Compact Astrophysical Objects Research Articles

Monte Carlo Simulations of Thermal‐Nonthermal Radiation from a Neutron Star Magnetospheric Accretion Shell

We discuss the space- and time-dependent Monte Carlo code we have developed to simulate the relativistic radiation output from compact astrophysical objects, coupled to a Fokker-Planck code to determine the self-consistent lepton populations. We have applied this code to model the emission from a magnetized neutron star accretion shell near the Alfven radius, reprocessing the radiation from the neutron star surface. We explore the parameter space defined by the accretion rate, stellar surface field, and level of wave turbulence in the shell. Our results are relevant to the emission from atoll sources, soft X-ray-transients containing weakly magnetized neutron stars, and recently suggested models of accretion-powered emission from anomalous X-ray pulsars.

The influence of free neutrons on dynamics and radiation of astrophysical plasmas

We present arguments in favor of the presence of free neutrons in plasmas generated by compact astrophysical objects and find conditions necessary for the formation of the neutron component. The broad range of phenomena caused by neutrons includes both dynamical (sources' variability, transition of fireballs to the two-flow regime) and kinetic (fission of helium nuclei by neutrons, electromagnetic cascade, emission in annihilation and nuclear lines, neutrino losses) effects. The presented theory can be applied to internal regions of accretion disks, jets in microquasars, and gamma-ray burst (GRB) fireballs.

WIMPs and MACHOs

WIMP is an acronym for weakly interacting massive particle and MACHO is an acronym for massive (astrophysical) compact halo object. WIMPs and MACHOs are two of the most popular DARK MATTER candidates. They represent two very different but reasonable possibilities of what the dominant component of the universe may be.

Gravitational waves from long-duration simulations of the dynamical bar instability

Compact astrophysical objects that rotate rapidly may encounter the dynamical ``bar instability.'' The bar-like deformation induced by this rotational instability causes the object to become a potentially strong source of gravitational radiation. We have carried out a set of long-duration simulations of the bar instability with two Eulerian hydrodynamics codes. Our results indicate that the remnant of this instability is a persistent bar-like structure that emits a long-lived gravitational radiation signal.

Meson Synchrotron Emission from Central Engines of Gamma-Ray Bursts with Strong Magnetic Fields.

Gamma-ray bursts (GRBs) are presumed to be powered by the still unknown central engines with timescales in the range from 1 ms to approximately a few seconds. We propose that the GRB central engines would be a viable site for strong meson synchrotron emission if they were compact astrophysical objects, such as neutron stars or rotating black holes with extremely strong magnetic fields (H approximately 1012-1017 G), and if protons or heavy nuclei were accelerated to ultrarelativistic energies on the order of approximately 1012-1022 eV. We show that the charged scalar mesons like pi+/- and heavy vector mesons like rho, which have several decay modes onto pi+/-, could be emitted, with a high intensity that is a thousand times larger than photons, through strong couplings to ultrarelativistic nucleons. These meson synchrotron emission processes eventually produce a burst of very high energy cosmic neutrinos with 1012 eV</=Enu. These neutrinos are to be detected during the early-time duration of short GRBs.

Kaon production in heavy-ion collisions and kaon condensation in neutron stars

The recent past witnesses the growing interdependence between the physics of hadrons, the physics of relativistic heavy-ion collisions, and the physics of compact objects in astrophysics. A notable example is the kaon which plays a special roles in all the three fields. In this contribution, we present a transport model study of kaon production and flow in heavy-ion collisions at SIS energies. We also discuss the effects of the kaon in-medium properties extracted from heavy-ion data on neutron star properties.

