Astrophysical Problems Research Articles

Magnetospheric radius of an inclined rotator in the magnetically threaded disk model

The estimate of the magnetospheric radius in a disk-fed neutron star X-ray binary is a long standing problem in high energy astrophysics. We have reviewed the magnetospheric radius calculations in the so-called magnetically threaded disk model, comparing the simplified approach originally proposed by Ghosh & Lamb (1979, ApJ, 232, 259) with the revised version proposed by Wang (1987, A&A, 183, 257), Wang (1995, ApJ, 449, L153), and Wang (1997, ApJ, 475, L135). We show that for a given set of fixed parameters (assuming also a comparable screening factor of the neutron star magnetic field by the currents induced on the disk surface) the revised magnetically threaded disk model predicts a magnetospheric radius that is significantly smaller than that derived from the Ghosh & Lamb (1979) treatment. For a fixed value of the neutron star magnetic field and a wide range of mass accretion rates, the inclusion of a large inclination angle between the neutron star rotation and magnetic field axes (χ ≳ 60 deg) leads to a further decrease of the magnetospheric radius. To illustrate the relevance of these calculations, we consider, as an example, the case of the transitional pulsars. During the so-called high mode of their sub-luminous accretion disk state, these sources have shown X-ray pulsations interpreted as due to accretion at an unprecedented low luminosity level compared to other neutron stars in X-ray binaries. In the context of the magnetic threaded disk model, we show that accretion at luminosities of ∼1033 erg s−1 (and thus accretion-driven X-ray pulsations) can be more easily explained when the prescription of the magnetospheric radius provided by Wang (1997) is used. This avoids the need to invoke very strong propeller outflows in the transitional pulsars, as proposed in other literature works.

Global N-body simulation of galactic spiral arms

The origin of galactic spiral arms is one of the fundamental problems in astrophysics. Based on the local analysis, Toomre (1981) proposed the swing amplification mechanism in which the self-gravity forms spiral arms as leading waves of stars rotate to trailing ones due to galactic shear. The structure of spiral arms is characterized by their number and pitch angle. We perform global N-body simulations of spiral galaxies to investigate the dependence of the spiral structure on disc parameters and compare the simulation results with the swing amplification model. We find that the spiral structure in the N-body simulations agrees well with that predicted by the swing amplification for the wide range of parameters. The pitch angle decreases with increasing the shear rate and is independent of the disc mass fraction. The number of spiral arms decreases with both increasing the shear rate and the disc mass fraction. If the disc mass fraction is fixed, the pitch angle increases with the number of spiral arms.

Early Chemical Evolution of Zn Driven by Magnetorotational Supernovae and the Pathway to the Solar Zn Composition

Abstract The site of Zn production remains an elusive and challenging problem in astrophysics. A large enhancement of the [Zn/Fe] ratios of very metal-poor stars in the Galactic halo suggests the death of short-lived massive stars, i.e., core-collapse supernovae (CCSNe), as one major site for Zn production. Previous studies have claimed that some specific CCSNe can produce Zn in sufficient quantities. However, it remains unclear which models can withstand the critical test of observations. Using a Zn abundance feature similar to that of r-process elements in faint satellite galaxies, we find evidence that Zn production took place through events that are much rarer than canonical CCSNe. This finding can be unified with the implied decrease in the rate of Zn production with an increasing metallicity for Galactic halo stars, which narrows down the major site of Zn production in the early galaxy to magnetorotational SNe (MR-SNe). On the other hand, in the later phase of galactic evolution, we predict that the major Zn-production site switched from MR-SNe to thermonuclear SNe (SNe Ia). According to this scenario, an accumulation of the contributions from two types of SNe eventually led to the solar isotope composition of Zn, which mainly owes 66,68Zn to MR-SNe and 64Zn to SNe Ia triggered by He-detonation. The requirement of Zn production in SNe Ia sheds a new light on the debate concerning the scenario for SN Ia progenitors, suggesting that a He-detonation model might be one major channel for SNe Ia.

