Vertical structure of the outer accretion disk in persistent low-mass X-ray binaries

We have investigated the influence of X-ray irradiation on the vertical structure of the outer accretion disk in low-mass X-ray binaries by performing a self-consistent calculation of the vertical structure and X-ray radiation transfer in the disk. Penetrating deep into the disk, the field of scattered X-ray photons with energy E ≳ 10 keV exerts a significant influence on the vertical structure of the accretion disk at a distance R ≳ 10¹⁰ cm from the neutron star. At a distance R ∼ 10¹¹ cm, where the total surface density in the disk reaches Σ₀ ∼ 20 g cm⁻², X-ray heating affects all layers of an optically thick disk. The X-ray heating effect is enhanced significantly in the presence of an extended atmospheric layer with a temperature T _atm ≈ (2–3) × 10⁶ K above the accretion disk. We have derived simple analytic formulas for the disk heating by scattered X-ray photons using an approximate solution of the transfer equation by the Sobolev method. This approximation has a ≲10% accuracy in the range of X-ray photon energies E < 20 keV.     

Supermassive binary black holes among cosmic gamma-ray sources

Supermassive binary black holes (SBBHs) are a natural outcome of galaxy mergers. Here we show that low-frequency (f≤10⁻⁶ Hz) quasi-periodic variability observed from cosmic blazar sources can provide substantial inductive support for the presence of close (d≲0.1 pc) SBBHs at their centers. It is argued on physical grounds that such close binary systems are likely to give rise to different (although not independent) periodicities in the radio, optical and X-ray/TeV regime, and, hence that detection of appropriate period ratios significantly corroborates the SBBH interpretation. This is illustrated for a binary model where optical longterm periodicity is related to accretion disk interactions, radio periodicity to Newtonian jet precession, and periodicities in the high energy bands to the orbital motion of the jet. We use the observed periodicities to constrain the properties for a sample of SBBH candidates including OJ 287 and AO 0235+16, and discuss the results within the context of jet activity and binary evolution.     

Disk wind in young binaries with low-mass secondary components: Optical observational manifestations

We consider a model of a young binary with a low-mass secondary component. Mass accretion from the remnants of the protostellar cloud onto the binary components is assumed to take place in accordance with current models; i.e., it proceeds mainly onto the low-mass component. The accretion is accompanied by mass outflow (disk wind), whose low-velocity component can be partially captured by the primary component. As a result, an asymmetric common envelope is formed. Its densest part is involved in the orbital motion of the secondary and can periodically shield the primary component of the binary from the observer. Assuming a standard dust-to-gas ratio for the disk wind (1: 100), we calculated the possible photometric effects from such eclipses and showed that they could be observed even at moderate accretion rates onto the low-mass binary component, ∼10⁻⁸–10⁻⁹ M _⊙ per year. In this case, the parameters of the minima depend on the model of the disk wind, on the ratio of its characteristic velocity to the orbital velocity of the secondary, and on its orbital inclination to the line of sight. These results can form the basis for interpreting a wide range of phenomena observed in young stars, such as the activity cycles in UX Ori stars, the unusually broad minima in some young eclipsing systems, etc., and for searching for substellar objects and massive protoplanets. In addition, the peripheral parts of the gas and dust disk around a young binary can fall within the shadow zone produced by the opaque part of the common envelope. In such cases, a shadow from the common envelope must be observed on the disk; this shadow must move over the disk following the orbital motion of the low-mass component. Detection and investigation of such structures in the images of protoplanetary disks may become a method of searching for protoplanets and studying binaries at early stages of their evolution.     

