Showing papers in "Astronomy Reports in 2006"

Instantaneous Radio Spectra of Giant Pulses from the Crab Pulsar from Decimeter to Decameter Wavelengths

Abstract: The results of simultaneous multifrequency observations of giant radio pulses from the Crab pulsar, PSR B0531+21, at 23, 111, and 600 MHz are presented and analyzed. Giant pulses were detected at a frequency as low as 23 MHz for the first time. Of the 45 giant pulses detected at 23 MHz, 12 were identified with counterparts observed simultaneously at 600 MHz. Of the 128 giant pulses detected at 111 MHz, 21 were identified with counterparts observed simultaneously at 600 MHz. The spectral indices for the power-law frequency dependence of the giant-pulse energies are from −3.1 to −1.6. The mean spectral index is −2.7 ± 0.1 and is the same for both frequency combinations (600–111 MHz and 600–23 MHz). The large scatter in the spectral indices of the individual pulses and the large number of unidentified giant pulses suggest that the spectra of the individual giant pulses do not actually follow a simple power law. The observed shapes of the giant pulses at all three frequencies are determined by scattering on interstellar plasma inhomogeneities. The scatter-broadening of the pulses and its frequency dependence were determined as τsc = 20(ν/100)−3.5±0.1 ms, where frequency ν is in MHz.

The role of close passages of galaxies and the asymmetry of their dark haloes in the formation of their spiral patterns

Abstract: The influence of close passages of galaxies on the shapes of disk galaxies and the distribution of stars in them is studied for several types of interactions in the framework of the restricted N-body problem. Depending on the conditions adopted, either two spiral density waves or ring structures are formed in the stellar disk of the galaxy. These structures can generate star formation fronts with the corresponding shape, as are observed in disk galaxies. Our calculations can also be applied to study the influence of the passage of a nearby star on a protoplanetary disk. The formation of ring structures there could specify the type of planet formation in the outer regions of the planetary system and the distribution of semimajor axes for the planetary orbits. We use the same model to study the generation and evolution of spiral density waves in the stellar disks of galaxies as a result of the recently found asymmetry of the gravitational potential in the massive dark haloes in disk galaxies. The dipole component of the gravitational field of the halo can continuously permanently generate the spiral structure in disk galaxies.

Variations of the dipole magnetic moment of the sun during the solar activity cycle

Abstract: Observations of the large-scale solar magnetic field (synoptic maps) and measurements of the magnetic field of the Sun as a star (the total magnetic field) are used to determine the dipole magnetic moment and direction of the dipole field for three successive solar cycles. Both the magnetic moment and its vertical and horizontal components vary regularly during the cycle, but never disappear completely. A wavelet analysis of the total magnetic field shows that the amplitude of the 27-day variations of this field is very closely related to the magnetic moment of the horizontal dipole. The reversal of the global dipole field corresponds to a change in the inclination of its axis and occurs in a series of steps lasting one to two years rather than continuously. Before the onset of the reversal, the dipole axis precesses relative to the solar rotational axis, then shifts in a meridianal plane, reaching very low latitudes, where a substantial shift in longitude then begins. These results are discussed in connection with helioseismological data indicating the existence of oscillations with a period of about 1.3 yr and properties of dynamo processes for the case of an inclined rotator.

