Astrophysical Systems Research Articles

Large amplitude electrostatic solitary waves in anisotropic plasma

The propagation of nonlinear ion-acoustic solitary waves (IASWs) is investigated in a magnetized anisotropic electron ion plasma with Cairns distributed electrons. The ions are warm and exhibit pressure anisotropy due to the external magnetic field. The energy integral equation for the wave potential is derived via Sagdeev approach. The effect of various physical plasma parameters (nonthermality, ion pressure anisotropy) on the characteristics of IASWs is investigated in detail. The present investigation could be of interest to space and astrophysical plasma systems having ion pressure anisotropy, for instance, in the magnetosphere and near earth magnetosheath.

The Symmetry Energy: Current Status of Ab Initio Predictions vs. Empirical Constraints

Infinite nuclear matter is a suitable laboratory to learn about nuclear forces in many-body systems. In particular, modern theoretical predictions of neutron-rich matter are timely because of recent and planned experiments aimed at constraining the equation of state of isospin-asymmetric matter. For these reasons, we have taken a broad look at the equation of state of neutron-rich matter and the closely related symmetry energy, which is the focal point of this article. Its density dependence is of paramount importance for a number of nuclear and astrophysical systems, ranging from neutron skins to the structure of neutron stars. We review and discuss ab initio predictions in relation to recent empirical constraints. We emphasize and demonstrate that free-space nucleon–nucleon data pose stringent constraints on the density dependence of the neutron matter equation of state, which essentially determines the slope of the symmetry energy at saturation.

Multimessenger emission from tidal waves in neutron star oceans

ABSTRACT Neutron stars in astrophysical binary systems represent exciting sources for multimessenger astrophysics. A potential source of electromagnetic transients from compact binary systems is the neutron star ocean, the external fluid layer encasing a neutron star. We present a groundwork study into tidal waves in neutron star oceans and their consequences. Specifically, we investigate how oscillation modes in neutron star oceans can be tidally excited during compact binary inspirals and parabolic encounters. We find that neutron star oceans can sustain tidal waves with frequencies between 0.01 and 20 Hz. Our results suggest that tidally resonant neutron star ocean waves may serve as a never-before studied source of precursor electromagnetic emission prior to neutron star–black hole and binary neutron star mergers. If accompanied by electromagnetic flares, tidally resonant neutron star ocean waves, whose energy budget can reach 1046 erg, may serve as early warning signs (≳1 min before merger) for compact binary mergers. Similarly, excited ocean tidal waves will coincide with neutron star parabolic encounters. Depending on the neutron star ocean model and a flare emission scenario, tidally resonant ocean flares may be detectable by Fermi and Nuclear Spectroscopic Telescope Array (NuSTAR) out to ≳100 Mpc with detection rates as high as ∼7 yr−1 for binary neutron stars and ∼0.6 yr−1 for neutron star–black hole binaries. Observations of emission from neutron star ocean tidal waves along with gravitational waves will provide insight into the equation of state at the neutron star surface, the composition of neutron star oceans and crusts, and neutron star geophysics.

Synchrotron Pair Production Equilibrium in Relativistic Magnetic Reconnection

Magnetic reconnection is ubiquitous in astrophysical systems, and in many such systems the plasma suffers from significant cooling due to synchrotron radiation. We study relativistic magnetic reconnection in the presence of strong synchrotron cooling, where the ambient magnetization, σ, is high and the magnetic compactness, ℓ B , of the system is of order unity. In this regime, e± pair production from synchrotron photons is inevitable, and this process can regulate the magnetization σ surrounding the current sheet. We investigate this self-regulation analytically and find a self-consistent steady state for a given magnetic compactness of the system and initial magnetization. This result helps estimate the self-consistent upstream magnetization in systems where plasma density is poorly constrained, and can be useful for a variety of astrophysical systems. As illustrative examples, we apply it to study the properties of reconnecting current sheets near the supermassive black hole of M87, as well as the equatorial current sheet outside the light cylinder of the Crab pulsar.

