On the oscillations of the plasma atmosphere in gamma-ray bursts

One of the possible hypotheses implies that cosmic gamma-ray bursts can arise when two neutron stars or black holes merge together. These bursts sometimes continue for several tens of seconds, but the time dependence of their intensity often exhibits ～10²–10³ almost periodic small peaks with a period of ～10 ms. A model of oscillations in the lower plasma shell, which arises in cosmic gamma-ray bursts and is located near a neutron star, is proposed; the greater part of arising plasma in the form of an “upper” shell continues to expand into the surroundings. Other possible interpretations of periodicity of the “small peaks” are also analyzed.     

Cosmological γ-ray bursts from a neutron star collapse induced by a primordial black hole

A detailed analysis is presented for a novel scenario in which gamma-ray bursts are of intergalactic origin and arise from the induced collapse of an isolated neutron star triggered by a primordial black hole. The energy released from the phase transition of accreted nucleon matter into a quark-gluon plasma is transferred by degenerate neutrinos to the star’s surface, where neutrinos annihilate into an electron-positron plasma and produce an inverted temperature layer that preserves a fire-ball from undue baryonic pollution. Possible observational tests include the absence of apparent cosmological time dilation, the location of γ-ray bursts primarily outside of galaxies, a specific shape of the log N-log S curve, with a large peak near red shift z∼10, the emission of ∼10⁻³ of the total energy in the form of 100-GeV photons, a bimodal distribution of durations, a very weak accompanying pulse of gravitational radiation, etc. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 10, 642–647 (25 November 1999) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Phase transition to color-flavor-locked matter and condensation of Goldstone bosons in neutron star matter

Deconfinement phase transition and condensation of Goldstone bosons in neutron star matter are investigated in a chiral hadronic model (also referred as to the FST model) for the hadronic phase (HP) and in the color-flavor-locked (CFL) quark model for the deconfined quark phase. It is shown that the hadronic-CFL mixed phase (MP) exists in the center of neutron stars with a small bag constant, while the CFL quark matter cannot appear in neutron stars when a large bag constant is taken. Color superconductivity softens the equation of state (EOS) and decreases the maximum mass of neutron stars compared with the unpaired quark matter. The K⁰ condensation in the CFL phase has no remarkable contribution to the EOS and properties of neutron star matter. The EOS and the properties of neutron star matter are sensitive to the bag constant B, the strange quark mass m_s and the color superconducting gap Δ. Increasing B and m_s or decreasing Δ can stiffen the EOS which results in the larger maximum masses of neutron stars.     

A neutron star collapse induced by a primordial black hole as the source of cosmological γ-ray bursts

A novel scenario is proposed for the origin of cosmological γ-ray bursts relating them with the induced collapse of an isolated neutron star under the action of a primordial black hole inside it. A mechanism is pointed out for black hole capturing into bounded orbits in a contracting protostellar cloud (which further evolves to a neutron star), and it is shown that this mechanism is most efficient in the pregalactic epoch. The qualitative results of neutrino transfer calculations are presented; these neutrinos originate from the quark phase transition in the nucleon matter which takes place in the accretion flow in the interior of the star. The neutrinos and antineutrinos escaping from a dense nucleon matter are degenerate and annihilate in the immediate vicinity of the star surface where an inverse temperature layer in the outstreaming electron-positron wind is produced. This layer acts as a natural barrier against baryon pollution and gives rise to a very high (≈ 10³) value of the Lorentz factor in the expanding plasma, in agreement with the observed energy and duration of the process. This makes it possible to explain the main properties of the γ-ray bursts. We also consider other important features of this scenario, including the predominantly extragalactic origin of the bursts, the apparent absence of the cosmological time dilation, the excess drop in the number of bursts—luminosity dependence for z>0.7, and the unlikely corrllation between the burst and the gravitational wave pulse.     