The OSACA database and a kinematic analysis of stars in the solar neighborhood

Abstract: We transformed radial velocities compiled from more than 1400 published sources, including the Geneva-Copenhagen survey of the solar neighborhood (CORAVEL-CfA), into a uniform system based on the radial velocities of 854 standard stars in our list. This enabled us to calculate the average weighted radial velocities for more than 25000 HIPPARCOS stars located in the local Galactic spiral arm (Orion arm) with a median error of ±1 km/s. We use these radial velocities together with the stars’ coordinates, parallaxes, and proper motions to determine their Galactic coordinates and space velocities. These quantities, along with other parameters of the stars, are available from the continuously updated Orion Spiral Arm Catalogue (OSACA) and the associated database. We perform a kinematic analysis of the stars by applying an Ogorodnikov-Milne model to the OSACA data. The kinematics of the nearest single and multiple main-sequence stars differ substantially. We used distant (-r ≈ 0.2 kpc) stars of mixed spectral composition to estimate the angular velocity of the Galactic rotation, ωo = −25.7 ± 1.2 kms−1 kpc−1, and the vertex deviation, l = 13° ± 2°, and detected a negative K effect. This negative K effect is most conspicuous in the motion of A0–A5 giants and is equal to K = −13.1 ± 2.0 kms−1 kpc−1.

The detection of class I methanol masers towards regions of low-mass star formation

Abstract: Six young bipolar outflows in regions of low-intermediate-mass star formation were observed in the 70-61A+, 80-71A+, and 5−1-40E methanol lines at 44, 95, and 84 GHz, respectively. Narrow features were detected towards NGC 1333-IRS4A, HH 25MMS, and L1157-B1. The flux densities of the detected lines are not higher than 11 Jy, which is much lower than the flux densities of strong maser lines in regions of high-mass star formation. Analysis shows that the narrow features are most likely masers.

Fine structure of convective motions in the solar photosphere: Observations and theory

Abstract: The granulation brightnesses and convective velocities in the solar photosphere between the levels of formation of the continuum radiation and the temperature minimum are examined. Spectral images of the granulation observed in lines of neutral and ionized iron with high spatial (0.5″) and temporal (9 s) resolutions were obtained using the German Vacuum Tower Telescope in Izana (Tenerife, Spain). A correlation analysis shows that the granules and intergranules change their relative brightness at a height near 250 km, and a general reversal of the velocity occurs near a height of 490 km, where the material above granules begins to predominantly descend, and the material above intergranules, to ascend. The maximum correlation coefficient between the velocity and the line brightnesdoesnot exceed 0.75. The properties of the brightness and velocity are analyzed in a sixteen-column model. Four sorts of motions are most typical and efficient. In the first two, only the sign of the relative contrast of the material changes (an efficiency of 46%). This occurs, on average, at a height of 270 km. In the last two motions, both the sign of the contrast and the direction of the motion are reversed near a height of 350 km (an efficiency of 28%). All the observed dependences are compared with theoretical relations obtained in a three-dimensional hydrodynamical model, with deviations from local thermodynamic equilibrium included in the calculation of the spectral-line profiles. This model can satisfactorily reproduce all the basic features of the convective velocities and intensities. It is concluded that the convective motions maintain their column structure throughout the photosphere, right to the level of the temperature minimum. This makes a separation of the photosphere into two regions with different granulation brightnesses and convective motions unjustified.

Giant Pulses—the Main Component of the Radio Emission of the Crab Pulsar

Abstract: The paper presents an analysis of dual-polarization observations of the Crab pulsar obtained on the 64-m Kalyazin radio telescope at 600 MHz with a time resolution of 250 ns. A lower limit for the intensities of giant pulses is estimated by assuming that the pulsar radio emission in the main pulse and interpulse consists entirely of giant radio pulses; this yields estimates of 100 and 35 Jy for the peak flux densities of giant pulses arising in the main pulse and interpulse, respectively. This assumes that the normal radio emission of the pulse occurs in the precursor pulse. In this case, the longitudes of the giant radio pulses relative to the profile of the normal radio emission turn out to be the same for the Crab pulsar and the millisecond pulsar B1937+21, namely, the giant pulses arise at the trailing edge of the profile of the normal radio emission. Analysis of the distribution of the degree of circular polarization for the giant pulses suggests that they can consist of a random mixture of nanopulses with 100% circular polarization of either sign, with, on average, hundreds of such nanopulses within a single giant pulse.

