, we find large density fluctuations in ions and neutrals and thus spatially inhomogeneous ionization fractions. As a result, the neutral-ion decoupling and damping of MHD turbulence take place over a range of length scales.

astro-ph.GA↗

arXiv2023

Implementation of chemistry in the Athena++ code

Chemistry plays a key role in many aspects of astrophysical fluids. Atoms and molecules are agents for heating and cooling, determine the ionization fraction, serve as observational tracers, and build the molecular foundation of life. We present the implementation of a chemistry module in the publicly available magneto-hydrodynamic code Athena++. We implement several chemical networks and heating and cooling processes suitable for simulating the interstellar medium (ISM). A general chemical network framework in the KIDA format is also included, allowing the user to easily implement their own chemistry. Radiation transfer and cosmic-ray ionization are coupled with chemistry and solved with the simple six-ray approximation. The chemical and thermal processes are evolved as a system of coupled ODEs with an implicit solver from the CVODE library. We perform and present a series of tests to ensure the numerical accuracy and convergence of the code. Many tests combine chemistry with gas dynamics, including comparisons with analytic solutions, 1D problems of the photo-dissociation regions and shocks, and realistic 3D simulations of the turbulent ISM. We release the code with the new public version of Athena++, aiming to provide a robust and flexible code for the astrochemical simulation community.

astro-ph.GA↗

arXiv2023

Toward Horizon-scale Accretion Onto Supermassive Black Holes in Elliptical Galaxies

We present high-resolution, three-dimensional hydrodynamic simulations of the fueling of supermassive black holes in elliptical galaxies from a turbulent medium on galactic scales, taking M87* as a typical case. The simulations use a new GPU-accelerated version of the Athena++ AMR code, and span more than 6 orders of magnitude in radius, reaching scales similar to the black hole horizon. The key physical ingredients are radiative cooling and a phenomenological heating model. We find that the accretion flow takes the form of multiphase gas at radii less than about a kpc. The cold gas accretion includes two dynamically distinct stages: the typical disk stage in which the cold gas resides in a rotationally supported disk and relatively rare chaotic stages ($\lesssim 10\%$ of the time) in which the cold gas inflows via chaotic streams. Though cold gas accretion dominates the time-averaged accretion rate at intermediate radii, accretion at the smallest radii is dominated by hot virialized gas at most times. The accretion rate scales with radius as $\dot{M}\propto r^{1/2}$ when hot gas dominates and we obtain $\dot{M}\simeq10^\mathrm{-4}-10^\mathrm{-3}\,M_\odot\,\mathrm{yr^{-1}}$ near the event horizon, similar to what is inferred from EHT observations. The orientation of the cold gas disk can differ significantly on different spatial scales. We propose a subgrid model for accretion in lower-resolution simulations in which the hot gas accretion rate is suppressed relative to the Bondi rate by $\sim (r_\mathrm{g}/r_{\rm Bondi})^{1/2}$. Our results can also provide more realistic initial conditions for simulations of black hole accretion at the event horizon scale.

astro-ph.HE↗