Constraints on the symmetry energy using the mass-radius relation of neutron stars

The nuclear symmetry energy is intimately connected with nuclear astrophysics. This contribution focuses on the estimation of the symmetry energy from experiment and how it is related to the structure of neutron stars. The most important connection is between the radii of neutron stars and the pressure of neutron star matter in the vicinity of the nuclear saturation density n_s. This pressure is essentially controlled by the nuclear symmetry energy parameters S_v and L , the first two coefficients of a Taylor expansion of the symmetry energy around n_s. We discuss constraints on these parameters that can be found from nuclear experiments. We demonstrate that these constraints are largely model-independent by deriving them qualitatively from a simple nuclear model. We also summarize how recent theoretical studies of pure neutron matter can reinforce these constraints. To date, several different astrophysical measurements of neutron star radii have been attempted. Attention is focused on photospheric radius expansion bursts and on thermal emissions from quiescent low-mass X-ray binaries. While none of these observations can, at the present time, determine individual neutron star radii to better than 20% accuracy, the body of observations can be used with Bayesian techniques to effectively constrain them to higher precision. These techniques invert the structure equations and obtain estimates of the pressure-density relation of neutron star matter, not only near n_s, but up to the highest densities found in neutron star interiors. The estimates we derive for neutron star radii are in concordance with predictions from nuclear experiment and theory.     

Constraints on the symmetry energy from observational probes of the neutron star crust

A number of observed phenomena associated with individual neutron star systems or neutron star populations find explanations in models in which the neutron star crust plays an important role. We review recent work examining the sensitivity to the slope of the symmetry energy L of such models, and constraints extracted on L from confronting them with observations. We focus on six sets of observations and proposed explanations: i) The cooling rate of the neutron star in Cassiopeia A, confronting cooling models which include enhanced cooling in the nuclear pasta regions of the inner crust; ii) the upper limit of the observed periods of young X-ray pulsars, confronting models of magnetic field decay in the crust caused by the high resistivity of the nuclear pasta layer; iii) glitches from the Vela pulsar, confronting the paradigm that they arise due to a sudden recoupling of the crustal neutron superfluid to the crustal lattice after a period during which they were decoupled due to vortex pinning; iv) the frequencies of quasi-periodic oscillations in the X-ray tail of light curves from giant flares from soft gamma-ray repeaters, confronting models of torsional crust oscillations; v) the upper limit on the frequency to which millisecond pulsars can be spun-up due to accretion from a binary companion, confronting models of the r-mode instability arising above a threshold frequency determined in part by the viscous dissipation timescale at the crust-core boundary; and vi) the observations of precursor electromagnetic flares a few seconds before short gamma-ray bursts, confronting a model of crust shattering caused by resonant excitation of a crustal oscillation mode by the tidal gravitational field of a companion neutron star just before merger.     

The Casimir effect as a possible source of cosmic energy

Energy production due to the Casimir effect is considered for the case of a superdense state of matter, which can appear in such cosmological objects as white dwarfs, neutron stars, quasars and so on. The energy output produced by the Casimir effect during the creation of a neutron star turns out to be sufficient to explain nova and supernova explosions. It is shown that the Casimir effect might be a possible source of the huge energy output of quasars.     

Quark matter in astrophysics and cosmology

We discuss the role of quark matter in astrophysics and cosmology. The implications of the dynamics of the quark-hadron phase transition in the early universe for the element abundances from big gang nucleosynthesis and the composition of the dark matter in the universe are addressed. We discuss the possibility of deciding on an equation of state for high density matter by observing the cooling of a neutron star remnant of SN1987A. Quark matter models for the Centauros events, Cygnus X-3 cosmic ray events, high energy gamma-ray bursts and the solar neutrino problem are described.     

High energy neutrinos from gamma-ray bursts with precursor supernovae

The high energy neutrino signature from proton-proton and photo-meson interactions in a supernova remnant shell ejected prior to a gamma-ray burst provides a test for the precursor supernova, or supranova, model of gamma-ray bursts. Protons in the supernova remnant shell and photons entrapped from a supernova explosion or a pulsar wind from a fast-rotating neutron star remnant provide ample targets for protons escaping the internal shocks of the gamma-ray burst to interact and produce high energy neutrinos. We calculate the expected neutrino fluxes, which can be detected by current and future experiments.     