A study of the outburst development in the classical symbiotic star Z and within the colliding-winds model

Abstract: Two-dimensional gas-dynamical modeling of the mass-flow structures in binary systems is used to study the outburst development in the classical symbiotic star Z And. The stage-by-stage rise of the light during the outburst can be explained using a model with colliding winds. We suggest a scenario for the development of the outburst and study the influence of possible changes in the flow structure on the brightness of the system. The model variations of the luminosity due to the formation of a system of shocks are in good agreement with the observed brightness variations.

Photometry and spectroscopy of CI camelpardalis from 1998–2005

Abstract: Our long-time monitoring of the B[e] star and transient X-ray source CI Cam during quiescence following the 1998 outburst demonstrates that the complex, stratified circumstellar envelope has tended to stabilize after this structure was perturbed by the passage of a shock wave from the outburst. The star’s U BV R brightness shows slow, possibly cyclic, variations with an amplitude of about 0.2m. We determined the spectral type of the primary, B4III-V, based on the widths of the absorption wings of high-numbered Balmer lines. A Doppler shift of 460 km/s was detected for the Hell λ4686 A emission line. The shifts in this line yield an orbital period of 19.41 days, which is also manifested itself in the photometric data as a wave with a V amplitude of 0.034m. The orbit is elliptical, with an eccentricity of 0.62. It is most likely that the secondary is a white dwarf surrounded by an accretion disk. The primary’s mass exceeds 12 M ⊙ . The system may be at a late stage of its evolution, after the stage of mass exchange.

Optical and IR monitoring of the BL Lac object S5 0716+714 from 2001–2004

Abstract: Multicolor (BV RIJHK) observations of the BL Lac object S5 0716+714 carried out irom 2001–2004 indicate that the spectral energy distribution of the slowly varying component remained constant over the entire wavelength interval and over the three-year observation period. The distribution can be represented by the power law F v ∼ v −1.12, providing evidence that this component has a synchrotron nature. The color characteristics of very rapid variability detected on several nights are the same as those for the slowly varying component. An analysis of published data on the color variability of the object for previous years indicates that the color characteristics of the slowly varying component are stable, independent of the time intervals considered and the characteristic variability time scales.

An iterative method for the construction of equilibrium N-body models for stellar disks

Abstract: One widely used technique for the construction of equilibrium models of stellar disks is based on the Jeans equations and the moments of velocity distribution functions computed using these equations. Stellar disks constructed using this technique are shown to be “not entirely” in equilibrium. Our attempt to abandon the epicyclic approximation and the approximation of infinite isothermal layers, which are commonly adopted in this technique, failed to improve the situation substantially. We conclude that the main drawback of techniques based on the Jeans equations is that the system of equations employed is not closed and, therefore, requires adopting an essentially ad hoc additional closure condition. A new iterative approach to constructing equilibrium N-body models with a given density distribution is proposed. The main idea behind this approach is that a model is first constructed using some approximation method, and is then allowed to adjust to an equilibrium state with the specified density distribution—if necessary, with the required parameters of the velocity distribution remaining fixed in the process. This iterative approach was used to construct isotropic, spherically symmetric models and models of stellar disks embedded in an external potential. The numerical models constructed prove to be close to equilibrium. It is shown that the commonly adopted assumption that the profile of the radial-velocity dispersion is exponential may be wrong. The technique proposed can be applied to a wide range of problems involving the construction of models of stellar systems with various geometries.