On the Role of Dynamical Cooling in the Dynamics of Circumbinary Disks

Hydrodynamical interactions between binaries and circumbinary disks (CBDs) play an important role in a variety of astrophysical systems, from young stellar binaries to supermassive black hole binaries. Previous simulations of CBDs have mostly employed locally isothermal equations of state. We carry out 2D viscous hydrodynamic simulations of CBDs around equal-mass, circular binaries, treating the gas thermodynamics by thermal relaxation toward equilibrium temperature (the constant-β cooling ansatz, where β is the cooling time in units of the local Keplerian time). As an initial study, we use the grid-based code Athena++ on a polar grid, covering an extended disk outside the binary co-orbital region. We find that with a longer cooling time, the accretion variability is gradually suppressed, and the morphology of the CBD becomes more symmetric. The disk also shows evidence of hysteresis behavior depending on the initial conditions. Gas cooling also affects the rate of angular momentum transfer between the binary and the CBD, where given our adopted disk thickness and viscosity (H/r ∼ 0.1 and α ∼ 0.1), the binary orbit expands while undergoing accretion for most β values between 0 and 4.0 except over a narrow range of intermediate β values. The validity of using a polar grid excising the central domain is also discussed.

Narrowing the allowed mass range of ultralight bosons with the S2 star

Aims.It is well known thatN-body simulations of ultralight bosons display the formation of a solitonic dark matter core in the innermost part of the halo. The scale-length of such a soliton depends on the inverse of the mass of the boson. On the other hand, the orbital motion of stars in the Galactic Center depends on the distribution of matter, no matter whether it is baryonic or dark, providing an excellent probe for the gravitational field of the region. In this Letter, we propose the S-stars in the Galactic Center as a new observational tool, complementary to other astrophysical systems, to narrow down the range of allowed values for an ultralight dark matter candidate boson mass.Methods.We built mock catalogs mirroring the forthcoming astrometric and spectroscopic observations of S2 and we used a MCMC analysis to predict the accuracy down to which the mass of an ultralight boson may be bounded. We show that once complementary constraints are considered, this analysis will help to restrict the allowed range of the boson mass.Results.Our analysis forecasts the boundary limit on the mass of an ultralight boson to be < 10−19eV at a 95% confidence level.

The role of cooling induced by mixing in the mass and energy cycles of the solar atmosphere

ABSTRACT In many astrophysical systems, mixing between cool and hot temperature gas/plasma through Kelvin–Helmholtz-instability-driven turbulence leads to the formation of an intermediate temperature phase with increased radiative losses that drive efficient cooling. The solar atmosphere is a potential site for this process to occur with interaction between either prominence or spicule material and the solar corona allowing the development of transition region material with enhanced radiative losses. In this paper, we derive a set of equations to model the evolution of such a mixing layer and make predictions for the mixing-driven cooling rate and the rate at which mixing can lead to the condensation of the coronal material. These theoretical predictions are benchmarked against 2.5D MHD simulations. Applying the theoretical scalings to prominence threads or fading spicules, we found that as a mixing layer grows on their boundaries this would lead to the creation of transition region material with a cooling time of ∼100 s, explaining the warm emission observed as prominence threads or spicules fade in cool spectral lines without the requirement for any heating. For quiescent prominences, dynamic condensation driven by the mixing process could restore ∼18 per cent of the mass lost from a prominence through downflows. Overall, this mechanism of thermal energy loss through radiative losses induced by mixing highlights the importance for considering dynamical interaction between material at different temperatures when trying to understand the thermodynamic evolution of the cool material in the solar corona.

Multi-spacecraft observations of shocklets at an interplanetary shock

ABSTRACT Interplanetary (IP) shocks are fundamental building blocks of the heliosphere, and the possibility to observe them in situ is crucial to address important aspects of energy conversion for a variety of astrophysical systems. Steepened waves known as shocklets are known to be important structures of planetary bow shocks, but they are very rarely observed related to IP shocks. We present here the first multi-spacecraft observations of shocklets observed by upstream of an unusually strong IP shock observed on 3 November 2021 by several spacecraft at L1 and near-Earth solar wind. The same shock was detected also by radially aligned Solar Orbiter at 0.8 au from the Sun, but no shocklets were identified from its data, introducing the possibility to study the environment in which shocklets developed. The Wind spacecraft has been used to characterize the shocklets, associated with pre-conditioning of the shock upstream by decelerating incoming plasma in the shock normal direction. Finally, using the Wind observations together with ACE and DSCOVR spacecraft at L1, as well as THEMIS B and THEMIS C in the near-Earth solar wind, the portion of interplanetary space filled with shocklets is addressed, and a lower limit for its extent is estimated to be of about 110RE in the shock normal direction and 25RE in the directions transverse to the shock normal. Using multiple spacecraft also reveals that for this strong IP shock, shocklets are observed for a large range of local obliquity estimates (9º–64º).