Absorption of gamma-ray photons in a vacuum neutron star magnetosphere: II. The formation of “lightnings”

The absorption of a high-energy photon from the external cosmic gamma-ray background in the inner neutron star magnetosphere triggers the generation of a secondary electron-positron plasma and gives rise to a lightning—a lengthening and simultaneously expanding plasma tube. It propagates along magnetic fields lines with a velocity close to the speed of light. The high electron-positron plasma generation rate leads to dynamical screening of the longitudinal electric field that is provided not by charge separation but by electric current growth in the lightning. The lightning radius is comparable to the polar cap radius of a radio pulsar. The number of electron-positron pairs produced in the lightning in its lifetime reaches 10²⁸. The density of the forming plasma is comparable to or even higher than that in the polar cap regions of ordinary pulsars. This suggests that the radio emission from individual lightnings can be observed. Since the formation time of the radio emission is limited by the lightning lifetime, the possible single short radio bursts may be associated with rotating radio transients (RRATs).     

Characteristics of the GAMMA-400 gamma-ray telescope for searching for dark matter signatures

The GAMMA-400 gamma-ray telescope currently under development is designed to measure fluxes of gamma rays and electron-positron cosmic-ray components, which could be associated with the annihilation or decay of dark matter particles, and to survey in detail the celestial sphere in order to search for and investigate discrete gamma-ray sources; to measure the energy spectra of Galactic and extragalactic dif- fuse gamma-ray emissions; and to study gamma-ray bursts and the gamma-ray emissions of active Sun. The GAMMA-400 energy range is 100 MeV to 3000 GeV. The gamma-ray telescope has an angular resolution of ～0.01°, an energy resolution of ～1%, and a proton rejection factor of ～10⁶. The GAMMA-400 will be installed on Russia’s Navigator space platform. Observations are planned to commence in 2018.     

Equation of State of Hot Neutrino Opaque Interior Matter of Neutron Star

The equation of state of hot neutrino opaque interior matter of the neutron star and some of its properties such as the free energy, effective mass, adiabatic index, and temperature are calculated along both isothermal and isentropic paths with the AV ₁₈ and AV ₁₄ potentials using the lowest order constrained variational method. We have shown that the calculated equation of state with the AV ₁₈ potential is harder than with the AV ₁₄ potential. It is found that there is no phase transition in the hot neutrino opaque interior matter of the neutron star. We have shown that for all values of density and entropy, the adiabatic index of neutron star matter is greater than . It is shown that our calculated equations of state obey the causality condition.     

Scientific tasks and present status of the GAMMA-400 project

The GAMMA-400 telescope is designed to investigate discrete high-energy gamma-ray sources in the energy range of 0.1–3000 GeV, to measure the energy spectra of galactic and extragalactic diffuse gammaray emissions, and to study gamma-ray bursts and gamma-ray emissions from an active Sun. The gamma-ray telescope has an angular resolution of ～0.01°, an energy resolution of ～1%, and a proton rejection factor of ～10⁶. Its special assignment is to measure fluxes of gamma rays, electrons, and positrons that could be associated with the annihilation or decay of dark matter particles.     

Spectra and luminosities in gamma-ray bursts

Summary A spatial-distribution analysis of 124 gamma-ray bursts recorded by the soviet experiment Konus has been done in a previous paper. The physical parameters used in this analysis were the flux intensity and the spectral parameterKT, derived fitting the spectra with a thermal bremsstrahlung law. In this paper an analogous analysis is performed with the spectral parameterv _c (the Larmor frequency times the square of the electron temperature) derived fitting the spectra with a thermal synchrotron of semi-relativistic electron model. In this second case, differently from the first one, we must consider also the spectrum dependence on the angle between the observer direction and the neutron star magnetic-field direction. We analyse the possible effects of this dependence on the logN-logP and on the event distributionvs. v _c . The peak luminosity of the bursts as a function ofv _c is also derived. Paper presented at the 2^o Convegno Nazionale di Fisica Cosmica, held at L'Aquila 29 May–2 June 1984.     