Three-dimensional hydrodynamical modeling of mass transfer in the close binary system β Lyr

Abstract: We present a three-dimensional hydrodynamical modeling of mass transfer in the close binary system β Lyr taking radiative cooling into account explicitly. The assumed mass-transfer rate through the first Lagrangian point L1 is 3.0 × 10−5M⊙/yr. A flow with a radius of 0.14–0.16 (in units of orbital separation) is formed in the vicinity of L1. This flow forms an accretion disk with a radius close to 23 R⊙ and a thickness of about 10 R⊙. The accretion disk is surrounded by an outer envelope that extends beyond the computational domain. A spiral shock forms at the outer boundary of the disk at orbital phase 0.25. Geometrically, the disk is toruslike, while the outer envelope is cylinder-like. In this model, which has low temperatures inside the computational domain, no jetlike structures form in the disk. It is possible that the jetlike structure in β Lyr arises due to the interaction of radiative wind from the accretor with the flow from L1. In the model considered, a hot region exists over the poles of the accretor at a height of about 0.21. The amount of matter lost by the system is close to 10% of the mass flowing through L1; i.e., the mass transfer in the system is almost conservative. For a mass-transfer rate of 3.0 × 10−5M⊙/yr, the orbital period varies by 40.4 s/yr. This means that the observed variation of the orbital period of 19 s/yr should correspond to a mass-transfer rate close to 1.0 × 10−5M⊙/yr.

Stellar structure of irregular galaxies: Edge-on galaxies

Abstract: Stellar photometry obtained using the Hubble Space Telescope is used to study the distributions of the number densities of stars of various ages in 12 irregular and dwarf spiral galaxies viewed edge-on. Two subsystems can be distinguished in all the galaxies: a thin disk comprised of young stars and a thick disk containing a large fraction of old stars (primarily red giants) in the system. Variations of the stellar number density in the thin and thick disks in the Z direction perpendicular to the plane of the galaxy follow an exponential law. The size of the thin disk corresponds to the visible size of the galaxy at the µ = 25 mag/arcsec2 isophote, while the thick disk is a factor of two to three larger. In addition to a thick disk, the massive irregular galaxy M82 also has a more extended stellar halo that is flattened at the galactic poles. The results of our previous study of 12 face-on galaxies are used together with the new results presented here to construct an empirical model for the stellar structure of irregular galaxies.

Cyclic variations in the differential rotation of the solar corona

Abstract: The rotation of the solar corona is analyzed using the original database on the brightness of the FeXIV 530.3 nm coronal green line covering six recent activity cycles. The rate of the differential rotation of the corona depends on the cycle phase. In decay phases, there are only small differences in the rotation, which are similar to that of a rigid body. The differences are more significant (though less pronounced than in the photosphere) during rise phases, just before maxima, and sometimes at maxima. The total rate of the coronal rotation is represented as a superposition of two, i.e., fast and slow modes. The synodic period of the fast mode is approximately 27 days at the equator and varies slightly with time. This mode displays weak differences in rotation and is most pronounced in the middle of decay phases. The slow mode is manifested only at high latitudes during the rise phases of activity, and displays a mean period of 31 days. The relative contribution of each mode to the total rotational rate is determined as a function of time and heliographic latitude. These results indicate that the structure of the velocity field in the convective zone must also vary with time. This conclusion can be verified by helioseismology measurements in the near future.

Magnetic Tunnels (Wormholes) in Astrophysics

Abstract: We consider models of a wormhole (i) maintained by an electromagnetic field, taking into account quantum vacuum corrections to the equation of state, (ii) maintained by a combination of the magnetic field and phantom energy, with a spherically symmetrical equation of state, and (iii) with a magnetic field and phantom matter with an anisotropic equation of state. It is shown that the quantum corrections and the density and exoticity of phantom energy or matter can be as small as is desired. For an external observer, the entrance to the tunnel appears to be a magnetic monopole of macroscopic size. The accretion of ordinary matter onto the entrance to the tunnel may result in the formation of a black hole with a radial magnetic field. We consider the possibility that some active galactic nuclei and Galactic objects may be current or former entrances to magnetic wormholes. We consider the possible existence of a broad mass spectrum for wormholes, from several billion solar masses to masses of the order of 2 kg. The Hawking effect (evaporation) does not operate in such objects due to the absence of a horizon, making it possible for them to be retained over cosmological time intervals, even if their masses are smaller than 1015 g. We also discuss a model for a binary system formed by the entrances of tunnels with magnetic fields, which could be sources of nonthermal radiation and γ-ray bursts.