Ultraluminous X-ray source populations in the Chandra Source Catalog 2.0

Abstract. The nature and evolution of ultraluminous X-ray sources (ULXs) is an open problem in astrophysics. They challenge our current understanding of stellar compact objects and accretion physics. The recent discovery of pulsar ULXs further demonstrates the importance of this intriguing and rare class of objects.In order to overcome the difficulties of directly studying the optical associations of ULXs, we generally resort in statistical studies of the stellar properties of their host galaxies. We present the largest such study based on the combination of Chandra archival data with the most complete galaxy catalog of the Local Universe. Incorporating robust distances and stellar population parameters based on associated multi-wavelength information, and we explore the association of ULXs with galaxies in the (star formation rate, stellar mass, metallicity) space.We confirm the known correlation with morphology, star formation rate and stellar mass, while we find an excess of ULXs in dwarf galaxies, indicating dependence on age and metallicity.

GaiaData Release 2

Context.The secondGaiadata release (GaiaDR2) provides precise five-parameter astrometric data (positions, proper motions, and parallaxes) for an unprecedented number of sources (more than 1.3 billion, mostly stars). This new wealth of data will enable the undertaking of statistical analysis of many astrophysical problems that were previously infeasible for lack of reliable astrometry, and in particular because of the lack of parallaxes. However, the use of this wealth of astrometric data comes with a specific challenge: how can the astrophysical parameters of interest be properly inferred from these data?Aims.The main focus of this paper, but not the only focus, is the issue of the estimation of distances from parallaxes, possibly combined with other information. We start with a critical review of the methods traditionally used to obtain distances from parallaxes and their shortcomings. Then we provide guidelines on how to use parallaxes more efficiently to estimate distances by using Bayesian methods. In particular we also show that negative parallaxes, or parallaxes with relatively large uncertainties still contain valuable information. Finally, we provide examples that show more generally how to use astrometric data for parameter estimation, including the combination of proper motions and parallaxes and the handling of covariances in the uncertainties.Methods.The paper contains examples based on simulatedGaiadata to illustrate the problems and the solutions proposed. Furthermore, the developments and methods proposed in the paper are linked to a set of tutorials included in theGaiaarchive documentation that provide practical examples and a good starting point for the application of the recommendations to actual problems. In all cases the source code for the analysis methods is provided.Results.Our main recommendation is to always treat the derivation of (astro-)physical parameters from astrometric data, in particular when parallaxes are involved, as an inference problem which should preferably be handled with a full Bayesian approach.Conclusions.Gaiawill provide fundamental data for many fields of astronomy. Further data releases will provide more data, and more precise data. Nevertheless, to fully use the potential it will always be necessary to pay careful attention to the statistical treatment of parallaxes and proper motions. The purpose of this paper is to help astronomers find the correct approach.

Density-Dependence of Nuclear Symmetry Energy: Role of QCD Chiral Phase Transition

The density-dependence of symmetry energy is of particular importance to many problems in nuclear physics and astrophysics. By using the functional path integral method, we show explicitly the relation between nuclear symmetry energy and isospin susceptibility. The latter one is found to be a probe to the QCD chiral phase transition. We further found in the Nambu-Jona-Lasinio model calculations that, the nuclear symmetry energy has an abrupt change at the critical nuclear density where the chiral symmetry restores partially.

Neutron Star Equation of State from the Quark Level in Light of GW170817

Abstract The matter state inside neutron stars (NSs) is an exciting problem in astrophysics, nuclear physics, and particle physics. The equation of state (EOS) of NSs plays a crucial role in the present multimessenger astronomy, especially after the event of GW170817. We propose a new NS EOS, “QMF18,” from the quark level, which describes robust observational constraints from a free-space nucleon, nuclear matter saturation, heavy pulsar measurements, and the tidal deformability of the very recent GW170817 observation. For this purpose, we employ the quark mean-field model, which allows us to tune the density dependence of the symmetry energy and effectively study its correlations with the Love number and the tidal deformability. We provide tabulated data for the new EOS and compare it with other recent EOSs from various many-body frameworks.

