arXiv subjects

Peter Anninos

Publications and source records attributed to Peter Anninos.

At least 19 recordsLinked to original sources

arXiv2026

Strong Polarization Signatures from Magnetically Stabilized Luminous Thin Accretion Disks

We utilize the Monte Carlo radiation transport capabilities of Cosmos++ to explore the polarization and Faraday rotation of radiation emitted from a set of general relativistic radiation magnetohydrodynamic simulations of magnetically stabilized, thin, black hole accretion disks. The guiding question is whether or not the Faraday rotation depolarizes the radiation to such a degree as to be inconsistent with the relatively high polarization measurements coming from the Imaging X-ray Polarimetry Explorer (IXPE). After first confirming that our code reproduces expected polarization results for electron clouds, planar scattering atmospheres, and AGN disks, we demonstrate the polarization and Faraday rotation capabilities using a Novikov-Thorne disk threaded with a purely toroidal magnetic field. We then analyze temporal snapshots from three different simulations of luminous, thin accretion disks threaded with different magnetic field configurations. We find that the effects of Faraday rotation are generally modest over the energy range of interest, since the strongest fields present in these simulations ($\gtrsim 10^8$ G) are mostly hidden beneath the photosphere. We can easily produce polarization degrees $\ge 4$\% for sources seen mostly edge-on. As one example of a relevant target, we compare our results to an IXPE observation of Cyg X-1, finding that we can match the polarization degree quite well, though our polarization angle is rotated approximately $90^\circ$ with respect to the observed data. We speculate on a few possible explanations for this.

astro-ph.HE↗

arXiv2025

r-Process nucleosynthesis from hyperaccreting neutron stars in common envelopes

We investigate nuclear reactions and feedback in hyperaccreting neutron star environments, considering accretion rates in the range 0.3 - $3\times10^4$ $M_\odot$ yr$^{-1}$, typical of short-period compact object binaries in common envelopes. Our mode ls account for weak reactions, neutrino energy loss, nuclear energy release, pair production, degenerate equations of state, and general relativistic hydrodynamics. Depending on accretion rates, these systems can develop both proton and neutron-rich atmospheres with strong convective instabilities linking the neutrino sphere to the outgoing accretion shock inside the radia tion trapping zone. Convection drives nucleons through multiple heating and cooling cycles, with photodisintegration dominat ing during the heating phase and heavy element synthesis during the cooling phase, ejecting material with abundances that dep end on the accretion rate and depth of the final decompression trajectory. The turbulent nature of convective currents is con ducive to creating a wide range of nuclear products through a variety of effects, including NSE freezeout and $r$, $p$ and $\ gamma$ processes. We also observe a novel multi-step process in reheated trajectories, consisting of proton-capture and photo-dissociation reactions operating on $r$-process seeds, producing overall neutron-deficient isotopes. A significant amount of infalling gas experiences high entropy and short (millisecond) freezeout timescales capable of making $r$-process elements w ith high over-abundances through a disequilibrium effect between neutrons and $α$-particles that does not require an excess of neutrons.

astro-ph.HE↗

arXiv2025

X-ray Spectra from General Relativistic RMHD Simulations of Thin Disks

We compare X-ray emission from several general relativistic, multi-frequency, radiation magnetohydrodynamic simulations of thin black hole accretion disks with different accretion rates and spins. The simulations were performed using the M1 closure scheme, resolved with twelve frequency (energy) bins logarithmically spaced from $5 \times 10^{-3}$ to $5 \times 10^3$ keV. We apply a general relativistic Monte Carlo transport code to post-process the simulation data with greater fidelity in frequency resolution and Compton scattering treatment. Despite the relatively few energy bins and Kompaneets approximation to Compton scattering utilized in the M1 method, we find generally good agreement between the methods. Both produce prominent thermal profiles with peaks around 2 - 2.5 keV, where agreement is particularly strong and representative of the soft state. Both also find weaker (lower luminosity) thermally sourced emission extending out to 100 keV due to the hotter innermost regions of the disks. Inverse Compton scattering becomes increasingly effective at hardening spectral outputs with increasing black hole spin, and becomes the dominant mechanism for photons that escape with energies between 10 to several hundred keV. At very high rates of spin the radiation flux in this upscattered component becomes comparable to the thermal flux, a phenomenon typically associated with intermediate states. Beyond $10^4$ keV, we observe faint, free-free emission from hot, optically thin coronal regions developing near the horizon, common to both spinning and nonspinning black holes.

