The dwarf nova MN Dra: Periodic processes at various phases of the supercycle

We analyze photometry of the dwarf nova MN Dra carried out using various instruments at four observatories on 18 nights between May 20 and June 28, 2009. The observations cover a variety of activity states of the system: a superoutburst, three normal outbursts, and quiescence. Analysis of the system’s light curve during the superoutburst decline reveals positive superhumps that recur, on average, with a period of 0.105 days and are due to the direct apsidal precession of the accretion disk. These are observed until the end of the superoutburst, but their period decreases at a rate of −24.5 × 10⁻⁵ of the period per period. Both the positive-superhump period and its derivative are in good agreement with estimates made during previous superoutbursts. At the brightness minimum and in normal outbursts, MN Dra displays brightness variations with a period of 0.096 days, whose amplitude is much larger during the brightness minimum (0.8 ^m –1.5 ^m ) than during normal outbursts (0.1 ^m –0.2 ^m ). We suggest that these brightness variations could be negative superhumps due to nodal precession of the oblique accretion disk.     

Light-curve syntheses for close binary systems: Modeling spiral waves in an elliptical disk around a white dwarf

We present an algorithm for synthesizing the light curve of a close binary consisting of a normal star (a red dwarf that fills its Roche lobe) and a spherical star (a white dwarf). The spherical component is surrounded by an elliptical accretion disk with a complex shape: it is geometrically thin near the spherical star and geometrically thick at the edge of the disk. An additional complication is presented by the presence of a one-or two-armed spiral pattern at the inner surface of the disk. The maximum height of the spiral arm above the disk surface is located at ～9 R _d , and the height decreases exponentially as the arm approaches the inner regions of the disk. Shielding of the inner hot parts of the disk by the crests of the spirals results in the formation of “steps” in out-of-eclipse parts of the orbital light curves. The algorithm takes into account the presence of a “hot line” by the lateral surface of the disk, making it possible to model binary systems in both quiescence and outburst. In the latter case, the hot line degenerates into a small bulge at the outer lateral surface of the disk, which can be considered an analog of a hot spot. The algorithm was applied to the orbital light curve of the cataclysmic binary IP Peg during its October 30, 2000, outburst. To explain the variations of the out-of-eclipse brightness of the system during the outburst, it is necessary to include the presence of a one-armed spiral wave at the inner surface of the disk, close to the periastron of the elliptical disk. We have obtained the parameters of IP Peg during the outburst for various models of the system.     

A photometric study of the eclipsing dwarf nova GY Cnc in quiescence and during an outburst

The results of photometric observations of the dwarf nova GY Cnc in the Rc filter acquired in 2013–2015 (~3900 orbital cycles, 19 nights in total) are presented, including observations during its outburst in April 2014. The binary’s orbital elements have been refined. The orbital period has changed only insignificantly during the ~30 000P_orb since the earlier observations; no systematic O–C variations were detected, only fluctuations within 0.004^d on time scales of 1500–2000P_orb. A “combined” model is used to solve for the parameters of GY Cnc during two states of the system. The flux from the white dwarf is negligible due to the star’s small size. The temperature of the donor star, T₂ ~ 3667 K (Sp M0.2 V), varies between 3440 and 3900 K (Sp K8.8–M1.7 V). The semi-major axis of the disk is a ~ 0.22a₀, on average. In quiescence, a varies within ~40%. The disk has a considerable eccentricity (e ~ 0.2?0.3) for a < 0.2a₀. The disk shape becomes more circular (e < 0.1) with increasing a. The outburst of GY Cnc was associated with increased luminosity of the disk due to the parameter α_g (related to the viscosity of the disk material) decreasing to 0.1–0.2 and the temperature in the inner parts of the disk increasing twofold, to T_in ~ 95 000 K. These changes were apparently due to the infall of matter onto the surface of the white dwarf as the outburst developed. All parameters of the accretion disk in quiescence display considerable variations about their mean values.     