Heun Functions and Some of Their Applications in Physics

Most of the theoretical physics known today is described by using a small number of differential equations. For linear systems, different forms of the hypergeometric or the confluent hypergeometric equations often suffice to describe the system studied. These equations have power series solutions with simple relations between consecutive coefficients and/or can be represented in terms of simple integral transforms. If the problem is nonlinear, one often uses one form of the Painlevé equations. There are important examples, however, where one has to use higher order equations. Heun equation is one of these examples, which recently is often encountered in problems in general relativity and astrophysics. Its special and confluent forms take names as Mathieu, Lamé, and Coulomb spheroidal equations. For these equations whenever a power series solution is written, instead of a two-way recursion relation between the coefficients in the series, we find one between three or four different ones. An integral transform solution using simpler functions also is not obtainable. The use of this equation in physics and mathematical literature exploded in the later years, more than doubling the number of papers with these solutions in the last decade, compared to time period since this equation was introduced in 1889 up to 2008. We use SCI data to conclude this statement, which is not precise, but in the correct ballpark. Here this equation will be introduced and examples for its use, especially in general relativity literature, will be given.

Modeling of Galactic Wind Formation from Supernovae Using High-Performance Computations

Fundamental processes in the dynamics of the interstellar medium, namely, galactic wind, i.e., ejections of interstellar matter from central regions of galaxies, which are presumably caused by the formation of supernovae, are mathematically modeled in detail on high-performance parallel computer systems. The mathematical simulation is based on a kinetically consistent gasdynamic approach developed for such class of problems in astrophysics. A kinetically consistent algorithm is well adapted to the architecture of high-performance computer systems with massive parallelism, so that complex large-scale astrophysical phenomena can be efficiently studied with a high resolution. The approach, method, and algorithms are described, and numerical results are presented.

Identification of activity peaks in time-tagged data with a scan-statistics driven clustering method and its application to gamma-ray data samples

Context.The investigation of activity periods in time-tagged data samples is a topic of great interest. Among astrophysical samples, gamma-ray sources are widely studied, due to the huge quasi-continuum data set available today fromFermi-LAT (FermiLarge Area Telescope) and the AGILE-GRID (Astro Rivelatore Gamma a Immagini LEggero-Gamma Ray Imaging Detector)Aims.To reveal flaring episodes of a given gamma-ray source, researchers make use of binned light curves. This method suffers from several drawbacks: the results depend on time-binning and the identification of activity periods is difficult for bins with a low signal-to-noise ratio. A different approach is investigated in this paper.Methods.We developed a general temporal-unbinned method to identify flaring periods in time-tagged data and discriminate statistically significant flares. We propose an event clustering method in one dimension to identify flaring episodes, and scan statistics to evaluate the flare significance within the whole data sample. This is a photometric algorithm. The comparison of the photometric results (e.g. photometric flux, gamma-ray spatial distribution) for the identified peaks with the standard likelihood analysis for the same period is mandatory to establish if source confusion is spoiling results.Results.The procedure can be applied to reveal flares in any time-tagged data sample. The result of the proposed method is similar to a photometric light curve, but peaks are resolved, they are statistically significant within the whole period of investigation, and peak detection capability does not suffer time-binning related issues. The study of the gamma ray activity of 3C 454.3 and of the fast variability of the Crab Nebula are shown as examples. The method can be applied for gamma-ray sources of known celestial position, for example, sources taken from a catalogue. Furthermore the method can be used when it is necessary to assess the statistical significance within the whole period of investigation of a flare from an unknown gamma-ray source. Extensive results based on this analysis method for some astrophysical problems are the subject of a forthcoming paper.

