High velocity neutron stars and γ-ray bursts

The observations of the BATSE instrument on the Compton Gamma-Ray Observatory show that the spatial distribution of -ray burst sources is isotropic but radially non-uniform. As is well known, the spectral features, the time histories and the X-ray tails present in some -ray bursts suggest that they may arise from galactic neutron stars. But, low velocity neutron stars born in the Galactic disk would concentrate toward the galactic plane and center, and could not fit the BATSE results. However, the high velocity neutron stars with velocity 1000 km s^–1 may escape from the Galactic gravitational field and form a nearly isotropic distribution. Here we calculate the three statistical values of V/V _max, sin² b and cos as functions of the intensitiesC _max/C _min and find that they could be fitted by the distribution of high velocity neutron stars under the assumption that the high velocity neutron stars should turn on as -ray burst sources only after some time (perhaps after they have ceased to be radio pulsars). Our calculation shows that the statistical value of cos is more sensitive than sin² b to the angular distribution of high velocity neutron stars, i.e. the deviation of cos from 0 is more readily detected than the deviation of sin² b from 1/3, so we expect that with the increasing sensitivity of instrument and the more exact value of cos , it is possible to determine whether this high velocity neutron star model is correct. Some results are discussed in the text.     

Hidden mass in the solar neighborhood

It is suggested that the minimum mass of a star at the time of its formation is approximately 0.01M . Making use of this fact and the stellar mass functionF(M) M ^–, it is found that the hidden mass (or the missing mass) in the solar neighborhood may be explained by the presence of a large number of invisible stars of very low mass (0.01M M<0.07M ).     

Evidence for a low-density inflationary universe?

The inflationary unvierse model predicts the density parameter ₀ to be 1.0 with the cosmological constant ₀ usually taken to be zero, whereas observational estimates give ₀0.2 and ₀10^-57 cm^–2. It was found, however, that the observed variation of angular diameter with redshift for extragalactic radio sources could be interpreted in terms of a low density universe with linear size evolution of the sources for either an inflationary model with 0 or an open model with =0.     

Neutron-phonon interaction in neutron stars: Phonon spectrum of Coulomb lattice

The phonon excitation spectrum of Coulomb lattice in the neutron star crusts is studied by solving Dyson's equation for phonons. It is shown that a strong renormalization of the phonon spectrum occurs at densities _s /4 for the crustal matter compositions with spherical nuclei, which imply relatively small nuclear mass numbers and charges. It is shown that, the lattice becomes unstable against density fluctuations above a critical density of the order of _s /3, where _s 2.6x10¹⁴ g cm^–3 is the nuclear saturation density. The neutron quasiparticle spectrum and the virtual mass of a nucleus are briefly discussed.     

Hubble expansion in a euclidean framework

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The first Byurakan Spectral Sky Survey. Stars of later spectral classes. V. The zone +65°⩽δ⩽+69°

We give the fifth list of red stars of the First Byurakan Spectral Sky Survey. The objects are located in the zone 5^h 18^h 30^m and +65 +69. The list contains the data on 18 new objects, one of which is a star of class R. We also give the first determination of the spectral class of 11 variable objects, for four of which we give the spectrograms in the range of wavelengths 4700–6700å.Translated fromAstrofizika, Vol. 37, No. 4, 1994.     

Pulsar radio emission

A new version of the theory of pulsar radio emission is developed for the case of a coaxial rotator. It is based on the electric field that we established [G. S. Sahakian, Astrofizika, 37, 97 (1994)] for the radiation channel (the channel of open magnetic field lines) and on convenient approximations for the electron energy obtained in [G. S. Sahakian and É. S. Chubarian, Astrofizika, 37, 255 (1994)]. It is shown that, owing to the emission of photons of curvature radiation by particles, e e+_c', and photon annihilation, _c e⁺e^– in the lower part of the radiation channel, a special region (the magnetic funnel) is formed in which vigorous cascade multiplication of particles occurs. The height of the magnetic funnel is h 6R^0.2, where R is the radius of the neutron star and is its angular rotation rate. As a result of supersaturation of the plasma density in the magnetic funnel, a discharge occurs after each time intervalt5·10^–7–0.8B ₁₂ ^–1.4 R ₆ ^–0.2 , i.e., the longitudinal electric field disappears (B is the magnetic induction in the star). During the active radiative processes in the magnetic funnel, two main fluxes of particles with high ultrarelativistic energies are formed: an upward flux of electrons and a positron flux falling onto the star's magnetic cap. These fluxes are accompanied by narrow strips of positron and electron fluxes, respectively, of considerably lower energy, which are fairly powerful, coherent radio sources. The pulsar's radio luminosity is calculated to be L7.4·10^223.8 ₃₀ ³ R ₆ ^–2 erg/sec, where =BR 3/2 is the star's magnetic moment. Comparing this result with observations, we conclude that the magnetic moment and hence the mass of the neutron star evidently must be considerably smaller, on the average, for fast pulsars than for slow ones. It is shown that the magnetic moment of the neutron star can be determined from the intervals between micropulses in the pulse profiles. The problem of the origin of the macrostructure of the radio pulse is discussed.Translated from Astrofizika, Vol. 38, No. 1, pp. 141–185, January – March, 1995.     