astro-ph.HE↗

arXiv2025

Stability and convergence of nuclear detonations in white dwarf collisions

We investigate the numerical stability of thermonuclear detonations in 1D accelerated reactive shocks and 2D binary collisions of equal mass, magnetized and unmagnetized white dwarf stars. To achieve high resolution at initiation sites, we devised geometric gridding and mesh velocity strategies specially adapted to the unique requirements of head-on collisional geometries, scenarios in which one expects maximum production of iron-group products. We study effects of grid resolution and the limiting of temperature, energy generation, and reactants for different stellar masses, separations, magnetic fields, initial compositions, detonation mechanisms, and limiter parameters across a range of cell sizes from 1 to 100 km. Our results set bounds on the parameter space of limiter amplitudes for which both temperature and energy limiting procedures yield consistent and monotonically convergent solutions. Within these bounds we find grid resolutions of 5 km or better are necessary for uncertainties in total released energy and iron-group products to drop below 10%. Intermediate mass products (e.g., calcium) exhibit similar convergence trends but with somewhat greater uncertainty. These conclusions apply equally to pure C/O WDs, multi-species compositions (including helium shells), magnetized and unmagnetized cores, and either single or multiple detonation scenarios.

astro-ph.HE↗

arXiv2024

Graviton-Photon Oscillations in an Expanding Universe

Through the Gertsenshtein effect, the presence of a large external B-field may allow photons and gravitons to mix in a way that resembles neutrino oscillations and is even more similar to axion-photon mixing. Assuming a background B-field (or any field that behaves like one), we examine the Gertsenshtein mechanism in the context of FLRW expanding universe models, as well as de Sitter space. The conformal invariance of Maxwell's equations and the conformal noninvariance of the Einstein equations preclude the operation of the Gertsenshtein effect at all scales. In general we find for the matter- and radiation-dominated cases, graviton-oscillations are possible only at late conformal times or when the wavelengths are much shorter than the horizon ($kη\gg 1$), but that the time-dependent oscillations eventually damp out in any case. The presence of charged particles additionally damps out the oscillations. For the de Sitter universe, we find that oscillations are possible only at early conformal times ($η\ll H^{-1}$) and for wavelengths short compared to the Hubble radius, but eventually freeze in when wavelengths become longer than the Hubble radius. In principle a Gertsenshtein-like mechanism might influence the balance of particle species in an inflationary phase before freezing in; however, we find that in all our models the mixing length is larger than the Hubble radius. We discuss several possible remedies to this situation.

gr-qc↗

arXiv2023

Multi-Frequency General Relativistic Radiation-Magnetohydrodynamic Simulations of Thin Disks

We present a set of six general relativistic, multi-frequency, radiation magnetohydrodynamic simulations of thin accretion disks with different target mass accretion rates around black holes with spins ranging from non-rotating to rapidly spinning. The simulations use the $\mathbf{M}_1$ closure scheme with twelve, independent frequency (or energy) bins ranging logarithmically from $5\times 10^{-3}$ to $5\times 10^3$ keV. The multi-frequency capability allows us to generate crude spectra and energy-dependent light curves directly from the simulations without a need for special post-processing. While we generally find roughly thermal spectra with peaks around 1 to 4 keV, our high-spin cases showed harder than expected tails for the soft or thermally dominant state. This leads to radiative efficiencies that are up to five times higher than expected for a Novikov-Thorne disk at the same spin. We attribute these high efficiencies to the high-energy, coronal emission. These coronae mostly occupy the effectively optically thin regions near the inner edges of the disks and also cover or sandwich the inner $\sim 15 GM/c^2$ of the disks.

astro-ph.HE↗

arXiv2022

Resolution Study of Thermonuclear Initiation in White Dwarf Tidal Disruption Events