Constraining the relative inclinations of the planets B and C of the millisecond pulsar PSR B1257+12

We investigate on the relative inclination of the planets B and C orbiting the pulsar PSR B1257+12. First, we show that the third Kepler’s law does represent an adequate model for the orbital periods P of the planets, because other Newtonian and Einsteinian corrections are orders of magnitude smaller than the accuracy in measuring P _B/C. Then, on the basis of available timing data, we determine the ratio sin i _C/ sin i _B = 0.92±0.05 of the orbital inclinations i _B and i _C independently of the pulsar’s mass M. It turns out that coplanarity of the orbits of B and C would imply a violation of the equivalence principle. Adopting a pulsar mass range 1 ≲ M ≲ 3, in solar masses (supported by present-day theoretical and observational bounds for pulsar’s masses), both face-on and edge-on orbital configurations for the orbits of the two planets are ruled out; the acceptable inclinations for B span the range 36 deg ≲ i _B ≲ 66 deg, with a corresponding relative inclination range 6 deg ≲ (i _C − i _B) ≲ 13 deg.     

Measurement of the orbital period of the X-ray burster GS 1826-238 based on observations of its optical brightness variations

The variability of the optical and X-ray fluxes from the binary GS 1826-238 is investigated. An epoch-folding analysis of the optical data obtained with the RTT-150 telescope in 2003–2004 has revealed periodic brightness variations in the source with a period P _orb = 2.24940 ± 0.00015 h with a high statistical significance. When estimating the detection significance of the periodic signal, we have specially taken into account the presence of a powerful aperiodic component (“red noise”) in the source’s brightness variability. The source’s power density spectra in the frequency range ∼10⁻⁵–0.01 Hz have been obtained. We have detected a statistically significant break in the power density spectrum of GS 1826-238 at a frequency ν _br ≈ (8.48 ± 0.14) × 10⁻⁵ Hz in both optical and X-ray energy bands. We have estimated the orbital period of the binary GS 1826-238 using the correlation between the break frequency in the power density spectrum and the orbital period of binaries, P _orb ∝ 1/ν _br, found by Gilfanov and Arefiev (2005): P _orb = 3.7 ± 0.8 h and P _orb = 11.3 ± 5.9 h when using Sco X-1 and 1H 16267-273, respectively, as reference sources. It seems to us that the method for estimating the orbital periods of low-mass X-ray binaries using the correlation P _orb ∝ 1/ν _br may turn out to be very promising, especially for persistent low-luminosity X-ray binaries.     

On the change of the inner boundary of an optically thick accretion disk around white dwarfs using the dwarf nova SS Cyg as an example

We present the results of our studies of the aperiodic optical flux variability for SS Cyg, an accreting binary systemwith a white dwarf. The main set of observational data presented here was obtained with the ANDOR/iXon DU-888 photometer mounted on the RTT-150 telescope, which allowed a record (for CCD photometers) time resolution up to 8 ms to be achieved. The power spectra of the source’s flux variability have revealed that the aperiodic variability contains information about the inner boundary of the optically thick flow in the binary system. We show that the inner boundary of the optically thick accretion disk comes close to the white dwarf surface at the maximum of the source’s bolometric light curve, i.e., at the peak of the instantaneous accretion rate onto the white dwarf, while the optically thick accretion disk is truncated at distances 8.5 × 10⁹ cm ∼10R _WD in the low state. We suggest that the location of the inner boundary of the accretion disk in the binary can be traced by studying the parameters of the power spectra for accreting white dwarfs. In particular, this allows the mass of the accreting object to be estimated.     

Modulation of circumstellar extinction in a young binary system with a low-mass companion in a noncoplanar orbit

We consider a model of cyclic brightness variations in a young star with a low-mass (q = M ₂/M ₁ ≤ 0.1) companion that accretes matter from the remnants of a protostellar cloud (circumbinary disk). We assume that the orbit of the companion is circular and that its plane does not coincide with the disk plane. We have computed grids of hydrodynamic models for such a binary by the SPH method based on which we have investigated the circumstellar extinction variations produced by the streams of matter and density waves excited in the circumbinary disk by the orbital motion of the companion. We show that, depending on the inclination and orientation of the binary’s line of nodes relative to the observer, the brightness of the primary component can undergo various (in shape and depth) oscillations with a period equal to the orbital one. In contrast to the models with coplanar circular orbits, the accretion rate onto the components of a binary with a noncoplanar orbit depends on the orbital phase. The results of our computations can be used to study the cyclic activity of UX Ori stars and young eclipsing binaries with anomalously long eclipses.     