Spatial field reconstruction with INLA

Aims. Monte Carlo radiative transfer (MCRT) simulations are a powerful tool for understanding the role of dust in astrophysical systems and its influence on observations. However, due to the strong coupling of the radiation field and medium across the whole computational domain, the problem is non-local and non-linear, and such simulations are computationally expensive in the case of realistic 3D inhomogeneous dust distributions. We explore a novel technique for post-processing MCRT output to reduce the total computational run time by enhancing the output of computationally less expensive simulations of lower-quality. Methods. We combined principal component analysis (PCA) and non-negative matrix factorisation (NMF) as dimensionality reduction techniques together with Gaussian Markov random fields and the integrated nested Laplace approximation (INLA), an approximate method for Bayesian inference, to detect and reconstruct the non-random spatial structure in the images of lower signal-to-noise ratios or with missing data. Results. We tested our methodology using synthetic observations of a galaxy from the SKIRT Auriga project - a suite of high-resolution magnetohydrodynamic Milky Way-sized galaxies simulated in cosmological environment using a ‘zoom-in' technique. With this approach, we are able to reproduce high-photon-number reference images ~5 times faster with median residuals below ~20%.

Comparing energy and entropy formulations for cosmic ray hydrodynamics

ABSTRACT Cosmic rays (CRs) play an important role in many astrophysical systems. Acting on plasma scales to galactic environments, CRs are usually modelled as a fluid, using the CR energy density as the evolving quantity. This method comes with the flaw that the corresponding CR evolution equation is not in conservative form as it contains an adiabatic source term that couples CRs to the thermal gas. In the absence of non-adiabatic changes, instead evolving the CR entropy density is a physically equivalent option that avoids this potential numerical inconsistency. In this work, we study both approaches for evolving CRs in the context of magnetohydrodynamic (MHD) simulations using the massively parallel moving-mesh code Arepo. We investigate the performance of both methods in a sequence of shock-tube tests with various resolutions and shock Mach numbers. We find that the entropy-conserving scheme performs best for the idealized case of purely adiabatic CRs across the shock while both approaches yield similar results at lower resolution. In this set-up, both schemes operate well and almost independently of the shock Mach number. Taking active CR acceleration at the shock into account, the energy-based method proves to be numerically much more stable and significantly more accurate in determining the shock velocity, in particular at low resolution, which is more typical for astrophysical large-scale simulations. For a more realistic application, we simulate the formation of several isolated galaxies at different halo masses and find that both numerical methods yield almost identical results with differences far below common astrophysical uncertainties.

A glimpse of axion phenomenology in astrophysics

The old and successful idea of using astrophysical systems as laboratories for fundamental physics is becoming extremely popular nowadays. Axions are a remarkable example of Feebly Interacting Particles playing an important role in astrophysical phenomena. It is well-known that stars are powerful axion factories, giving strong constraints on their properties. Aspects of the axion phenomenology in horizontal-branch stars and supernovae are reviewed.

Head on collision of ion–acoustic shock waves in weakly relativistic electron–positron–ion quantum plasma

In this work, we explore analytically the head on collision (HoC) of two ion acoustic shock wave in relativistic quantum plasma. Extended Poincaré Lighthill Kuo(PLK) method has been applied in this problem in a fruitful manner to study the HoC of waves which are rare in literature. Introducing the strained coordinates and expanding the field quantities into the parameter ϵ, a set of differential equations is obtained. Tangent Hyperbolic method is also used while investigating nonlinear interaction of this mode. These techniques are proven to be efficient enough to extract the essence of physics. Our major focus is to study the influence of plasma characteristics on shock wave phase shifts upon a head-on collision. The effects of the ratio of electrons to positron temperature, fractional concentration of positrons, quantum parameter, kinematic viscosity on the phase shift are studied. The findings suggest that shock interactions are distinct from those of solitons. It is also observed that the dispersion and dissipation of the colliding nonlinear structure have a combined role in the occurrence and modification of trajectory variations. The results of this work might be beneficial in studying the propagation and interaction of quantum ion–acoustic shock waves in dense astrophysical plasma systems (white dwarf star) and intense laser-solid density plasma investigations.

Reconnection-driven energy cascade in magnetohydrodynamic turbulence.

Magnetohydrodynamic turbulence regulates the transfer of energy from large to small scales in many astrophysical systems, including the solar atmosphere. We perform three-dimensional magnetohydrodynamic simulations with unprecedentedly large magnetic Reynolds number to reveal how rapid reconnection of magnetic field lines changes the classical paradigm of the turbulent energy cascade. By breaking elongated current sheets into chains of small magnetic flux ropes (or plasmoids), magnetic reconnection leads to a previously undiscovered range of energy cascade, where the rate of energy transfer is controlled by the growth rate of the plasmoids. As a consequence, the turbulent energy spectra steepen and attain a spectral index of -2.2 that is accompanied by changes in the anisotropy of turbulence eddies. The omnipresence of plasmoids and their consequences on, for example, solar coronal heating, can be further explored with current and future spacecraft and telescopes.