The lead nucleus as a miniature surrogate for a neutron star

The nucleus of ²⁰⁸Pb, a system 18 orders of magnitude smaller and 55 orders of magnitude lighter than a neutron star, may be used as a miniature surrogate to establish important correlations between its neutron skin and several neutron-star properties. Indeed, models with a thicker neutron skin in ²⁰⁸Pb generate larger neutron stars that have a lower liquid-to-solid transition density. Further, we illustrate how the correlation between the neutron skin in ²⁰⁸Pb and the radius of a 1.4 solar-mass neutron star may be used to place important constraints on the equation of state of neutron-rich matter and how it may help elucidate the existence of a phase transition at the core of the star.     

Gravity waves accompanying supernova explosions

Gravitational radiation arising during the formation of a protoneutron star is studied. Here it is mainly large-scale nonuniformities that develop inside the star. The entropy and density profiles of such nonuniformities resemble the “mushroom cloud” of a nuclear explosion. A bubble of hot neutron matter floats to the surface of the star, like the “mushroom cloud” of an explosion in the earth’s atmosphere. Depending on the symmetry of the problem, from two to six bubbles can float upward at the same time. The characteristic masses of such bubbles are 0.01M _⊗ and the radial velocities reach ∼0.1c. The energy radiated in the form of gravitational waves in one cycle of bubbles floating to the surface is ∼10⁻² M _⊗ c ²−10⁻¹⁰ M _⊗ c ². Such cycles occur repeatedly as the neutron star cools. This phase can last up to seconds. The total energy radiated in the form of gravitational radiation can reach 10⁻¹ M _⊗ c ². Pis’ma Zh. éksp. Teor. Fiz. 64, No. 12, 817–822 (25 December 1996)     

The spin-isospin decomposition of the nuclear symmetry energy from low to high density

The energy per particle BA in nuclear matter is calculated up to high baryon density in the whole isospin asymmetry range from symmetric matter to pure neutron matter.The results,obtained in the framework of the Brueckner-Hartree-Fock approximation with two-and three-body forces,confirm the well-known parabolic dependence on the asymmetry parameterβ=(N?Z)/A(β^2 law)that is valid in a wide density range.To investigate the extent to which this behavior can be traced back to the properties of the underlying interaction,aside from the mean field approximation,the spin-isospin decomposition of BA is performed.Theoretical indications suggest that theβ^2 law could be violated at higher densities as a consequence of the three-body forces.This raises the problem that the symmetry energy,calculated according to theβ^2 law as a difference between BA in pure neutron matter and symmetric nuclear matter,cannot be applied to neutron stars.One should return to the proper definition of the nuclear symmetry energy as a response of the nuclear system to small isospin imbalance from the Z=N nuclei and pure neutron matter.     

Nuclear symmetry energy effects in finite nuclei and neutron star

Within a relativistic mean-field model with nonlinear isoscalar–isovector coupling, we explore the possibility of constraining the density dependence of nuclear symmetry energy from a systematic study of the neutron skin thickness of finite nuclei and neutron star properties. We find the present skin data supports a rather stiff symmetry energy at subsaturation densities that corresponds to a soft symmetry energy at supranormal densities. Correlation between the skin of ²⁰⁸Pb and the neutron star masses and radii with kaon condensation has been studied. We find that ²⁰⁸Pb skin estimate suggest star radii that reveals considerable model dependence. Thus precise measurements of neutron star radii in conjunction with skin thickness of heavy nuclei could provide significant constraint on the density dependence of symmetry energy.     

Relativistic Mean-Field Approach to Infinite Nuclear Matter and Neutron Star

The properties of infinite nuclear matter and neutron star are studied theoretically in relativistic mean-field (RMF) approach with three typical parameter sets NL1, NL-SH and TM1. It is found that all these new RMF parameter sets can very satisfactorily reproduce the properties of high density matter. Among these parameter sets, TM1, with a nonlinear ω term, reproduces a slightly smaller energy, piessure and neutron star mass than NL-SH and NL1. The ρ meson field has a large influence on the properties of neutron star and infinite nuclear matter. A detailed discussion for the significance of numerical results is also given.