Brightness oscillations in models of young binary systems with low-mass secondary components

We consider a model for the cyclic brightness variations of a young star with a low-mass companion that accretes matter from the remnants of a protostellar cloud. At small inclinations of the binary orbit to the line of sight, the streams of matter and the density waves excited in the circumbinary disk can screen the primary component of the binary from the observer. To study these phenomena, we have computed grids of hydrodynamic models for binary systems by the SPH method based on which we have calculated the phase light curves for the different orientations of the orbit. The model parameters were varied within the following ranges: the component mass ratio q = 0.01–0.1 and the eccentricity e = 0–0.5. We adopted optical grain characteristics typical of circumstellar dust. Our computations have shown that the brightness oscillations with orbital phase can have a complex structure. The amplitudes and shapes of the light curves depend strongly on the inclination of the binary orbit and its orientation relative to the observer and on the accretion rate. The results of our computations are used to analyze the cyclic activity of UX Ori stars.     

Investigation of the new cataclysmic variable 1RXS J180834.7+101041

We present the results of our photometric and spectroscopic studies of the new eclipsing cataclysmic variable star 1RXS J180834.7+101041. Its spectrum exhibits double-peaked hydrogen and helium emission lines. The Doppler maps constructed from hydrogen lines show a nonuniform distribution of emission in the disk similar to that observed in IP Peg. This suggests that the object can be a cataclysmic variable with tidal density waves in the disk. We have determined the component masses (M _WD = 0.8 ± 0.22M _⊙ and M _RD = 0.14 ± 0.02M _⊙) and the binary inclination (i = 78° ± 1.5°) based on well-known relations between parameters for cataclysmic variable stars. We have modeled the binary light curves and showed that the model of a disk with two spots is capable of explaining the main observed features of the light curves.     

Spectral Features of Photon Bubble Models of Ultraluminous X-ray Sources

The nature of Ultraluminous X-ray Sources – X-ray sources which exceed the Eddington luminosity for a ∼10 M _⊙ black hole – remains a mystery. One possible explanation is an inhomogeneous accretion disk around a solar mass black hole where photon transport through radiation-pressure dominated “photon bubbles” can lead to super-Eddington accretion. While previous studies of this model have focused primarily on its radiation-hydrodynamics aspects, here we explore some observational implications of such a model with a Monte Carlo–Fokker Planck radiation transport code.     

The black-hole candidate XTE J1817-330 as seen by XMM-Newton and INTEGRAL

The new black hole candidate XTE J1817-330, discovered on 26 January 2006 with RXTE, was observed with XMM-Newton and INTEGRAL in February and March 2006, respectively. The X-ray spectrum is dominated by the thermal emission of the accretion disk in the soft band, with a low absorption column density (N _H=1.77(±0.01)×10²¹ cm⁻²) and a maximum disk temperature kT _max=0.68(±0.01) keV, plus a power law component, with the photon index decreasing from 2.66±0.02 to 1.98±0.07 between the two observations. Several interstellar absorption lines are detected in the X-ray spectrum, corresponding to O I, O II, O III, O VII and Fe XXIV. We constrain the distance to the system to be in the range 1–5 kpc.      

Mass-luminosity relation for massive stars

A catalog of massive (⩾10 M _⊙) stars in binary and multiple systems with well-known masses and luminosities has been compiled. The catalog is analyzed using a theoretical mass-luminosity relation. This relation allows both normal main-sequence stars and stars with peculiarities: with clear manifestations of mass transfer, mass accretion, and axial rotation, to be identified. Least-squares fitting of the observational data in the range of stellar masses 10M _⊙ ⩽ M ≲ 50 M _⊙ yields the relation L ∼ M ^2.76. An erratum to this article is available at .     