Photometric parameters of the dwarf nova SS Cygni in the quiescent state

The mean 1983–1996 UBV light curves of the dwarf nova SS Cyg are used to derive the binary parameters in the quiescent state. Solutions are obtained for a classical hot-spot model and a model with an energy source lying outside the accretion disk. Photometric and spectroscopic data are combined to infer the masses and radii of the binary components. The white dwarf in SS Cyg is one and a half times as massive as the red dwarf, q=M _wd /M _rd ～1.45, M _rd ～0.46M _⊙ and M _wd ～0.66M _⊙. The orbital inclination of the system is i?51°–54°. The contribution of the accretion disk to the total flux in the quiescent state is estimated to be ～47–49% and ～54% in the VU and B filters, respectively. The hot spot contributes less than ～3% to the total optical flux. In the “non-classical” hot-spot model, the disk and bulge contributions are 27 and 2–8%, respectively, depending on the orbital phase. The shape of the mean light curves of SS Cyg suggests asymmetric heating of the red-dwarf surface in the quiescent state by high-temperature radiation generated in the hot-spot region.     

Analysis of Photometric Observations of the New Cataclysmic Variable ASASSN-13cx

Photometric observations of the variable star ASASSN-13cx acquired in the course of a program of studies of cataclysmic variables and their parameters recently carried out at the Sternberg Astronomical Institute (SAI) are presented. The star was observed with the 50-cm and 60-cm telescopes of the SAI Crimean Astronomical Station and a CCD photometer (～1800 images in the V and Rc filters) during the variable’s outburst of August–September 2014 and in a period of quiescence in October–November 2016. The ASASSN-13cx system is confirmed to be a SU UMa variable. Parameters of the system are derived from eight light curves using a “composite” model that takes into account the presence of a hot spot on the lateral surface of the geometrically thick disk and of a region of enhanced energy release near the disk edge, at the base of the gas flow (the so-called “hot line”). Parameters of the system for three light curves during the outburst were obtained in the framework of a “spiral” model that additionally takes into account the presence of geometric perturbations on the accretion-disk surface. The parameters of ASASSN-13cx determined using these models provide good accuracy in reproducing the system’s light curves in both states. The basic parameters of the system have been determined for the first time: the component mass ratio q = M₁/M₂ = 7.0 ± 0.2, the orbital inclination i = 79.9°?80.1°, the distance between the components’ centers of mass a₀ = 0.821(1) R_?, and the sizes and temperatures of the stars: R₁ = 0.0124(5)a₀ = 0.0102(4) R_?, T₁ = 12 500 ± 280 K, 〈R₂〉 = 0.236(4)a₀ = 0.194(3) R_?, T₂ = 2550 ± 400 K, corresponding to M4–9V for the spectral type of the secondary. Parameters of the accretion disk have been derived for both activity states. The mass of matter in the accretion disk increased by almost a factor of two during ～400 orbital periods in quiescence.     

Accretion disk of the cataclysmic variable IY UMa in quiescence and its active state

We present the results of a study of the eclipsing binary IY UMa (type SU UMa) during its superoutburst of 2004 and in quiescence.We have refined the orbital period of the system. Light curves are presented for various states of activity.We estimate the parameters of IYUMa during the superoutburst and in quiescence for hot-line and spiral-wave models. The spiral-wave model, which takes into account the presence of vertical perturbations in outer parts of the disk, is able to reproduce the light-curve shapes and phases of dips in the out-of-eclipse parts of the binary’s light curves during the outburst both qualitatively and quantitatively.     

Synthetic light curves of close binaries in a cool-disk model. The “hot line” and “hot spot” as visual indicators of interaction between the flow and disk

We present a “combined” model taking into account visual manifestations of the interaction between the gas flow and the accretion disk in a close binary system in the form of a “hot line” and a “hot spot.” The binary consists of a red dwarf that fills its Roche lobe and a compact spherical star (a white dwarf or neutron star) surrounded with a thick ellipsoidal accretion disk of a complex shape. The disk thickness is not large near the compact star but increases according to a parabolic law towards its outer edge. The oblique collision of the gaseous flow with matter of the cool, rotating disk, whose outer edge has a temperature <10 000 K, creates an extended region of enhanced energy release. In the combined model, this region is represented with a hot line that coincides with the optically opaque part of the flow and is located outside the disk, together with a hot spot at the outer surface of the disk, on the leeward side of the flow. The synthetic light curves for the combinedmodel and a hot-line model demonstrate that both models are able to fairly accurately reproduce the shapes of both classical and atypical light curves of cataclysmic variables in quiescence. Our determination of the parameters of the cataclysmic variable OY Car from an analysis of its light curves using the two models shows that the basic characteristics of the close binary, such as the component mass ratio q = M ₁/M ₂, orbital inclination i, effective temperatures of the red dwarf (T ₂) and white dwarf (T ₁), and orientation of the disk α _e , remain the same within the errors. The parameters describing the size of the slightly elliptical disk and the radiation flux from the disk differ by several percent (∼ 2–8%). A more significant difference is detected in the parameters of the hot line, due to the different shape and alignment of the flow and the presence of an additional radiation source—the hot spot—on the disk.     