X-Ray Spectral Analysis of the Jet Termination Shock in Pictor A on Subarcsecond Scales with Chandra

Hot spots observed at the edges of extended radio lobes in high-power radio galaxies and quasars mark the position of mildly relativistic termination shock, where the jet bulk kinetic energy is converted to the internal energy of the jet particles. These are the only astrophysical systems where mildly relativistic shocks can be directly resolved at various wavelengths of the electromagnetic spectrum. The western hot spot in the radio galaxy Pictor A is an exceptionally good target in this respect, due to the combination of its angular size and high surface brightness. In our previous work, after a careful Chandra image deconvolution, we resolved this hot spot into a disk-like feature perpendicular to the jet axis, and identified it as the front of the jet termination shock. We argued for a synchrotron origin of the observed X-ray photons, which implied electron energies reaching at least 10–100 TeV at the shock front. Here, we present a follow-up on that analysis, proposing, in particular, a novel method for constraining the shape of the X-ray continuum emission with subarcsecond resolution. The method is based on a Chandra hardness map analysis, using separately deconvolved maps in the soft and hard X-ray bands. In this way, we have found there is a systematic, yet statistically significant gradient in the hardness ratio across the shock, such that the implied electron energy index ranges from s ≤ 2.2 at the shock front to s > 2.7 in the near downstream. We discuss the implications of the obtained results for a general understanding of particle acceleration at mildly relativistic shocks.

Experimental and numerical investigation of bistability in rotating permanent magnet-generated electrolyte flow in a ring-shaped container

Here, the stability of a transversely magnetized rotating permanent magnet-generated flow in a concentric cylindrical ring channel is studied. Numerical calculations show that the steady-state solution becomes asymmetric through a pitchfork bifurcation at a Reynolds number (Re) of 60. The two new antisymmetric steady-state solutions become cyclic at Re = 90. Nonlinearities develop at larger Re values and the limit cycle solutions are destabilized at Re = 250, enabling random transition events between the two pitchfork branches. Such transitions have been observed in all kinds of natural phenomena, spanning from neuroscientific to astrophysical systems, which are often too complex to be directly computed. Our presented system is physical yet simple enough to be used to conduct a parametric study with full three-dimensional direct numerical simulations. It raises the possibility of numerically and experimentally analyzing transitions in more detail. Experimental measurements indicated the existence of long-lived states and suitability for the proposed system for future studies of such phenomenon. However, the experimental results did not conclusively observe bistability.

A new residual distribution hydrodynamics solver for astrophysical simulations

ABSTRACT Many astrophysical systems can only be accurately modelled when the behaviour of their baryonic gas components is well understood. The residual distribution (RD) family of partial differential equation (PDE) solvers produce approximate solutions to the corresponding fluid equations. We present a new implementation of the RD method. The solver efficiently calculates the evolution of the fluid, with up to second order accuracy in both time and space, across an unstructured triangulation, in both 2D and 3D. We implement a novel variable time stepping routine, which applies a drifting mechanism to greatly improve the computational efficiency of the method. We conduct extensive testing of the new implementation, demonstrating its innate ability to resolve complex fluid structures, even at very low resolution. We can resolve complex structures with as few as 3–5 resolution elements, demonstrated by Kelvin–Helmholtz and Sedov blast tests. We also note that we find cold cloud destruction time scales consistent with those predicted by a typical PPE solver, albeit the exact evolution shows small differences. The code includes three residual calculation modes, the LDA, N, and blended schemes, tailored for scenarios from smooth flows (LDA), to extreme shocks (N), and both (blended). We compare our RD solver results to state-of-the-art solvers used in other astrophysical codes, demonstrating the competitiveness of the new approach, particularly at low resolution. This is of particular interest in large scale astrophysical simulations, where important structures, such as star forming gas clouds, are often resolved by small numbers of fluid elements.

