Oscillation structure of gamma-ray bursts and their possible origin

It is hypothesized that thermonuclear burning of the matter from the envelope of a massive compact star accreting onto a hot neutron star produced by spherically symmetric collapse of a stellar iron core can proceed in oscillation mode (much as is the case during thermal explosions of carbon-oxygen cores in lower mass stars). Local density oscillations near the neutron-star surface can generate shock waves; in these shocks, the electron-positron plasma is stratified from the remaining matter, and shells of an expanding relativistic fireball with an oscillation time scale in cosmological gamma-ray bursts (GRBs) of ～10^?2 s are formed. It is pointed out that the GRB progenitors can be nonrotating massive Wolf-Rayet (WR) stars whose collapse, according to observational data, can proceed without any substantial envelope ejection.     

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.     

Velocity Distributions of Runaway Stars Produced by Supernovae in the Galaxy

Using a method of population synthesis, we investigate the runaway stars produced by disrupted binaries via asymmetric core collapse supernova explosions (CC-RASs) and thermonuclear supernova explosions (TN-RASs). We find the velocities of CC-RASs in the range of about 30–100 km s ^?1. The runaway stars observed in the galaxy are possibly CC-RASs. Due to differences in stellar chemical components and structures, TN-RASs are divided into hydrogen-rich TN-RASs and helium-rich TN-RASs. The velocities of the former are about 100–500 km s ^?1, while the velocities of the latter are mainly between 600 and 1100 km s ^?1. The hypervelocity stars observed in the galaxy may originate from thermonuclear supernova explosions. Our results possibly cover the US 708 which is a compact helium star and travels with a velocity of 1157 ±53 km s^?1 in our galaxy.     

Hydrodynamical models of type II supernovae

A series of hydrodynamical models of type-II supernova outbursts (SNII) has been calculated. Approximate relations connecting the total outburst energy ε, the mass of envelope ejectedM, the presupernova radiusR, and the amount of ionizing quanta radiated by the supernovaeN _H with such values as the duration of the light curve plateau Δt, and absolute magnitude in the wavelength bandV and photospheric velocityU _PH observed near the middle of the plateau have been established. Advantage has been taken of the relations to obtain a preliminary evaluation for the characteristics of the average SN II: ε=7×10⁵⁰ erg,M=6M _⊙,R=500R _⊙,N _H=2×10⁵⁸. The SNIIs with plateau-like light curves seem to be accounted for by thermonuclear explosions of degenerate cores of red giant stars and result in a total disruption of the star without any stellar remnant. To the contrary, SNIIs with linear light curves have substantially different properties (in particular, they throw considerably less massive envelopes off). These SNII must signify the birth of collapsed objects—neutron stars (pulsars) or black holes.     

Neutron stars and the dense matter equation of state

Neutron stars provide a unique laboratory with which to study cold, dense matter. The observational quantities of primary astrophysics interest are the maximum mass and the typical radius of a neutron star. These quantities are related to the relative stiffness of neutron-rich matter at supernuclear densities and the density dependence of the nuclear symmetry energy near the nuclear saturation density. The measurements of these nuclear properties via nuclear systematics and structure, heavy-ion collisions and parity-violating electron scattering from neutron-rich nuclei, are discussed. Several new observations, including mass measurements of binary pulsars and a confirmed distance determination for a nearby cooling neutron star, will be summarized. Additionally addressed will be observations of thermal emissions from cooling neutron stars in globular clusters and thermonuclear explosions from accreting stars. It will be demonstrated how this astrophysical data is shedding light on the pressure-density relation of extremely dense matter.     

On the relationship between pulsars and supernova remnants

We propose that single stars in the mass range 4–6·5M _⊙, that explode as Supernovae of Type I, are totally disrupted by the explosion and form shell-type remnants. More massive single stars which explode as Supernovae of Type II also give rise to shell-type remnants, but in this case a neutron star or a black hole is left behind. The first supernova explosion in a close binary also gives rise to a shell-type supernova remnant. The Crab-like filled-centre supernova remnants are formed by the second supernova explosion in a close binary. The hybrid supernova remnants, consisting of a filled centre surrounded by a shell, are formed if there is an active neutron star inside the shell.     

Nuclear fission in the neutron stars andγ-ray bursts

Nuclear explosions near the surface of a neutron star occur because of the nuclear fission of superheavy nuclei which is overabundant in neutrons. Such nuclei exist in the nonequilibrium layer of the neutron stars solid envelope and are transported close to the surface in starquake events. These explosions may be observed as γ-ray bursts.     