On the common relativistic origin of the neutron lifetime discrepancy, the slight superluminality of neutrinos at Fermilab, and several astrophysical problems

On the common relativistic origin of the neutron lifetime discrepancy, the slight superluminality of neutrinos at Fermilab, and several astrophysical problems

Propagation dynamics of successive emissions in laboratory and astrophysical jets and problem of their collimation

This paper presents the results of numerical simulation of the propagation of a sequence of plasma knots in laboratory conditions and in the astrophysical environment. The physical and geometric parameters of the simulation have been chosen close to the parameters of the PF-3 facility (Kurchatov Institute) and the jet of the star RW Aur. We found that the low-density region formed after the first knot propagation plays an important role in the collimation of the subsequent ones. Assuming only the thermal expansion of the subsequent emissions, qualitative estimates of the time taken to fill this area with the surrounding matter and the angle of jet scattering have been made. These estimates are consistent with observations and results of our modeling.

Computation of Atomic Astrophysical Opacities

The revision of the standard Los Alamos opacities in the 1980–1990s by a group from the Lawrence Livermore National Laboratory (OPAL) and the Opacity Project (OP) consortium was an early example of collaborative big-data science, leading to reliable data deliverables (atomic databases, monochromatic opacities, mean opacities, and radiative accelerations) widely used since then to solve a variety of important astrophysical problems. Nowadays the precision of the OPAL and OP opacities, and even of new tables (OPLIB) by Los Alamos, is a recurrent topic in a hot debate involving stringent comparisons between theory, laboratory experiments, and solar and stellar observations in sophisticated research fields: the standard solar model (SSM), helio and asteroseismology, non-LTE 3D hydrodynamic photospheric modeling, nuclear reaction rates, solar neutrino observations, computational atomic physics, and plasma experiments. In this context, an unexpected downward revision of the solar photospheric metal abundances in 2005 spoiled a very precise agreement between the helioseismic indicators (the radius of the convection zone boundary, the sound-speed profile, and helium surface abundance) and SSM benchmarks, which could be somehow reestablished with a substantial opacity increase. Recent laboratory measurements of the iron opacity in physical conditions similar to the boundary of the solar convection zone have indeed predicted significant increases (30–400%), although new systematic improvements and comparisons of the computed tables have not yet been able to reproduce them. We give an overview of this controversy, and within the OP approach, discuss some of the theoretical shortcomings that could be impairing a more complete and accurate opacity accounting.

Meeting the Challenges of Modeling Astrophysical Thermonuclear Explosions: Castro, Maestro, and the AMReX Astrophysics Suite

We describe the AMReX suite of astrophysics codes and their application to modeling problems in stellar astrophysics. Maestro is tuned to efficiently model subsonic convective flows while Castro models the highly compressible flows associated with stellar explosions. Both are built on the block-structured adaptive mesh refinement library AMReX. Together, these codes enable a thorough investigation of stellar phenomena, including Type Ia supernovae and X-ray bursts. We describe these science applications and the approach we are taking to make these codes performant on current and future many-core and GPU-based architectures.

Parallel Algorithms for Astrophysics Problems

The algorithm and the mathematical modeling package for 3-D gravitational gasdynamics problems with ultra-high resolution meshes are described. The modeling results of filamentary formations, i.e. concentrated areas with high gas density in molecular clouds (MC), and the nonisothermic compression calculation data are discussed. The spatial mesh resolution required for satisfying Jeans conditions in modeling is substantiated. The programming code developed uses dynamic gridding called local adaptive mesh refinement (AMR) at several (up to 10) resolution levels. To provide adequate resolution themeshes are added automatically and dynamically as well as destroyed as needed. The computation paralleling algorithm withOpenMPand CUDA is given. The programming language chosen to compute the problems of gravitational gas dynamics efficiently is justified and substantiated. The practice of applying algorithms for modeling the MC fragmentation after collisions, the filament and protostellar clouds formation, the star formation stages is analyzed.