Interpretation of light curves of the cataclysmic variable OY Car in a model with shockless interaction between a gaseous stream and the disk

To determine the parameters of the accretion disk and shock-wave region responsible for the formation of the orbital peak in the light curve of the binary system OY Car (an SU UMa-type variable), we have analyzed its U BV R and JK light curves using two gas-dynamical models with different regions of shock interaction: one with a hot line along the stream from the Lagrange point L1 and one with a hot spot on the accretion disk. The hot-line model can better describe the quiescent state of the system: the maximum X² for the optical light curves does not exceed 207, whereas the minimum residual for the hot-spot model is X²>290. The shape of the eclipse is almost identical in both models; the main differences are in interpreting out-of-eclipse portions of the light curves, whose shape can varyin the transition from one orbital cycle to another. The hot-spot model is not able to describe variations of the system’s brightness at orbital phases ?～0.1–0.6. The rather complex behavior of the observed flux in this phase interval can be explained in the hot-line model as being due to variations of the temperature and size of the system. Based on the analysis of a sequence of 20 B curves of OY Car, we conclude that the flux variations in the primary minimum are due to variations of the luminosity of the accretion disk, whereas the flux variability in the vicinity of the orbital peak is due to the combined effect of the radiation of the disk and hot line. The JK light curves of OY Car in the quiescent state and during a small flare also indicate preference for the hot-line model, since the primaryminimum and the flux near quadratures calculated using the hot-spot model are not consistent with the observations.     

Photometry and spectroscopy of CI camelpardalis from 1998–2005

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 0.2^m. 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 Å 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 0.034^m. 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.     

Structure of the accretion disk in the cataclysmic variable UX UMa in the transition to its quiescent state

Our analysis of BV RI light curves for the cataclysmic variable UX UMa obtained in intermediate activity states, in the transition between the active and quiescent states of the system on March 12, 1997 and May 3, 2000, shows that the shapes of these light curves cannot be adequately described using the standard hot-spot model. A model with a “hot line” near the edge of the disk and a two-armed spiral structure on the disk surface reproduces much better out-of-eclipse variations in the light curves. The presence of an extended hot line can explain the anomalous shape of the I light curve on March 12, 1997. The decrease in the observed luminosity of the system between March 12, 1997 and May 3, 2000 could be due to a decrease in the disk luminosity by a factor of 2–2.5; the higher disk luminosity on the earlier date is associated with appreciable deviations of the radial temperature distribution of the disk material from that for the standard model. The phases and depths of dips in the out-of-eclipse sections of the UX UMa light curves are due primarily to the parameters of the complex shape of the accretion disk, which has a spiral structure located mainly near its outer edge. The contribution of the spiral arms in the V filter reaches 20–50% of the total disk radiation. The crest of the first spiral wave in our model maintains its approximate position in azimuth; this structure could represent a bulge in a halo at the outer edge of the disk near orbital phases φ ～ 0.7, in the direction of the continuation of the extended shock in the disk itself. The position of the crest of the second spiral arm changes with time. This structure may represent a one-armed spiral wave near the apastron of the weakly elliptical disk. Finally, the observations testify to the presence of another spiral arm that is les clearly manifest in terms of both its luminosity and its height above the unperturbed disk surface. Thus, in an intermediate activity state of UX UMa, the surface of the accretion disk is distorted by the action of a two-armed spiral structure in the outer regions of the disk, which is asymmetric in both its luminosity and dimensions, and a bulge at the disk edge in the region of its interaction with the inflow to the disk.     

Spectroscopy and doppler mapping of the binary SS Cyg during outburst

We carried out spectroscopy of the binary SSCyg in the Hα, Hβ, and Hγ lines in its active state in August and December 2006. We have estimated the parameters of the main flow elements contributing to the spectra. Profile variations during the orbital period are analyzed, and a Doppler tomogram computed for the Hα line. We consider the evolution of the line profiles with the development of the outburst. A phenomenological model explaining the observed outburst features is suggested. In this model, the main elements of the flow determining the shape of the spectral lines are the accretion disk, a toroidal shell formed in the inner parts of the disk, an expanding spherical shell around the accreting star, a region in front of the bow shock that forms due to the orbital motion of the disk in the circumbinary envelope, and the surface of the donor star near the inner Lagrange point, L1, which is heated by radiation from the accretor.     