A search for soft X-ray emission from stellar sources

With the soft X-ray detector (0.2–0.284 keV) aboard the Astronomical Netherlands Satellite (ANS) we have searched for X-ray emission from hot star coronae and peculiar stars. On Sirius ( CMa) and Capella ( Aur) X-ray emission has been measured at 6 and 5 level, respectively, above background. In all other cases the search revealed no evidence for soft X-ray emission. Upper limits to the luminosities of about 25 star coronae (main-sequence stars, (sub)giants, and supergiants) and of 4 peculiar stars ( Sco, Lyr, P Cyg, and Car) have been obtained.Paper presented at the COSPAR/IAU Symposium on Fast Transients in X-and Gamma-Rays, held at Varna, Bulgaria, 29–31 May, 1975.     

Gamma-ray bursts from high velocity neutron stars

We investigate the viability of the Galactic corona model of -ray bursts by calculating the spatial distribution of neutron stars born with high velocities in the Galactic disk, and comparing the resulting brightness and angular distribution with the BATSE data. We find that the Galactic corona model can reproduce the BATSE peak flux and angular distribution data for neutron star kick velocities 800 km s^–1, source turn-on ages 10 Myrs, and sampling depths 100 kpc d _max 400 kpc.     

THE RELATIONSHIP BETWEEN SOLAR FLARE GAMMA-RAY EMISSION AND NEUTRON PRODUCTION

We have examined six solar neutron events measured by satellite instruments and/or neutron monitors (NM) to understand the relationship between the intensity–time profiles of the -ray lines, the pion-related -rays, and the neutron production. In all six events the solar neutron production was clearly time-extended. We find that neutron emission as detected by NMs most closely follows the emission of pion-related -rays, whereas lower energy neutron production may follow that of nuclear -ray line emissions. Although this distinction is not unexpected, it is safe to say that the 2.223 MeV -ray line from neutron capture on hydrogen is a poor measure of the neutron production at energies >200 MeV. During the three events on 1982, June 3, 1990, May 24 and 1991, June 4 solar neutrons with energies greater than 200 MeV were recorded by NMs. The NM increases on 1982, June 3 and 1990, May 24 can be modeled using the time profile of the pion-related -rays. For the 1991, June 4 event the NM signal was small but lasted for 60 min and the high-energy -ray data available to us are insufficient to conclude unambiguously that the high-energy neutron production followed the pion-related -rays. In the other three events on 1991, June 9, 11, and 15 solar neutrons with energies 10–100 MeV were observed by the COMPTEL -ray instrument on the Compton Gamma Ray Observatory. The duration of the low-energy neutron production on 1991, June 9 corresponded clearly to the high-energy and not to the low-energy -ray emission.     

The first byurakan spectral sky survey. Late spectral class stars. VII. The zone +76° ≤ δ ≤ +80°

We present the seventh list of stars of the late M and C spectral classes taken from the plates of the First Byurakan Spectral Sky Survey (FBS). Data on 24 objects, 2 of which are carbon stars of early subclasses (R-stars), are given for the first time. The spectral membership of two known stars is established. The objects lie in the zone 3 ^h 40 ^m 18 ^h 30 ^m and +76° +80°.Translated fromAstrofizika, Vol. 38, No. 3, 1995.     