The unique BL lac object S5 0716+714

Abstract: A cluster of three galaxies has been observed around the unusual object S5 0716+714, which displays a lineless continuum throughout all wavelength intervals (from radio to gammarays) and rapid variability (even within a day). The galaxies have very similar redshifts: 0.264 ± 0.004, 0.257 ± 0.005, and 0.249 ± 0.003. for the first time, our observations with the 6-m telescope of the Special Astrophysical Observatory equipped with the multimode SCORPIO spectral camera have revealed some evidence for the host galaxy around the object—an elliptically shaped flux excessively extended by 3″–5″ from S5 0716+714.

Dynamics of bubbles in supernovae and turbulent vortices

Abstract: We consider the motion of a bubble in a central acceleration field created by gravity or a centrifugal force. In the former case, the bubble moves outwards from and, in the latter, towards the center. We have calculated the characteristic time needed for a bubble to leave or reach the center. The solution obtained provides insight into the processes of thermonuclear supernovae and combustion; in other words, into the interaction between a flame and a turbulent vortex. In the case of combustion, a light bubble of burnt material propagates towards the axis of a strong turbulent vortex faster than it drifts in the direction of rotation of the vortex. It is expected that the development of bubbles should prevent the formation of “pockets” at the flame front, similar to those predicted by a simplified model of turbulent combustion in a constant density flux. In the case of a thermonuclear supernova in a deflagration burning regime, it is shown that light products of burning rise from the center of the white dwarf substantially more rapidly than the thermonuclear flame front propagates. As a result, a flame cannot completely burn the central part of the star, and instead is pushed to the outer layers of the white dwarf. The effect of bubble motion (large-scale convection) makes spherically symmetric models for thermonuclear supernovae unrealistic, which is of prime importance for the supernova spectrum and energy. The motion of bubbles is even faster in the case of a rotating white dwarf; under certain conditions, the centrifugal force may dominate over the gravitational force. To test this theory, we have carried out numerical simulations of supernovae explosions for various sizes of the burned region in the core of the presupernova. We have derived a relation between the rate of large-scale convection and the size of the burned region, which is specified by the rate of the deflagration in the thermonuclear burning.

A model of a solar flare : Comparisons with observations of high-energy processes

Abstract: New data for the energy and location of the hard-emission centers of a solar flare agree with an electrodynamic model of a solar flare based on the idea of the accumulation of free magnetic energy in the field of a current sheet. Three-dimensional MHD simulations are used to show that the energy stored in the preflare magnetic field of the current sheet is sufficient for the development of a flare and a coronal mass ejection. The flare and coronal mass ejection result from the explosive decay of the current sheet. The position of the brightness-temperature maximum of the radio emission during the flare coincides with the maximum of the current in the current sheet. The exponential spectrum of relativistic protons generated during the flare is consistent with acceleration by the electric field during the current-sheet decay.

Methanol and other molecular tracers of outflows and dense gas in the molecular cloud G345.01+1.79

Abstract: The results of SEST millimeter observations of the molecular cloud G345.01+1.79 are presented. Spectra of CH3OH, SO2, SiO, HCO+, C18O, C33S, C34S, HCN, and DCN lines have been obtained. Mapping of the cloud in CH3OH, SiO, and C34S lines indicates that the maximum integrated intensity in the SiO and C34S lines and in low-excitation CH3OH transitions coincide with the northern group of methanol masers, while the corresponding maximum for high-excitation CH3OH transitions coincides with the southern methanol-maser group. The physical parameters are estimated from the quasi-thermal CH3OH lines under the large-velocity-gradient approximation, and their distribution on the sky derived. The density and temperature are higher toward the southern group of methanol masers than in the northern group. This may indicate that star formation is in an earlier stage of evolution in the northern than toward the southern group. A maser component can be distinguished in 14 (of 71) CH3OH lines. We have detected for the first time weak, probably maser, emission in the CH3OH lines at 148.11, 231.28, 165.05, 165.06, and 165.07 GHz. A blue wing is clearly visible in the CH3OH, SiO, C18O, and SO2 lines. The emission in this wing is probably associated with a compact source whose velocity is characteristic of the CH3OH maser emission in the southern group of masers.