Accretion activity of young binary systems with low-mass secondary components

The accretion activity of young binaries with low-mass (q = M ₂/M ₁ ≤ 0.1) secondary components is studied. The source of accreted matter is a common disk surrounding the binary system and coplanar with its orbit. Gas dynamic models of these systems are used to calculate the rates of accretion to the components and their dependence on the phase of the orbital period is studied. It is shown that, despite its low mass, the secondary accretes matter at a relatively higher rate than the primary. This result can be regarded as an extension of the work of Artymowicz and Lubow for young binaries with components that have unequal masses. Possible astrophysical applications of the theory are discussed.     

X-ray and optical orbital modulation of EXO 0748-676: A co-variability study using XMM-Newton

We present a multi-wavelength timing study of the eclipsing low mass X-ray binary EXO 0748-676 (UY Vol) using XMM-Newton when the source was in a hard spectral state. The orbital optical and X-ray light curves show a fairly large amount of intensity modulation in the 7 observations taken during September-November, 2003, covering 36 complete binary orbits of EXO 0748-676. While assessing the non-burst variability, simultaneously in the optical and X-ray light curves, we find that they are not correlated at reprocessing or orbital time-scales, but are weakly correlated at a few 1000s of seconds time-scales. Although a large fraction of the optical emission is likely to be due to reprocessing, the lack of significant correlation and presence of large variability in the orbital X-ray and optical light curves is probably due to structures and structural changes in the accretion disk that produce, and sometimes mask the reprocessed signal in varying amounts. These disk structures could be induced, at least partly, by irradiation. From the observed modulations seen in the optical light curves, there is strong evidence of accretion disk evolution at time scales of a few hours.     

Neutron star masses: dwarfs, giants and neighbors

We discuss three topics related to the neutron star (NS) mass spectrum. At first we discuss the possibility to form low-mass (M≲1M _⊙) objects. In our opinion this and suggest this is possible only due to fragmentation of rapidly rotating proto-NSs. Such low-mass NSs should have very high spatial velocities which could allow identification. A critical assessment of this scenario is given. However, the mechanism has its own problems, and so formation of such objects is not very probable. Secondly, we discuss mass growth due to accretion for NSs in close binary systems. With the help of numerical population synthesis calculations we derive the mass spectrum of massive (M>1.8M _⊙) NSs. Finally, we discuss the role of the mass spectrum in population studies of young cooling NSs. We formulate a kind of mass constraint which can be helpful, in our opinion, in discussing different competitive models of the thermal evolution of NSs. S.B.P. wants to thank the Organizers for support and hospitality. The work of S.B.P. was supported by the RFBR grant 06-02-16025 and by the “Dynasty” Foundation (Russia). The work of M.E.P.—by the RFBR grant 04-02-16720 and that of H.G. by DFG grant 436 ARM 17/4/05.     

Modeling the luminosity function of galactic low-mass X-ray binaries

The evolution of the family of binaries with a low-mass star and a compact neutron star companion (low-mass X-ray binaries (LMXBs) with neutron stars) ismodeled by the method of population synthesis. Continuous Roche-lobe filling by the optical star in LMXBs is assumed to be maintained by the removal of orbital angular momentum from the binary by a magnetic stellar wind from the optical star and the radiation of gravitational waves by the binary. The developed model of LMXB evolution has the following significant distinctions: (1) allowance for the effect of the rotational evolution of a magnetized compact remnant on themass transfer scenario in the binary, (2) amore accurate allowance for the response of the donor star to mass loss at the Roche-lobe filling stage. The results of theoretical calculations are shown to be in good agreement with the observed orbital period-X-ray luminosity diagrams for persistent Galactic LMXBs and their X-ray luminosity function. This suggests that the main elements of binary evolution, on the whole, are correctly reflected in the developed code. It is shown that most of the Galactic bulge LMXBs at luminosities L _x > 10³⁷ erg s^?1 should have a post-main-sequence Roche-lobe-filling secondary component (low-mass giants). Almost all of the models considered predict a deficit of LMXBs at X-ray luminosities near ～10^36.5 erg s^?1 due to the transition of the binary from the regime of angular momentum removal by a magnetic stellar wind to the regime of gravitational waves (analogous to the widely known period gap in cataclysmic variables, accreting white dwarfs). At low luminosities, the shape of the model luminosity function for LMXBs is affected significantly by their transient behavior-the accretion rate onto the compact companion is not always equal to the mass transfer rate due to instabilities in the accretion disk around the compact object. The best agreement with observed binaries is achieved in the models suggesting that heavy neutron stars with masses 1.4–1.9M _⊙ can be born.     