On the interior of the neutron star (II)

With the assumption, the physical 3-spacet = constant in a superdense star is spheroidal and the matter-density on the boundary surface of the configurationa = 2 × 10¹⁴ g cm^–3( the average matter density in a neutron star) Vaidya and Tikekar (1982) proposed an exact relativistic model for a neutron star. They suggested that their model can describe the hydrostatic equilibrium conditions in such a superdense star with densities in the range of 10¹⁴-10¹⁶ g cm^–3. Based on this model Parui and Sarma (1991) estimated the maximum limit of the density variation parameter for a stable neutron star (both for charged and uncharged) which is equal to 0.68, i.e. _max = 0.68.In this paper we have shown variation of central density per unit equilibrium radius (0/a), variation of mass, upper limit of density variation parameter both for charged and uncharged neutron stars at densities 10¹⁵ and 10¹⁶ g cm^–3, respectively. We have obtained _max = 0.68, i.e. the same as before. The important is that the duration of stability among the neutron star's constituents around _max will be shorter and shorter at higher densities as we proceed near the centre of the neutron star. In case of a charged neutron star, once stability among the constituents has been established, then unstability appears gradually maintaining linear relation between change in central density per unit equilibrium radius and change in mass whereas in case of uncharged neutron star, linear relation does not maintain.     

Neutron stars and general equation of nuclear matter

It has been shown that the mass of neutron stars obtained from equations of state based on nuclear theory depend upon the number of baryons assembled in it but not on the type of interactions considered. On examining the behaviour of different equations of state based on nuclear theories, a simple polytropic equation of state,P = (K/N)(p –p _s)^N is proposed. The results obtained forN=1.75 cover the entire range of neutron star masses obtained from the equations of state based on nuclear theories and give a maximum mass of 2.8M . Depending upon various mechanisms for energy output the mass of Crab pulsar is estimated to range from 0.32M to 1.5M . The relation connecting the coordinate mass,M, and the rest mass,M ₀, may be written asM/M 0.93 (M ₀/M)^0.9.     

Relativistic supermassive stars

A plausible scenario for the formation of a stable supermassive star in the relativistic regime has been discussed. The onset of the negativity of the `distribution function' in the stable sequences of the star clusters [the stability of star clusters is assured by using the variational method (Chandrasekhar, 1964a,b) for equivalent gas spheres] described by Tolman's type VII solution with vanishing surface density has been regarded as an indication of the conversion of the cluster structure into a supermassive star. For the critical values of the `adiabatic index', (4/3) < _crit (5/3) (forwhich a supermassive star represent neutrally stable system), the mass, and the size of this object comes out to be 6.87 × 10⁷ M M 1.7 × 10⁹M, and 2.74× 10¹⁴ cm a 1.43× 10¹⁵cm respectively, for the central temperature,T₀ = 6× 10⁷°K, which is sufficient for the release ofnuclear energy. The total energy released during their evolution rangesfrom 2.46× 10⁶⁰ - 3.18× 10⁶² erg, which is sufficient to power these objects at least for a period of 10⁶ - 10⁷years. These figures agrees quite well with those cited for Quasi Stellar Objects (QSOs) in the literature.     

On the evolution of obstars from the main sequence to the Helium exhaustion phase

The evolution of a 20M star of extreme Population I (X=0.602,Z=0.044) from the main sequence through the He-exhaustion has been calculated with particular attention to the treatment of the semiconvective zones. Two different relations as convective instability criteria have been adopted _{R a} +(/(4–3)) (d ln /d lnP) or _{R a} ). The He-burning phase is substantially different in the two cases. In the first evolutionary picture the star begins the He-burning phase as red supergiant, while in the second one as blue supergiant. The two evolutions are compared with the observational blue to red supergiants ration _B/n _R in connection with the problem of the existence of the neutrino processes and taking into account the possible presence of a high percentage of binary systems among supergiants.We draw the conclusion that the massive star evolution seems to be still completely uncertain and also not able to give a decisive test for or against the neutrino emission.     

Generation of γ-bursts by old magnetic neutron stars

A model of -bursts is considered that treats the flares of neutron stars as a result of convectiveoscillation instability associated with the stars having strong internal magnetic fields ( 10¹³ to 10¹⁴ G). In the context of this model only sufficiently old (10⁴ to 10⁷ yr), drastically cooled-down neutron stars may be sources of -bursts. The paper shows that major characteristics of a -burster in the Supernova N 49 remnant (energy release during burst up to 10⁴⁴ erg, age 10⁴ yr, burst-to-burst interval (I to 3)×10⁶s; rotation period P=8 s) may be explained under the assumption that the mass of the neutron star is about 0.14M _· while its mean magnetic field strength is 1.5×10¹⁴ G abd 10¹³ G within the star and on its surface, respectively. The observational tests of the model discussed conclude the paper.     