Microvariability of line profiles in the spectra of OB stars: δOri A

Abstract: We have studied variability of the spectral lines of the OB star δOri A—the brightest component of the δOri triple system. Forty spectra with signal-to-noise ratios ≈500–800 and a time resolution of four minutes were obtained. We detected variability in the HeIIλ4686, HeIλ4713, and Hβ absorption and the CIIIλ5696 emission profiles. The amplitude of the variability is ≈(0.5–1)% of the continuum intensity. The dynamical wavelet spectrum of the profile variations reveals large-scale components in the interval 25–50 km/s that move within the-V sin i to V sin i band for the primary star of the system, Aa1, with a band crossing time of 4h–5h. However, some of the variable features go outside the band, presumably due to either imhomogeneities in the stellar wind from δOri Aa1 or nonradial pulsations of the weaker components of the system, Aa2 or Ab. The detected variability may be cyclic with a period of ≈4h. We suggest that it is associated with nonradial pulsations of the primary in the sector mode (l,m) = (2, −2).

A search for periodicity in the light curves of selected blazars

Abstract: We present an analysis of multifrequency light curves of the sources 2223-052 (3C 446), 2230+114 (CTA 102), and 2251+158 (3C 454.3), which had shown evidence of quasiperiodic activity. The analysis made use of data from the University of Michigan Radio Astronomy Observatory (USA) at 4.8, 8, and 14.5 GHz, as well as the Metsahovi Radio Astronomy Observatory (Finland) at 22 and 37 GHz. Application of two different methods (the discrete autocorrelation function and the method of Jurkevich) both revealed evidence for periodicity in the flux variations of these sources at essentially all frequencies. The periods derived for at least two of the sources—2223-052 and 2251+158—are in good agreement with the time interval between the appearance of successive VLBI components. The derived periods for 2251+158 (P = 12.4 yr and 2223-052 (P = 5.8 yr) coincide with the periods found earlier by other authors based on optical light curves.

Integrated radio luminosities of pulsars

Abstract: The integrated radio luminosities of 311 long-period (P > 0.1 s) and 27 short-period (P < 0.1 s) pulsars have been calculated using a new compilation of radio spectra. The luminosities are in the range 1027 − 1030 erg/s for 88% of the long-period pulsars and 1028 − 1031 erg/s for 88% of the short-period pulsars. We find a high correlation between the luminosity L and the estimate L 1 = S 400 d 2 from the catalog of Taylor et al. The factor η for the transformation of the rotational energy of the neutron star into radio emission increases-decreases with increasing period for long-period and short-period pulsars. The mean value of η is −3.73 for the long-period and −4.85 for short-period pulsars. No dependence was found between L and the pulsar’s kinematic age t k = |z|/〈v z〉, where |z| and 〈v z〉 = 300 km/s are the pulsars’ height above the plane of the Galaxy and mean velocity. A dependence of L on the rate of rotational energy losses Ė was found for both groups of pulsars. It is shown that L ∝ Ė 1/3 for the entire sample. The pulsar luminosity function is constructed, and the total number and birth rate of pulsars in the Galaxy are calculated.