Astrometric Solution of the Multiple System Xy Leo

An astrometric solution, together with time of minimum analysis, has been made for the multiple system XY Leonis (HIP 49136) to identify the properties of the remote companion to the eclipsing pair (AB). From this solution, we derive the inclination of the wide orbit (AB-cd) as 94.4^∘± 0.2^∘, angle of nodes as 247.3^∘± 0.2^∘, and the mass of the wide component (the dwarf binary cd) as 0.98 ± 0.2 M_⊙. This study confirms that the light travel time effect can explain the sinusoidal O−C variation of the eclipsing system.     

Interaction of the X-ray source radiation with the atmosphere of the normal star in close binary systems

The structure of the stellar atmosphere irradiated by an X-ray source is calculated. On the basis of these numerical calculations an approximate theory of the X-ray reprocessing is formulated. The interaction of X-rays with the stellar atmosphere induces a considerable stellar wind. However, the main part of the X-ray energy is reemitted.The optical appearances of the close binary system including an X-ray source are discussed. The light curve of such a system is obtained. The mass-loss rate of a star with the size close to that of its Roche lobe is evaluated in the isothermal approximation. Most likely, the accretion of matter on to a neutron star, or a black hole, is the cause of the X-ray luminosity. The accreting matter is supplied with the mass outflow from the normal component induced by X-rays. The X-ray luminosity is shown to have an upper limit stipulated by the outflow saturation.The model of HZ Her=Her X1 system is constructed which accounts for the observed light curve. The optical appearances of the system are due to the X-ray heating of the face of the X-ray source area of the normal star. The radiation of this hot area is partly reflected by the surface of the disc around the X-ray source. The thin disc is formed by the accretion of matter by the X-ray source. The effective reflection of hard X-rays (hv15–30 keV) by the stellar surface is considered. This phenomenon makes it possible to detect those X-ray pulsars whose beam does not intercept the Earth.The model of Sco X1 as a black hole in a close binary system is discussed.     

Dynamical constraints on some orbital and physical properties of the WD0137-349A/B binary system

In this paper I deal with the WD0137-349 binary system consisting of a white dwarf (WD) and a brown dwarf (BD) in a close circular orbit of about 116 min. I, first, constrain the admissible range of values for the inclination i by noting that, from looking for deviations from the third Kepler law, the quadrupole mass moment Q would assume unlikely large values, incompatible with zero at more than 1-sigma level for i≲35 deg and i≳43 deg. Then, by conservatively assuming that the most likely values for i are those that prevent such an anomalous behavior of Q, i.e. those for which the third Kepler law is an adequate modeling of the orbital period, I obtain i=39±2 deg. Such a result is incompatible with the value i=35 deg quoted in literature by more than 2 sigma. Conversely, it is shown that the white dwarf’s mass range obtained from spectroscopic measurements is compatible with my experimental range, but not for i=35 deg. As a consequence, my estimate of i yields an orbital separation of a=(0.59±0.05)R_⊙ and an equilibrium temperature of BD of T _eq=(2087±154) K which differ by 10% and 4%, respectively, from the corresponding values for i=35 deg.