We study the initiation of thermonuclear detonations in tidally disrupted white dwarf stars by intermediate-mass ($10^3 M_\odot$) black holes. The length scales required to resolve the initiation mechanism are not easily reached in three-dimensions, so instead we have devised two-dimensional proxy models which, together with a logarithmic gridding strategy, can adequately capture detonation wave fronts as material undergoes simultaneous compression and stretching from tidal forces. We consider 0.15 and 0.6 solar mass white dwarf stars parameterized by tidal strengths in the range $β=4~\text{to}~23$. High spatial resolution elucidates the manner and conditions leading to thermonuclear detonation, linking the initiation sequence to stellar composition and tidal strength. All of our models suffer sustained detonations triggered by a combination of adiabatic compression, mild thermonuclear preconditioning, and collisional heating, in degrees depending primarily on tidal strength. We find many diagnostics, such as temperature, total released energy, and iron group products, are fairly well-converged (better than 10%) at resolutions below 10 km along the scale height of the orbital plane. The exceptions are intermediate mass transients like calcium, which remain uncertain up to factors of two even at 1 km resolution.

astro-ph.HE↗

arXiv2022

General Relativistic Implicit Monte Carlo Radiation-Hydrodynamics

We report on a new capability added to our general relativistic radiation-magnetohydrodynamics code, Cosmos++: an implicit Monte Carlo (IMC) treatment for radiation transport. The method is based on a Fleck-type implicit discretization of the radiation-hydrodynamics equations, but generalized for both Newtonian and relativistic regimes. A multiple reference frame approach is used to geodesically transport photon packets (and solve the hydrodynamics equations) in the coordinate frame, while radiation-matter interactions are handled either in the fluid or electron frames then communicated via Lorentz boosts and orthonormal tetrad bases attached to the fluid. We describe a method for constructing estimators of radiation moments using path-weighting that generalizes to arbitrary coordinate systems in flat or curved spacetime. Absorption, emission, scattering, and relativistic Comptonization are among the matter interactions considered in this report. We discuss our formulations and numerical methods, and validate our models against a suite of radiation and coupled radiation-hydrodynamics test problems in both flat and curved spacetimes.

astro-ph.IM↗

arXiv2020

Multi-Frequency General Relativistic Radiation-Hydrodynamics with $\bf{M}_1$ Closure

We report on recent upgrades to our general relativistic radiation-magnetohydrodynamics code, Cosmos++, which expands the two-moment, $\bf{M}_1$, radiation treatment from grey to multi-frequency transport, including Doppler and gravitational frequency shifts. The solver accommodates either photon (Bose-Einstein) or neutrino (Fermi-Dirac) statistical distribution functions with absorption, emission, and elastic scattering processes. An implicit scheme is implemented to simultaneously solve the primitive inversion problem together with the radiation-matter coupling source terms, providing stability over a broad range of opacities and optical depths where the interactions terms can be stiff. We discuss our formulations and numerical methods, and validate our methods against a wide variety of test problems spanning optically thin to thick regimes in flat, weakly curved, and strongly curved spacetimes.

astro-ph.IM↗

arXiv2019

Nuclear Ignition of White Dwarf Stars by Relativistic Encounters with Rotating Intermediate Mass Black Holes

We present results from general relativistic calculations of nuclear ignition in white dwarf stars triggered by near encounters with rotating intermediate mass black holes with different spin and alignment parameters. These encounters create thermonuclear environments characteristic of Type Ia supernovae capable of producing both calcium and iron group elements in arbitrary ratios, depending primarily on the proximity of the interaction which acts as a strong moderator of nucleosynthesis. We explore the effects of black hole spin and spin-orbital alignment on burn product synthesis to determine whether they might also be capable of moderating reactive flows. When normalized to equivalent impact penetration, accounting for frame dragging corrections, the influence of spin is weak, no more than 25% as measured by nuclear energy release and mass of burn products, even for near maximally rotating black holes. Stars on prograde trajectories approach closer to the black hole and produce significantly more unbound debris and iron group elements than is possible by encounters with nonrotating black holes or by retrograde orbits, at more than 50% mass conversion efficiency. The debris contains several radioisotopes, most notably Ni56, made in amounts that produce sub-luminous (but still observable) light curves compared to branch-normal SNe Ia.

astro-ph.HE↗

arXiv2019

The Event Horizon General Relativistic Magnetohydrodynamic Code Comparison Project