Parametric excitation of acoustic oscillations in closed coronal magnetic loops

Abstract: Quasi-periodic modulations of the microwave emission from solar outbursts at 37 GHz are studied based on 17 events observed in 1989–2000 at the Metsahovi Observatory. Low-frequency modulations with periods of ∼5 min were found in approximately 90% of the observed microwave outbursts. The most likely origin of this modulation is modulation of the current flowing along a closed coronal magnetic loop due to the five-minute oscillations of the photospheric-convection velocity. In approximately 70% of the cases, oscillations with periods ∼10 min were observed simultaneously with the five-minute oscillations in the same events. In 30% of the cases, simultaneous modulation of the microwave emission by three low-frequency signals with periods of 3, 5, and 10 min was observed. One possible origin of these “double” and “triple” modulations is parametric excitation of acoustic oscillations with periods of 10 and 3 min in a closed coronal magnetic loop as a result of coupling with the five-minute photospheric oscillations. This can occur when the period of the natural acoustic oscillations of the closed magnetic loop is about 10 min (the resonance condition). Since the ten-minute oscillations are excited more easily than the three-minute oscillations at the parametric instability, the latter are observed less frequently. For the same reason, the observed linewidth of the ten-minute oscillations is considerably greater than that of the three-minute oscillations.

The influence of ultrahigh-energy cosmic rays on star formation in the early Universe

Abstract: The presence of ultrahigh-energy (UHE) cosmic rays results in an increase in the degree of ionization in the post-recombination Universe, which stimulates the efficiency of the production of H2 molecules and the formation of the first stellar objects. As a result, the onset of the formation of the first stars is shifted to higher redshifts, and the masses of the first stellar systems decrease. As a consequence, a sufficient increase in the ionizing radiation providing the reionization of the Universe can occur. We discuss the possible observational manifestations of these effects and their dependence on the parameters of UHE cosmic rays.

Ballooning instability and oscillations of coronal loops

Abstract: The excitation of the ballooning instability by the eigenoscillations of coronal loops is analyzed using the energy method. The second variation of the potential energy for the case of a plasma—plasma boundary is obtained via the linearized ideal MHD equations. It is shown that the eigenmodes of a magnetic tube and of a toroidal coronal loop coincide in a first approximation. The bending oscillations of the loops are able to excite the ballooning instability when β ≪ 1. The effects of the instability in solar coronal loops are discussed.

The mass of the compact object in the low-mass X-ray binary 2S 0921-630

Abstract: We interpret the observed radial-velocity curve of the optical star in the low-mass X-ray binary 2S 0921-630 using a Roche model, taking into account the X-ray heating of the optical star and screening of X-rays coming from the relativistic object by the accretion disk. Consequences of possible anisotropy of the X-ray radiation are considered. We obtain relations between the masses of the optical and compact (X-ray) components, m v and m x , for orbital inclinations i = 60°, 75°, and 90°. Including X-ray heating enabled us to reduce the compact object’s mass by ∼0.5–1 M ⊙, compared to the case with no heating. Based on the K0III spectral type of the optical component (with a probable mass of m v ≃ 2.9 M ⊙), we concluded that m x ≃ 2.45−2.55 M ⊙ (for i = 75°−90°). If the K0III star has lost a substantial part of its mass as a result of mass exchange, as in the V404 Cyg and GRS 1905+105 systems, and its mass is m v ≃ 0.65−0.75 M ⊙, the compact object’s mass is close to the standard mass of a neutron star, m x ≃ 1.4 M ⊙ (for i = 75°−90°). Thus, it is probable that the X-ray source in the 2S 0921-630 binary is an accreting neutron star.

Possibilities for studying microlensing of distant quasars using the RADIOASTRON space interferometer

Abstract: Gravitational lensing is a powerful tool for studies of the distribution of matter (including dark matter) in the Universe. The characteristic angular separation of images and characteristic variability time scale depend on the characteristic masses of the gravitational lenses. The construction of the RADIOASTRON space interferometer in the coming years will provide qualitatively new conditions for investigations of microlensing of distant quasars in the radio, since it will become possible not only to resolve individual microimages of these quasars, but also to observe astrometric microlensing (i.e., the shift of an image of a distant quasar, sometimes called weak microlensing). Astrometric microlensing by compact objects in the Galactic halo is not a very significant effect for the RADIOASTRON interferometer, since the probability of such events is low and the characteristic time scales involved appreciably exceed the proposed operational lifetime of the interferometer.