Radial profiles of surface density in debris discs

ABSTRACT Resolved observations of debris discs can be used to derive radial profiles of azimuthally averaged surface density (ASD), which carries important information about the disc structure even in the presence of non-axisymmetric features and has improved signal-to-noise characteristics relative to the two-dimensional surface brightness measurements. We develop a (semi-)analytical formalism allowing one to relate ASD to the underlying semimajor axis and eccentricity distributions of the debris particles in a straightforward manner. This approach does not involve the distribution of particle apsidal angles, thus simplifying calculations. It is a much faster, more flexible and effective way of calculating ASD than the Monte Carlo sampling of orbital parameters of debris particles. We present explicit analytical results based on this technique for a number of particle eccentricity distributions, including two cases of particular practical importance: a prescribed radial profile of eccentricity, and the Rayleigh distribution of eccentricities. We then show how our framework can be applied to observations of debris discs and rings for retrieving either the semimajor axis distribution or (in some cases) the eccentricity distribution of debris, thus providing direct information about the architecture and dynamical processes operating in debris discs. Our approach also provides a fast and efficient way of forward modelling observations. Applications of this technique to other astrophysical systems, e.g. the nuclear stellar disc in M31 or tenuous planetary rings, are also discussed.

Exhaustive existence and non-existence results for Hardy–Hénon equations in {{,mathrm{{textbf{R}}},}}^n

This paper concerns solutions to the Hardy–Hénon equation -Δu=|x|σup\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\begin{aligned} -\\Delta u = |x|^\\sigma u^p \\end{aligned}$$\\end{document}in {{,mathrm{{textbf{R}}},}}^n with n ge 1 and arbitrary p, sigma in {{,mathrm{{textbf{R}}},}}. This equation was proposed by Hénon in 1973 as a model to study rotating stellar systems in astrophysics. Although there have been many works devoting to the study of the above equation, at least one of the following three assumptions p>1,sigma ge -2, and n ge 3 is often assumed. The aim of this paper is to investigate the equation in other cases of these parameters, leading to a complete picture of the existence/non-existence results for non-trivial, non-negative solutions in the full generality of the parameters. In addition to the existence/non-existence results, the uniqueness of solutions is also discussed.

Inverse cascades of kinetic energy and thermal variance in three-dimensional horizontally extended turbulent convection

Inverse cascades of kinetic energy and thermal variance in the subset of vertically homogeneous modes in spectral space are found to cause a slow aggregation to a pair of convective supergranules that eventually fill the whole horizontally extended, three-dimensional, turbulent Rayleigh-B\'{e}nard convection layer when a heat flux is prescribed at the top and bottom. An additional weak rotation of the layer around the vertical axis stops this aggregation at a scale that is smaller than the lateral domain extension and ceases the inverse cascade for the thermal variance. The inverse cascade for the kinetic energy remains intact, even for times at which the root mean square values of temperature and velocity have reached the statistically steady state. This kinetic energy inverse cascade sustains the horizontally extended convection patterns which are best visible in the temperature field. The resulting characteristic length of the aggregated convection patterns depends on the thermal driving and linearly on the strength of rotation. Our study demonstrates the importance of inverse energy cascades beyond the two-dimensional turbulence case in a three-dimensional convection flow that is subject to a multi-scale energy injection by thermal plumes and driven by boundary heat fluxes as typically present in natural geo- and astrophysical systems, such as solar convection.

Experimental evidence of early-time saturation of the ion-Weibel instability in counterstreaming plasmas of CH, Al, and Cu.

The collisionless ion-Weibel instability is a leading candidate mechanism for the formation of collisionless shocks in many astrophysical systems, where the typical distance between particle collisions is much larger than the system size. Multiple laboratory experiments aimed at studying this process utilize laser-driven (I≳10^{15} W/cm^{2}), counterstreaming plasma flows (V≲2000km/s) to create conditions unstable to Weibel-filamentation and growth. This technique intrinsically produces temporally varying plasma conditions at the midplane of the interaction where Weibel-driven B fields are generated and studied. Experiments discussed herein demonstrate robust formation of Weibel-driven B fields under multiple plasma conditions using CH, Al, and Cu plasmas. Linear theory based on benchmarked radiation-hydrodynamic FLASH calculations is compared with Fourier analyses of proton images taken ∼5-6 linear growth times into the evolution. The new analyses presented here indicate that the low-density, high-velocity plasma-conditions present during the first linear-growth time (∼300-500ps) sets the spectral characteristics of Weibel filaments during the entire evolution. It is shown that the dominant wavelength (∼300μm) at saturation persists well into the nonlinear phase, consistent with theory under these experimental conditions. However, estimates of B-field strength, while difficult to determine accurately due to the path-integrated nature of proton imaging, are shown to be in the ∼10-30T range, an order of magnitude above the expected saturation limit in homogenous plamas but consistent with enhanced B fields in the midplane due to temporally varying plasma conditions in experiments.