Recent developments in compact object astrophysics, especially the discovery of merging neutron stars by LIGO, the imaging of the black hole in M87 by the Event Horizon Telescope (EHT) and high precision astrometry of the Galactic Center at close to the event horizon scale by the GRAVITY experiment motivate the development of numerical source models that solve the equations of general relativistic magnetohydrodynamics (GRMHD). Here we compare GRMHD solutions for the evolution of a magnetized accretion flow where turbulence is promoted by the magnetorotational instability from a set of nine GRMHD codes: Athena++, BHAC, Cosmos++, ECHO, H-AMR, iharm3D, HARM-Noble, IllinoisGRMHD and KORAL. Agreement between the codes improves as resolution increases, as measured by a consistently applied, specially developed set of code performance metrics. We conclude that the community of GRMHD codes is mature, capable, and consistent on these test problems.

astro-ph.HE↗

arXiv2018

Divergence-Free Magnetohydrodynamics on Conformally Moving, Adaptive Meshes Using a Vector Potential Method

We present a new method for evolving the equations of magnetohydrodynamics (both Newtonian and relativistic) that is capable of maintaining a divergence-free magnetic field ($\nabla \cdot \mathbf{B} = 0$) on adaptively refined, conformally moving meshes. The method relies on evolving the magnetic vector potential and then using it to reconstruct the magnetic fields. The advantage of this approach is that the vector potential is not subject to a constraint equation in the same way the magnetic field is, and so can be refined and moved in a straightforward way. We test this new method against a wide array of problems from simple Alfven waves on a uniform grid to general relativistic MHD simulations of black hole accretion on a nested, spherical-polar grid. We find that the code produces accurate results and in all cases maintains a divergence-free magnetic field to machine precision.

physics.comp-ph↗

arXiv2018

Relativistic Tidal Disruption and Nuclear Ignition of White Dwarf Stars by Intermediate Mass Black Holes

We present results from general relativistic calculations of the tidal disruption of white dwarf stars from near encounters with intermediate mass black holes. We follow the evolution of 0.2 and $0.6 M_\odot$ stars on parabolic trajectories that approach $10^3$ - $10^4 M_\odot$ black holes as close as a few Schwarzschild radii at periapsis, paying particular attention to the effect tidal disruption has on thermonuclear reactions and the synthesis of intermediate to heavy ion elements. These encounters create diverse thermonuclear environments characteristic of Type I supernovae and capable of producing both intermediate and heavy mass elements in arbitrary ratios, depending on the strength (or proximity) of the interaction. Nuclear ignition is triggered in all of our calculations, even at weak tidal strengths $β\sim 2.6$ and large periapsis radius $R_P \sim 28$ Schwarzschild radii. A strong inverse correlation exists between the mass ratio of calcium to iron group elements and tidal strength, with $β\lesssim 5$ producing predominately calcium-rich debris. At these moderate to weak interactions, nucleosynthesis is not especially efficient, limiting the total mass and outflows of calcium group elements to $< 15$\% of available nuclear fuel. Iron group elements however continue to be produced in greater quantity and ratio with increasing tidal strength, peaking at $\sim 60$\% mass conversion efficiency in our closest encounter cases. These events generate short bursts of gravitational waves with characteristic frequencies 0.1-0.7 Hz and strain amplitudes $0.5\times10^{-22}$ - $3.5\times10^{-22}$ at 10 Mpc source distance.

astro-ph.HE↗

arXiv2018

Relativistic Viscous Radiation Hydrodynamic Simulations of Geometrically Thin Disks: I. Thermal and Other Instabilities

We present results from two-dimensional, general relativistic, viscous, radiation hydrodynamic numerical simulations of Shakura-Sunyaev thin disks accreting onto stellar mass Schwarzschild black holes. We consider cases on both the gas- and radiation-pressure-dominated branches of the thermal equilibrium curve, with mass accretion rates spanning the range from $\dot{M} = 0.01 L_\mathrm{Edd}/c^2$ to $10 L_\mathrm{Edd}/c^2$. The simulations directly test the stability of this standard disk model on the different branches. We find clear evidence of thermal instability for all radiation-pressure-dominated disks, resulting universally in the vertical collapse of the disks, which in some cases then settle onto the stable, gas-pressure-dominated branch. Although these results are consistent with decades-old theoretical predictions, they appear to be in conflict with available observational data from black hole X-ray binaries. We also find evidence for a radiation-pressure-driven instability that breaks the unstable disks up into alternating rings of high and low surface density on a timescale comparable to the thermal collapse. Since radiation is included self-consistently in the simulations, we are able to calculate lightcurves and power density spectra (PDS). For the most part, we measure radiative efficiencies (ratio of luminosity to mass accretion rate) close to 6%, as expected for a non-rotating black hole. The PDS appear as broken power laws, with a break typically around 100 Hz. There is no evidence of significant excess power at any frequencies, i.e. no quasi-periodic oscillations are observed.