Synchrotron neutrino-pair radiation in neutron stars

The neutrino-pair radiation by electrons in a non-quantizing magnetic field B is investigated. For a relativistic degenerate electron gas the emissivity of this process is mainly given by \documentclass{article}\pagestyle{empty}\begin{document}$ \varepsilon _r = 5 \times 10^{15} (pF/mc)^{4/3} \,B_{13}^{2/3} T_y^{12/8} \,{\rm erg} \times {\rm cm}^{ - 3} \times {\rm sec}^{- 1} $\end{document} where pF is the electron Fermi momentum. Under typical neutron star conditions at B ∼ 10¹³G neutrino synchrotron radiation appears to be one of the most effective mechanisms of neutrino energy loss in the envelopes of neutron stars; this mechanism may also compete with other known neutrino production mechanisms in the neutron star cores if pion condensate or quark matter is absent.     

The effects of strong interaction on the observational discrimination between neutron and strange stars

Strange stars are compact objects similar to neutron stars composed of strange matter. This paper investigates the observational effects of the strong interaction between quarks. We believe: 1) that the conversion of a neutron star to a strange star is a large “period glitch” which is determined by the strong interaction; 2) that the strong interaction results in effective damping of oscillation of hot strange stars, which could be a new mechanism of driving supernova explosions; 3) that the strong interaction increases the difference in rotation between strange and neutron stars under high temperatures, making the minimum period for strange stars lower than that for neutron stars.     

Expansion of neutron star matter and its final nuclear composition

The final nuclear composition of the matter expanding from the density of a neutron star is investigated. It is assumed that starquakes cause the cracks which penetrate the neutron star crust and that the neutron star fluid can flow out through the cracks into space. The change with time of the nuclear composition of this matter is calculated by use of the compressible nuclear mass formula, and the hydrodynamics of the system is followed by the effect of nuclear transformation with time of the second fission of heavy neutron-rich nuclei, which is followed by a rapid rise to above 10⁹ K. If the value of the -strength function exceeds about 10^–5.5 MeV^–1 s^–1, the system proceeds to a state of nuclear equilibrium in the later expansion stage and the nuclear composition is reshuffled, finally to be transformed into neutron-excess, stable nuclei within the atomic mass region 80A120. It also becomes clear that if the strength function has a value smaller than the above critical value, then the neutron-rich nuclides withA[200, 400] are copiously produced. These results will also be applied in the cases of a neutron-star-black-hole collision and the explosion of a neutron star associated with the catastrophic phase transition within the neutron star core. The astrophysical implications are briefly discussed.     

Effect of partial mixing of matter on the hydrodynamic angular momentum transport processes in massive main-sequence stars

We consider the evolution of a rotating star with a mass of 16M _⊙ and an angular momentum of 3.25 × 10⁵² g cm² s^?1, along with the hydrodynamic transport of angular momentum and chemical elements in its interiors. When the partial mixing of matter of the turbulent radiative envelope and the convective core is taken into account, the efficiency of the angular momentum transport by meridional circulation in the stellar interiors and the duration of the hydrogen burning phase increase. Depending on the Schmidt number in the turbulent radiative stellar envelope, the ratio of the equatorial rotational velocity to the circular one increases with time in the process of stellar evolution and can become typical of early-type Be stars during an additional evolution time of the star on the main sequence. Partial mixing of matter is a necessary condition under which the hydrodynamic transport processes can increase the angular momentum of the outer stellar layer to an extent that the equatorial rotational velocity begins to increase during the second half of the evolutionary phase of the star on the main sequence, as shown by observations of the brightest stars in open star clusters with ages of 10–25 Myr. When the turbulent Schmidt number is 0.4, the equatorial rotational velocity of the star increases during the second half of the hydrogen burning phase in the convective core from 330 to 450 km s^?1.     

Energetics of galactic nuclei

A model of compact galactic nuclei in statistical equilibrium was developed in [L. Sh. Grigorian and G. S. Sahakian, Astrofizika (in press)]. It was shown that they should consist predominantly of neutron stars (pulsars) and white dwarfs. The problem of the energy reserves of galactic nuclei is discussed in terms of this concept. The mechanism of conversion of a white dwarf into a neutron star due to the accretion of interstellar matter is considered. This means that a galactic nucleus has an energy reserve of some 5·10⁶⁰ N₈ erg (N is the number of stars in the nucleus). It is shown that galactic nuclei are powerful sources of hard γ radiation [power L » 2·10⁴⁴µ₃₀N₈(Ω/50)^17/7 erg/sec, where µ is the magnetic moment and Ω is the angular rotation rate of a neutron star ] due to curvature radiation from relativistic electron fluxes flowing along channels of open magnetic field lines of pulsars. The x-ray and ultraviolet emission are due to synchrotron emission from the same electron fluxes in the magnetic field of the galactic nucleus (L » 10⁴²-10⁴⁴ erg/sec). The optical (visible and infrared) and radio emission are due to bremsstrahlung from electrons in the interstellar medium [L » 6·10⁴⁶N ₈ ² (5/R_pc)³ erg/sec, where R is the radius of the galactic nucleus]. An equation is obtained for the magnetic moment of a pulsar: µ ≈ 3.4·10^-5L_γP^17/7, where P is the pulsar’s period and L_03B3; is the luminosity of the pulsar’s y radiation.     