Non-steady-state accretion disks in X-ray novae: Outburst models for Nova Monocerotis 1975 and Nova Muscae 1991

We have fit outbursts of two X-ray novae (Nova Monocerotis 1975=A0620-00 and Nova Muscae GS 1991=1124-683) using a non-steady-state accretion-disk model. The model is based on a new solution for a diffusion-type equation for non-steady-state accretion and describes the evolution of a viscous α disk in a binary system after the peak of the outburst, when the matter in the disk is totally ionized. The accretion rate in the disk decreases according to a power law. We derive formulas for the accretion rate and effective temperature of the disk. The model has three free input parameters: the mass of the central object M, the turbulence parameter α, and the normalization parameter δt. The results of the modeling are compared with the observed X-ray and optical B and V light curves. The estimates for the turbulence parameter α are similar: 0.2–0.4 for A 0620-00 and 0.45–0.65 for GS 1124-683, suggesting a similar nature for the viscosity in the accretion disks around the compact objects in these sources. We have also derived the distances to these systems as functions of the masses of their compact objects.     

A possible manifestation of spiral shock waves in the accretion disks of cataclysmic variables

We present the results of three-dimensional numerical simulations of flow structures in binary systems with spiral shock waves. Variations of the mass-transfer rate perturb the equilibrium state of the accretion disk; consequently, a condensation (blob) behind the shock breaks away from the shock front and moves through the disk with variable speed. Our computations indicate that the blob is a long-lived formation, whose mean parameters do not vary substantially on timescales of several tens of orbital periods of the system. The presence of the spiral shocks maintains the compact blob in the disk: it prevents the blob from spreading due to the differential motion of matter in the disk, and dissipative spreading on this timescale is negligible. A number of cataclysmic variables display periodic or quasi-periodic photometric variations in their light curves with characteristic periods ～0.1–0.2P _orb, where P _orb is the orbital period. The blobs formed in systems with spiral shock waves are examined as a possible origin for these variations. The qualitative (and, in part, quantitative) agreement between our computations and observations of IP Peg and EX Dra provides evidence for the efficacy of the proposed model.     

Three-dimensional gas-dynamical modeling of changes in the flow structure during the transition from the quiescent to the active state in symbiotic stars

The results of three-dimensional modeling of the flow structure in the classical symbiotic system Z Andromedae are presented. Outbursts in systems of this type occur when the accretion rate exceeds the upper limit of the steady-burning range. Therefore, in order to realize the transition from a quiescent to an active state, it is necessary to find a mechanism capable of sufficiently increasing the accretion rate on the time scales typical for outburst development. Our calculations provide support for a mechanism for the transition from quiescence to outburst in classical symbiotic systems suggested earlier based on two-dimensional calculations (Bisikalo et al., 2002). Our results show that an accretion disk forms in the system for a wind velocity of 20 km s^?1. The accretion rate for the solution with the disk is ～22.5–25% of the mass-loss rate of the donor, which is ～4.5?5 × 10^?8M_⊙ yr^?1 for Z And. This value is in agreement with the steady-burning range for the white-dwarf masses usually accepted for this system. When the wind velocity increases from 20 to 30 km s^?1, the accretion disk is destroyed and the disk material falls onto the accretor surface. This process is followed by an approximately twofold jump in the accretion rate. The resulting growth in the accretion rate is sufficient so as to exceed the upper limit of the steady-burning range, thus bringing the system into an active state. The time during which the accretion rate is above the steady-burning value is in very good agreement with observations. Our analysis leads us to conclude that small variations in the donor wind velocity can lead to the transition from disk accretion to wind accretion and, as a consequence, to the transition from a quiescent to an active state in classical symbiotic stars.     