Coalescing neutron stars as gamma ray bursters?

We investigate the dynamics and evolution of coalescing neutron stars. The three-dimensional Newtonian equations of hydrodynamics are integrated by the Piecewise Parabolic Method on an equidistant Cartesian grid. The code is purely Newtonian, but does include the emission of gravitational waves and their back-reaction. The properties of neutron star matter are described by the equation of state of Lattimer and Swesty (1991). Energy loss by all types of neutrinos and changes of the electron fraction due to the emission of electron neutrinos and antineutrinos are taken into account by an elaborate neutrino leakage scheme. We simulate the coalescence of two identical, cool neutron stars with a baryonic mass of 1.6M and a radius of 15 km and with an initial center-to-center distance of 42 km. The initial distributions of density and electron concentration are given from a model of a cold neutron star in hydrostatic equilibrium. We investigate three cases which differ by the initial velocity distribution in the neutron stars. The orbit decays due to gravitational-wave emission and after one revolution the stars are so close that dynamical instability sets in. Within 1 ms the neutron stars merge into a rapidly spinning (P 1 ms), high-density body ( 10¹⁴ g/cm³) with a surrounding thick disk of material with densities 10¹⁰ – 10¹² g/cm³ and orbital velocities of 0.3-0.5 c. The peak emission of gravitational waves has a maximum luminosity of a few times 10⁵⁵ erg/s and is reached for about 1 ms. The amplitudes of the gravitational waves are close to 3 10^–23 at a distance of 1 Gpc and the typical frequency is near the dynamical value of the orbital motion of the merging neutron stars of 2 KHz. In a post-processing step, the rate of neutrino-antineutrino annihilation is calculated from the neutrino luminosities generated during the hydrodynamical simulations. We find the integral annihilation rate to be a few 10⁵⁰ erg/s during the phase of strongest neutrino emission, which is too small to generate the observed bursts considering the fact that the merged object of about 3M will most likely collapse to a black hole within milliseconds.     

Effects of hyperons on the vibrations of neutron stars

We calculate the effects of hyperons and resonance particles on the vibrations of neutron stars. Vibrating neutron stars can store large amounts of energy in their vibrations; the interaction of the vibrations with the atmosphere would produce electromagnetic radiation. If any process damps out the vibrations rapidly on an astronomical time scale ( 1000 years) then vibrating neutron stars are not likely to be found. Previous work indicates that radiation by a neutrino URCA process (N+NP+N+e ^–+ ) does not rapidly damp many of the neutron star models. Some neutron stars are predicted to contain massive baryons; here we study thermal damping by nonequilibrium reactions involving these baryons.During vibrations the thermodynamic equilibrium state is changed and particle reactions attempt to restore equilibrium. If the reaction rates per particle are very rapid or slow compared to the frequency of vibration the system follows almost the same pressure-volume curve through both parts of the gas cycle, and very little work is done. In the intermediate case, when reaction rates are comparable to the frequency, damping is rapid.We find that the reaction rates for weak interactions such asN+NP+^– (the ^– is the first hyperon to appear with increasing density in degenerate neutron star matter) are of the right magnitude to cause rapid damping. If there is a hyperon region in the star then it cannot sustain vibrations. We also consider the much faster (and hence less important) processN+NP+^–.     

Plasma acceleration by radiation in X-ray pulsars

Charged particle acceleration is considered by a radiation flux from a star or hot spot in X-ray pulsars. It is shown that for any distance from the star there exists the upper velocity limit up to which a particle can be accelerated by radiation. This critical velocity does not depend on the luminosity of the spot. Near the hot spot surface the critical velocityv0.65c. These results are applied to plasma acceleration inX-ray pulsars. The mechanism is advanced, of -ray generation in the course of plasma accretion, onto a neutron star. It is shown that in the presence of a large magnetic field and high luminosity of the spot the relativistic electron-position avalanche may appear. The optical depth of the electron-positron cloud achieves the value of order one. The X-ray quanta emitted by the spot are scattered by relativistic (2.6) electron-positron pairs and are transformed into -radiation. Hard quanta with energy 1 MeV leave the generation region in the narrow cone 0.25.