Heat blanketing envelopes and thermal radiation of strongly magnetized neutron stars

Strong (B?10⁹ G) and superstrong (B?10¹⁴ G) magnetic fields profoundly affect many thermodynamic and kinetic characteristics of dense plasmas in neutron star envelopes. In particular, they produce strongly anisotropic thermal conductivity in the neutron star crust and modify the equation of state and radiative opacities in the atmosphere, which are major ingredients of the cooling theory and spectral atmosphere models. As a result, both the radiation spectrum and the thermal luminosity of a neutron star can be affected by the magnetic field. We briefly review these effects and demonstrate the influence of magnetic field strength on the thermal structure of an isolated neutron star, putting emphasis on the differences brought about by the superstrong fields and high temperatures of magnetars. For the latter objects, it is important to take proper account of a combined effect of the magnetic field on thermal conduction and neutrino emission at densities ρ?10¹⁰ g?cm^?3. We show that the neutrino emission puts a B-dependent upper limit on the effective surface temperature of a cooling neutron star.     

Multiple X-ray bursts and the model of a spreading layer of accreting matter over the neutron star surface

We report the detection of series of close type I X-ray bursts consisting of two or three events with a recurrence time much shorter than the characteristic (at the observed mean accretion rate) time of matter accumulation needed for a thermonuclear explosion to be initiated on the neutron star surface during the JEM-X/INTEGRAL observations of several X-ray bursters. We show that such series of bursts are naturally explained in the model of a spreading layer of accreting matter over the neutron star surface in the case of a sufficiently high (? ? 1 × 10^?9 M _⊙ yr^?1) accretion rate (corresponding to a mean luminosity L _tot ? 1 × 10³⁷erg s^?1). The existence of triple bursts requires some refinement of the model—the importance of a central ring zone is shown. In the standard model of a spreading layer no infall of matter in this zone is believed to occur.     

Single X-ray Bursts and the Model of a Spreading Layer of Accreting Matter over the Neutron Star Surface

The excess of the rate of type I X-ray bursts over that expected when the matter fallen between bursts completely burns out in a thermonuclear explosion which is observed in bursters with a high persistent luminosity (4 × 10³⁶ ? L_X ? 2 × 10³⁷ erg s^?1) is explained in terms of the model of a spreading layer of matter coming from the accretion disk over the neutron star surface. In this model the accreting matter settles to the stellar surface mainly in two high-latitude ring zones. Despite the subsequent spreading of matter over the entire star, its surface density in these zones turns out to be higher than the average one by 2–3 orders of magnitude, which determines the predominant ignition probability. The multiple events whereby the flame after the thermonuclear explosion in one ring zone (initial burst) propagates through less densematter to another zone and initiates a second explosion in it (recurrent burst) make a certain contribution to the observed excess of the burst rate. However, the localized explosions of matter in these zones, after which the burning in the zone rapidly dies out without affecting other zones, make a noticeably larger contribution to the excess of the burst rate over the expected one.     

Scenarios for the formation of binary and millisecond pulsars – A critical assessment