Light curve interpretation for “Quiescent” X-ray novae in a model with noncollisional interaction between the flow and disk. The system GU Mus = GRS 1124-68

We have analyzed optical and infrared light curves of GU Mus obtained during the system's quiescent state and carried out computations for “hot-line” and “hot-spot” models. The hot-line model describes the optical variability of GU Mus better than the hot-spot model. Season-to-season variations of the shape, amplitude, and mean levels of the optical and infrared light curves of GU Mus are due to changing parameters of the hot line and, to a lesser degree, of the accretion disk. Taking into account the contribution of the variability of the disk + hot line system to the variability of the system as a whole, we are able to reliably estimate the orbital inclination, \(i = 54^\circ \pm 1^\circ .3\), and the mass of the black hole, M _X =(6.7–7.6)M_⊙.     

A model for superoutbursts in SU UMa-type binaries

We suggest a new mechanism for the superoutbursts in SU UMa binaries, in which the increase in the accretion rate resulting in a superoutburst is associated with the formation of a spiral “precessional” wave in the inner parts of the disk, where gas-dynamical perturbations are negligible. The existence of such waves was suggested by us previously. The results of three-dimensional gas-dynamical simulations have shown that a considerable increase in the accretion rate (by up to an order of magnitude) is associated with the formation of the precessional wave. The features of the precessional spiral wave can explain both the energy release in the superoutburst and all its observational manifestations. One distinguishing feature of superoutbursts in SU UMa-type stars is the formation of a “superhump” in the light curve. Our model reproduces well both the formation of a superhump and its observational features, including its period, which is up to 3–7% longer than the orbital period, and the detectability of the superhump independent of the orbital inclination of the binary.     

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

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.     

Light-curve synthesis for close binaries: An extended shock as an analog of a hot spot in cataclysmic variables

An algorithm is presented for the synthesis of the light curve of a close binary system consisting of a red dwarf that fills its Roche lobe and a spherical white dwarf. The spherical component is surrounded by an elliptical, geometrically thick accretion disk. The code models an extended shock located along the edge of the stream near the outer boundary of the disk. The observational manifestations of the shock show that it can be considered as an analog of a hot spot at the edge of the disk. Synthetic light curves for the SU UMa system OY Car at various phases of its activity indicate that the model can describe both typical and peculiar light curves for this cataclysmic variable reasonably well.     

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

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^?5 M _⊙/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^?5 M _⊙/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^?5 M _⊙/yr.     

Formation of a radiative wind and accretion disk in microquasars. Generation of flare activity in the disk due to an increase in the supply of material to the Lagrange point L<Subscript>1</Subscript>. The system LMC X-3

Three-dimensional numerical hydrodynamical modeling of a radiative wind and accretion disk in a close binary system with a compact object is carried out, using the massive X-ray binary LMC X-3 as an example. This system contains a precessing disk, and may have relativistic jets. These computations show that an accretion disk with a radius of about 0.20 (in units of the component separation) forms from the radiative wind from the donor when the action of the wind on the central source is taken into account, when the accretion rate is equal to the observed value (about 3.0 × 10^?8 M _⊙/year, which corresponds to the case when the donor overflows its Roche lobe by nearly 1%). It is assumed that the speed of the donor wind at infinity is about 2200 km/s. The disk that forms is geometrically thick and nearly cylindrical in shape, with a low-density tunnel at its center extending from the accretor through the disk along the rotational axis. We have also modeled a flare in the disk due to short-term variations in the supply of material through the Lagrange point L₁, whose brightnesses and durations are able to explain flares in cataclysmic variables and X-ray binaries. The accretion disk is not formed when the donor underfills its Roche lobe by 0.5%, which corresponds to an accretion rate onto the compact object of 2.0 × 10^?9 M _⊙/year. In place of a disk, an accretion envelope with a radius of about 0.03 forms, within which gas moves along very steep spiral trajectories before falling onto the compact object. As in the accretion-disk case, a tunnel forms along the rotational axis of the accretion envelope; a shock forms behind the accretor, where flares occur in a compact region a small distance from the accretor at a rate of about six flares per orbital period, with amplitudes of about 10 ^m or more. The flare durations are two to four minutes, and the energies of individual particles at the flare maximum are about 100–150 keV. These flares appear to be analogous to the flares observed in gamma-ray and X-ray burst sources. We accordingly propose a model in which these phenomena are associated with massive, close X-ray binary systems with component-mass ratios exceeding unity, in which the donor does not fill its Roche lobe. Although no accretion disk forms around the compact object, an accretion region develops near the accretor, where the gamma-ray and X-ray flares occur.