astro-ph.HE↗

arXiv2017

Numerical Simulations of a Jet-Cloud Collision and Starburst: Application to Minkowski's Object

We present results of three-dimensional, multi-physics simulations of an AGN jet colliding with an intergalactic cloud. The purpose of these simulations is to assess the degree of "positive feedback," i.e. jet-induced star formation, that results. We have specifically tailored our simulation parameters to facilitate comparison with recent observations of Minkowski's Object (M.O.), a stellar nursery located at the termination point of a radio jet coming from galaxy NGC 541. As shown in our simulations, such a collision triggers shocks which propagate around and through the cloud. These shocks condense the gas and under the right circumstances may trigger cooling instabilities, creating runaway increases in density, to the point that individual clumps can become Jeans unstable. Our simulations provide information about the expected star formation rate, total mass converted to \ion{H}{1}, H$_2$, and stars, and the relative velocity of the stars and gas. Our results confirm the possibility of jet-induced star formation, and agree well with the observations of M.O.

astro-ph.GA↗

arXiv2017

CosmosDG: An hp-Adaptive Discontinuous Galerkin Code for Hyper-Resolved Relativistic MHD

We have extended Cosmos++, a multi-dimensional unstructured adaptive mesh code for solving the covariant Newtonian and general relativistic radiation magnetohydrodynamic (MHD) equations, to accommodate both discrete finite volume and arbitrarily high order finite element structures. The new finite element implementation, called CosmosDG, is based on a discontinuous Galerkin (DG) formulation, using both entropy-based artificial viscosity and slope limiting procedures for regularization of shocks. High order multi-stage forward Euler and strong stability preserving Runge-Kutta time integration options complement high order spatial discretization. We have also added flexibility in the code infrastructure allowing for both adaptive mesh and adaptive basis order refinement to be performed separately or simultaneously in a local (cell-by-cell) manner. We discuss in this report the DG formulation and present tests demonstrating the robustness, accuracy, and convergence of our numerical methods applied to special and general relativistic MHD, though we note an equivalent capability currently also exists in CosmosDG for Newtonian systems.

astro-ph.IM↗

arXiv2016

Grackle: a Chemistry and Cooling Library for Astrophysics

We present the Grackle chemistry and cooling library for astrophysical simulations and models. Grackle provides a treatment of non-equilibrium primordial chemistry and cooling for H, D, and He species, including H2 formation on dust grains; tabulated primordial and metal cooling; multiple UV background models; and support for radiation transfer and arbitrary heat sources. The library has an easily implementable interface for simulation codes written in C, C++, and Fortran as well as a Python interface with added convenience functions for semi-analytical models. As an open-source project, Grackle provides a community resource for accessing and disseminating astrochemical data and numerical methods. We present the full details of the core functionality, the simulation and Python interfaces, testing infrastructure, performance, and range of applicability. Grackle is a fully open-source project and new contributions are welcome.

astro-ph.CO↗

arXiv2014

Numerical Simulations of Optically Thick Accretion onto a Black Hole - II. Rotating Flow

In this paper we report on recent upgrades to our general relativistic radiation magnetohydrodynamics code, Cosmos++, including the development of a new primitive inversion scheme and a hybrid implicit-explicit solver with a more general closure relation for the radiation equations. The new hybrid solver helps stabilize the treatment of the radiation source terms, while the new closure allows for a much broader range of optical depths to be considered. These changes allow us to expand by orders of magnitude the range of temperatures, opacities, and mass accretion rates, and move a step closer toward our goal of performing global simulations of radiation-pressure-dominated black hole accretion disks. In this work we test and validate the new method against an array of problems. We also demonstrate its ability to handle super-Eddington, quasi-spherical accretion. Even with just a single proof-of-principle simulation, we already see tantalizing hints of the interesting phenomenology associated with the coupling of radiation and gas in super-Eddington accretion flows.

astro-ph.IM↗