The evolution of high-and low-mass X-ray binaries (HMXB and LMXB) into different types of binary radio pulsars, the ‘high-mass binary pulsars’(HMBP) and ‘low-mass binary pulsars’ (LMBP) is discussed. The HMXB evolve either into Thorne-Zytkow objects or into short-period binaries consisting of a helium star plus a neutron star (or a black hole), resembling Cygnus X-3. The latter systems evolve (with or without a second common-envelope phase) into close binary pulsars, in which the companion of the pulsar may be a massive white dwarf, a neutron star or a black hole ( some final systems may also consist of two black holes). A considerable fraction of the systems may also be disrupted in the second supernova explosion. We discuss the possible reasons why the observed numbers of double neutron stars and of systems like Cyg X-3 are several orders of magnitude lower than theoretically predicted. It is argued that the observed systems form the tip of an iceberg of much larger populations of unobserved systems, some of which may become observable in the future. As to the LMBP, we consider in some detail the origins of systems with orbital periods in the range 1–20 days. We show that to explain their existence, losses of orbital angular momentum (e.g., by magnetic braking) and in a number of cases: also of mass, have to be taken into account. The masses of the low-mass white dwarf companions in these systems can be predicted accurately. We notice a clear correlation between spin period and orbital period for these systems, as well as a clear correlation between pulsar magnetic field strength and orbital period. These relations strongly suggest that increased amounts of mass accreted by the neutron stars lead to increased decay of their magnetic fields: we suggest a simple way to understand the observed value of the ‘bottom’ field strengths of a few times 10⁸ G. Furthermore, we find that the LMBP-systems in which the pulsar has a strong magnetic field (> 10¹¹ G) have an about two orders of magnitude larger birth rate (i.e., about 4 × 10^-4 yr^-1 in the Galaxy) than the systems with millisecond pulsars (which have B < 10⁹ G). Using the observational fact that neutron stars receive a velocity kick of ∼450 km/s at birth, we find that some 90% of the potential progenitor systems of the strong-field LMBP must have been disrupted in the Supernovae in which their neutron stars were formed. Hence, the formation rate of the progenitors of the strong-field LMBP is of the same order as the galactic supernova rate (4 × 10^-3 yr^-1). This implies that a large fraction of all Supernovae take place in binaries with a close low-mass (< 2.3 M⊙) companion.     

Properties of Wolf-Rayet stars in eclipsing binary systems

The results of investigations of a number of eclipsing Wolf-Rayet binaries are presented. The ‘core’ radiuses, the ‘core’ temperatures and masses of WR stars in the eclipsing WR+OB binary systems V 444 Cyg, CX Cep, CQ Cep, and CV Ser are obtained (see Table I). The results obtained from the light curves analysis of the V 444 Cyg in the range λλ2460 Å-3.5μ give strong evidence for the Beals (1944) model of WR phenomenon. The chromospheric-coronal effects in the WN5 extended atmosphere are not observed up to a distance ofr?20R _⊙. In the Hertzsprung—Russell diagram all the WR stars lie on the left side from the main sequence between the main sequence and the sequence of uniform helium stars (see Figure 9). Their locations are close to those of the helium remnants formed as a result of mass exchange in massive close binary systems. The period variations in the systems V 444 Cyg and CQ Cep have been discovered and a reliable value of the mass loss rateM=10^?5 M _⊙yr^?1 is obtained, for the two WR stars. The results of the photometric and spectroscopic investigations of the WR stars with low mass companions (post X-ray binary stage?) are presented too (see Table II). The masses of the companions are (1–2)M _⊙, their optical luminosity is ～10³⁶, erg s^?1 which implies that these companions cannot be the normal stars. It is possible that these companions are neutron stars accreting from the stellar wind of the WR stars. Low values of the X-ray luminosities of such WR stars with low mass companions imply that the accretion of matter in such systems is distinct from the accretion process in classical X-ray binary systems. It is noted also that the parameters of low massive companions coupled with WR stars are close to those of helium stars.     

Symmetry of the neutron and proton superfluidity effects in cooling neutron stars

We investigate the combined effect of neutron and proton superfluidities on the cooling of neutron stars whose cores consist of nucleons and electrons. We consider the singlet state paring of protons and the triplet pairing of neutrons in the cores of neutron stars. The critical superfluid temperatures T _c are assumed to depend on the matter density. We study two types of neutron pairing with different components of the total angular momentum of a Cooper pair along the quantization axis (|m _J |=0 or 2). Our calculations are compared with the observations of thermal emission from isolated neutron stars. We show that the observations can be interpreted by using two classes of superfluidity models: (1) strong proton superfluidity with a maximum critical temperature in the stellar core T _c ^max ?4×10⁹ K and weak neutron superfluidity of any type (T _c ^max ?2×10⁸ K); (2) strong neutron superfluidity (pairing with m _J =0) and weak proton superfluidity. The two types of models reflect an approximate symmetry with respect to an interchange of the critical neutron and proton pairing temperatures.     

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+^–.     

The influence of magnetic field and rotation on the onset of convection in the envelopes of neutron stars

The influence of magnetic field and rotation on the occurrence of convective instabilities in the liquid layer of neutron star envelopes has been investigated. The critical wavelength _c , which denotes the boundary between stable and unstable behaviour of convective disturbances, is calculated for a neutron star model as a function of magnetic field and rotation. It is shown that the strength of the magnetic fields of neutron stars strongly suppresses the onset of convection, whereas the limiting effect of rotation acts only if the magnetic field vanishes.