Magnetohydrodynamic Origin of Jets from Accretion Disks

A review is made of recent magnetohydrodynamic (MHD) theory and simulations of origin of jets from accretion disks. Many compact astrophysical objects emit powerful, highly-collimated, oppositely directed jets. Included are the extra galactic radio jets of active galaxies and quasars, and old compact stars in binaries, and emission line jets in young stellar objects. It is widely thought that these different jets arise from rotating, conducting accretion disks threaded by an ordered magnetic field. The twisting of the B field by the rotation of the disk drives the jets by magnetically extracting matter, angular momentum, and energy from the accretion disk. Two main regimes have been discussed theoretically, hydromagnetic winds which have a significant mass flux, and Poynting flux jets where the mass flux is negligible. Over the past several years, exciting new developments on models of jets have come from progress in MHD simulations which now allow the study of the origin -the acceleration and collimation - of jets from accretion disks. Simulation studies in the hydromagnetic wind regime indicate that the outflows are accelerated close to their region of origin whereas the collimation occurs at much larger distances.

Probing the Mass Fraction of MACHOs in Extragalactic Halos

Current microlensing searches calibrate the mass fraction of the Milky Way halo that is in the form of massive astrophysical compact halo objects (MACHOs). We show that surveys like the Sloan Digital Sky Survey (SDSS) can probe the same quantity in halos of distant galaxies. Microlensing of background quasars by MACHOs in intervening galaxies would distort the equivalent width distribution of the quasar emission lines by an amplitude that depends on the projected quasar-galaxy separation. For a statistical sample of ~105 quasars as expected in the SDSS, this distortion is detectable at the 2 σ level out to a quasar-galaxy impact parameter of several tens of kpc, as long as extragalactic halos are made of MACHOs. Detection of this signal would test whether the MACHO fraction inferred for the Milky Way halo is typical of other galaxies.

Self‐consistent Fokker‐Planck Treatment of Particle Distributions in Astrophysical Plasmas

High-energy, multicomponent plasmas in which pair creation and annihilation, lepton-lepton scattering, lepton-proton scattering, and Comptonization all contribute to establishing the particle and photon distributions are present in a broad range of compact astrophysical objects. The different constituents are often not in equilibrium with each other, and this mixture of interacting particles and radiation can produce substantial deviations from a Maxwellian profile for the lepton distributions. Earlier work has included much of the microphysics needed to account for electron-photon and electron-proton interactions, but little has been done to handle the redistribution of the particles as a result of their Coulomb interaction with themselves. The most detailed analysis thus far for finding the exact electron distribution appears to have been done within the framework of nonthermal models, where the electron distribution is approximated as a thermal one at low energy with a nonthermal tail at higher energy. Recent attention, however, has been focused on thermal models.

Kaon Production in Heavy-Ion Collisions and Maximum Mass of Neutron Stars

We determine an ``empirical'' kaon dispersion relation by analyzing and fitting recent experimental data on kaon production in heavy-ion collisions. We then investigate its effects on the hadronic equation of state at high densities and on neutron star properties. We find that the maximum mass of neutron stars can be lowered by about ${0.4M}_{\ensuremath{\bigodot}}$ once kaon condensation as constrained by our empirical dispersion relation is introduced. We emphasize the growing interplay between hadron physics, relativistic heavy-ion physics, and the physics of compact objects in astrophysics.

Kaons in dense matter, kaon production in heavy-ion collisions, and kaon condensation in neutron stars

The recent past witnesses the growing interdependence between the physics of hadrons, the physics of relativistic heavy-ion collisions, and the physics of compact objects in astrophysics. A notable example is the kaon which plays special roles in all the three fields. In this paper, we first review the various theoretical investigations of kaon properties in nuclear medium, focusing on possible uncertainties in each model. We then present a detailed transport model study of kaon production in heavy-ion collisions at SIS energies. We shall discuss especially the elementary kaon and antikaon production cross sections in hadron-hadron interactions, that represent one of the most serious uncertainties in the transport model study of particle production in heavy-ion collisions. The main purpose of such a study is to constrain kaon in-medium properties from the heavy-ion data. This can provide useful guidances for the development of theoretical models of the kaon in medium. In the last part of the paper, we apply the kaon in-medium properties extracted from heavy-ion data to the study of neutron star properties. Based on a conventional equation of state of nuclear matter that can be considered as one of the best constrained by available experimental data on finite nuclei, we find that the maximum mass of neutron stars is about 2 M ⊙, which is reduced to about 1.5 M ⊙ once kaon condensation as constrained by heavy-ion data is introduced.