Three-dimensional hydrodynamical modeling of mass transfer in the close binary system β Lyr with an accretor wind

Abstract: We present three-dimensional hydrodynamical modeling of mass transfer in the close binary system β Lyr taking into account explicitly radiative cooling and the stellar wind of the accretor. Our computations show that flow forces wind out from the orbital plane, where an accretion disk with a radius of 0.4–0.5 and a height of about 0.15–0.17 (in units of orbital separation) is formed. Gas motions directed upward from the orbital plane are initiated in the region of interaction of the flow from L1 and the accretor wind (x = 0.91, y = −0.17); i.e., a jetlike structure forms. This structure has the shape of a gas pillar above the orbital plane, where gas moves with the velocity of stellar wind. The number density of the gas in this structure is about 1014 cm−3, and its temperature is 20 000–45 000 K. At heights of about 0.15–0.20 above the orbital plane, in the region between the jetlike structure and the disk, two spiral shocks form. It is possible that the emission lines observed in the spectrum of β Lyr binary originate in this region.

The role of plasma ejections in the development of large solar flares of various durations

Abstract: Recent observations indicate that relatively strong plasma ejections are accompanied by the formation of systems of coronal loops with two glowing ribbons near their footpoints. However, while two-ribbon flares can sometimes last for many hours, for example, soft X rays, they sometimes decay within tens of minutes. We study here factors affecting the durations of flares using four major flares occurring in July 15–18, 2002, as examples. Various ground-based and satellite observations are used to show that short-duration events involved collimated (narrow) plasma ejections directed to the north and the subsequent formation of compact loops in the leading part of the active region. During one event, a powerful eastward ejection in a wide solid angle was followed by the formation of an extended arch system in the trailing part, which determined the long duration of the flare. It is proposed that in events involving collimated jets and corresponding narrow features in coronal mass ejections (CMEs), systems of coronal loops do form, but post-eruptive energy release either does not occur or is expressed very faintly. So the energy does not go downward from this region, and the plasma is emitted free in the coronal loops. In contrast to such rapid flares, wide ejections and bright, large-scale CMEs are accompanied by the formation and prolonged existence of an extended arch system. Thus, powerful nonstationary solar processes involve a large-scale CME and the flare itself, with the pattern of a particular event determined by the reconnection scenario and the evolution of the ejected plasma.

The activity of late-type stars: the Sun among stars with cyclic activity

Abstract: The coronal and chromospheric emission of several hundred late-type stars whose activity was recently detected are analyzed. This confirms the previous conclusion for stars of HK project that there exist three groups of objects: active red M dwarfs, G-K stars with cyclic activity, and stars exhibiting high but irregular activity. The X-ray fluxes, EUV-spectra, and X-ray cycles can be used to study the main property of stellar coronas—the gradual increase in the number of high-temperature (T ≥ 10 MK) regions in the transition from the Sun to cyclically active K dwarfs and more rapidly rotating F and G stars with irregular activity. The level of X-ray emission is closely related to the spottedness of the stellar surface. The correlation between the chromospheric and coronal emission is weak when the cycles are well-defined, but becomes strong when the activity is less regular. Unexpectedly, stars whose chromospheric activity is even lower than that of the Sun are fairly numerous. Common and particular features of solar activity among the activity of other cyclically active stars are discussed. Our analysis suggests a new view of the problem of heating stellar coronas: the coronas of stars with pronounced cycles are probably heated by quasistationary processes in loops, while prolonged nonstationary coronal events are responsible for heating the coronas of F and G stars with high but irregular activity.