On Nearly Newtonian Potentials and Their Implications to Astrophysics

We review the concept of the slow motion problem in General relativity. We discuss how the understanding of this process may imprint influence on the explanation of astrophysical problems

A wide and collimated radio jet in 3C84 on the scale of a few hundred gravitational radii

Understanding the launching, acceleration, and collimation of jets powered by active galactic nuclei remains an outstanding problem in relativistic astrophysics. This is partly because observational tests of jet formation models suffer from the limited angular resolution of ground-based very long baseline interferometry that has thus far been able to probe the transverse jet structure in the acceleration and collimation zone of only two sources. Here we report radio interferometric observations of 3C 84 (NGC 1275), the central galaxy of the Perseus cluster, made with an array including the orbiting radio telescope of the RadioAstron mission. The obtained image transversely resolves the edge-brightened jet in 3C 84 only 30 microarcseconds from the core, which is ten times closer to the central engine than what has been possible in previous ground-based observations, and it allows us to measure the jet collimation profile from ~ 100 to ~10000 gravitational radii from the black hole. The previously found, almost cylindrical jet profile on scales larger than a few thousand r_g is now seen to continue at least down to a few hundred r_g from the black hole and we find a broad jet with a transverse radius larger than about 250 r_g at only 350 r_g from the core. If the bright outer jet layer is launched by the black hole ergosphere, it has to rapidly expand laterally on scales smaller than 100 r_g. If this is not the case, then this jet sheath is likely launched from the accretion disk.

On the Pressure of a Neutron Gas Interacting with the Non-Uniform Magnetic Field of a Neutron Star

On the basis of simple arguments, practically not going beyond the scope of an undergraduate course in general physics, we estimate the additional pressure (at zero temperature) of degenerate neutron matter due to its interaction with the non-uniform magnetic field of a neutron star. This work has methodological and possibly scientific value as an intuitive application of the content of such a course to a solution of topical problems of astrophysics.

Methods of Laser, Non-Linear, and Fiber Optics in Studying Fundamental Problems of Astrophysics

Precise measurements of Doppler shifts of lines in stellar spectra allowing the radial velocity to be measured are an important field of astrophysical studies. A remarkable feature of the Doppler spectroscopy is the possibility to reliably measure quite small variations of the radial velocities (its acceleration, in fact) during long periods of time. Influence of a planet on a star is an example of such a variation. Under the influence of a planet rotating around a star, the latter demonstrates periodic motion manifested in the Doppler shift of the stellar spectrum. Precise measurements of this shift made it possible to indirectly discover planets outside the Solar system (exoplanets). Along with this, searching for Earth-type exoplanets within the habitable zone is an important challenge. For this purpose, accuracy of spectral measurements has to allow one to determine radial velocity variations at the level of centimeters per second during the timespans of about a year. Suchmeasurements on the periods of 10–15 years also would serve as a directmethod for determination of assumed acceleration of the Universe expansion. However, the required accuracy of spectroscopic measurements for this exceeds the possibilities of the traditional spectroscopy (an iodine cell, spectral lamps). Methods of radical improvement of possibilities of astronomical Doppler spectroscopy allowing one to attain the required measurement accuracy of Doppler shifts are considered. The issue of precise calibration can be solved through creating a system of a laser optical frequency generator of an exceptionally high accuracy and stability.

Direct measurement of nuclear cross-section of astrophysical interest: Results and perspectives

Stellar evolution and nucleosynthesis are interconnected by a wide network of nuclear reactions: the study of such connection is usually known as nuclear astrophysics. The main task of this discipline is the determination of nuclear cross-section and hence of the reaction rate in different scenarios, i.e. from the synthesis of a few very light isotopes just after the Big Bang to the heavy element production in the violent explosive end of massive stars. The experimental determination of reaction cross-section at the astrophysical relevant energies is extremely difficult, sometime impossible, due to the Coulomb repulsion between the interacting nuclei which turns out in cross-section values down to the fbar level. To overcome these obstacles, several experimental approaches have been developed and the adopted techniques can be roughly divided into two categories, i.e. direct and indirect methods. In this review paper, the general problem of nuclear astrophysics is introduced and discussed from the point of view of experimental approach. We focus on direct methods and in particular on the features of low-background experiments performed at underground laboratory facilities. The present knowledge of reactions involved in the Big Bang and stellar hydrogen-burning scenarios is discussed as well as the ongoing projects aiming to investigate mainly the helium- and carbon-burning phases. Worldwide, a new generation of experiment in the MeV range is in the design phase or at the very first steps and decisive progresses are expected to come in the next years.