Осесимметричные стационарные течения в компактных астрофизических объектах

Осесимметричные стационарные течения в компактных астрофизических объектах

Effect of binary sources on the search for massive astrophysical compact halo objects via microlensing

view Abstract Citations (113) References (19) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Effect of Binary Sources on the Search for Massive Astrophysical Compact Halo Objects via Microlensing Griest, Kim ; Hu, Wayne Abstract If the dark matter in the Galactic halo consists of compact objects in the 10^-6^-10^2^2 M_sun_ mass range, it can be detected as it gravitationally microlenses stars in neighboring galaxies and the Galactic bulge. Though extremely rare, microlensing has several powerful signatures: the light curves follow a well-known function, are time symmetric, and are the same in all filter bands. These signatures, however, may be lost if the source star is a member of a binary system. Since most stars are, in fact, members of binaries, the true microlensing events may be rejected as background. We study the effect of binary sources by using the event geometry and resulting light curves to define several categories of binary microlensing events, and then calculate the probability of each category occurring. We average these probabilities over measured distributions of binary orbital periods and mass ratios, finding ~10%-20% of events on binary sources should be distinguishable from single-source microlensing events (for the LMC). The bulk of binary events should be achromatic and similar to single source light curves, but 2%-5% should have truly unusual light curves and color shifts greater than 0.1 mag. The total microlensing rate is larger by 5%-15% for binary sources and in deriving the MACHO mass from a binary light curve an error of a factor of 2 or more may be made if single source formulae are used. Publication: The Astrophysical Journal Pub Date: October 1992 DOI: 10.1086/171793 Bibcode: 1992ApJ...397..362G Keywords: Binary Stars; Dark Matter; Galactic Structure; Gravitational Lenses; Point Sources; Light Curve; Stellar Color; Stellar Mass; Astrophysics; COSMOLOGY: GRAVITATIONAL LENSING full text sources ADS | Related Materials (1) Erratum: 1993ApJ...407..440G

Strange baryon matter

Using the non-linear SU(3) L × SU(3) R chiral lagrangian coupled to a field theory of nuclear forces, we show that a bound state of baryons with a well-defined surface may conceivably form in the presence of kaon condensation. This state is of similar density to ordinary nuclei, but has net strangeness equal to about two thirds the baryon number. We discuss the properties of lumps of strange baryon matter with baryon number between ∼ 20 and ∼ 10 57 where gravitational effects become important. The possibility that the ground state of baryonic matter at zero pressure is strange and that ordinary nuclei may only be metastable has important consequences for laboratory nuclear physics, the early universe and astrophysical compact objects.

Dynamo Driven Mean Magnetic Field in Accretion Disks of Compact Astrophysical Objects and its Manifestations

The simplest case of the nonlinear turbulent dynamo mechanism is proposed. It is shown that under certain conditions the generated mean magnetic field can become stronger than the small-scale one. Some manifestations (the model of Cyg X-1 bimodal behaviour, asymmetric accretion onto the magnetized rotating compact star) of this mean field are discussed.

Wave effects in gravitational lensing of electromagnetic radiation.

Wave effects in the gravitational lensing of electromagnetic radiation by compact objects in astrophysics are treated. In the regime where the influence of diffraction is severe, solutions of the wave equation appropriate for lensing in astrophysics (which is of the form of the Schroumldinger equation for Coulomb scattering) are obtained. In the WKB regime, previous formulations are refined and extended to the case in which a part of the source is directly behind the lensing mass. Wave effects tend to be largest for this geometry which requires that radiation at a caustic be treated. The autocorrelation function of the electric field and the related frequency dependence of the radiation are emphasized, in addition to the diffraction pattern, to measure the deviations from the solutions obtained in the approximation of geometrical optics. In contrast, gravitational lensing is independent of frequency in the usual treatment which is based upon geometrical optics. Numerical computations are performed to indicate how the wave effects depend upon the relevant parameters including especially the size of the source. Wave effects are most significant at radio frequencies and for lensing by stellar and planetary masses. The importance of wave effects is governed by the ratio of the Schwarzschild radius of the lensing mass to the wavelength of the radiation.