E. A. Karitskaya

Distribution of polarization and intensity of radiation across the stellar disk and numberical values of atmospheric characteristics governing this distribution

Computations of polarization and intensity of radiation from a unit stellar surface area are presented, as well as a study of the numerical characteristics of atmospheres — single-scattering albedo Ωλ and the initial source functionλ(δλ), which define the polarization behaviour of atmospheres. The radiatively stable models of stellar atmospheres presented by Kuruczet al. (1974) and Kurucz (1979) have been used for calculations. Since the Ωλ versus optical depth τλ dependence is rather weak, it has been assumed that Ωλ(τλ=cost. With a fixed effective temperatureTeff maximum values of Ω are characteristic of stars featuring the lowest surface gravity accelerationg. Among stars with radiatively stable atmospheres, maximum values of Ω (λ=5000 Å) ≈ 0.4–0.6 are exhibited by supergiants withTeff=8000–20 000 K. The plot of Ω(λ) is characterized by discontinuities at the boundaries of spectral series for hydrogen and, sometimes, for helium. Maximum Ωλ are attained in the Lyman region of λ=912–1200 Å, where Ωλ can reach the value 0.7–0.9 for supergiants, this value being ≳ 0.3 for Main-Sequence stars. For stars withTeff ≳ 35 000 K, high values of Ωλ also are attained for λ<912 Å. Within the infrared region, Ωλ is always small because of bremsstrahlung absorption.A rapid growth of the source functionBλ with <λ typical for ultraviolet range (within the Wien part of spectrum), together with high values of Ωλ results in the strong polarization of emission from a unit stellar surface element, sometimes exceeding the values for the case of a pure electron scattering. For longer wavelengths, where the limb-darkening coefficient is smaller, the plane of polarization abruptly turns 90° in the central parts of the visible stellar disk.

Analysis of optical spectra of V1357 Cyg≡Cyg X-1

Optical spectra and light curves of the massive X-ray binary V1357 Cyg are analyzed. The calculations were based on models of irradiated plane-parallel stellar atmospheres, taking into account reflection of the X-ray radiation, asphericity of the stellar surface, and deviations from LTE for several ions. Comparison of observed spectra obtained in 2004–2005 at the Bohyunsan Observatory (South Korea) revealed variations of the depths of HI lines by up to 18% and of HeI and heavy elements lines by up to 10%. These variations are not related to the orbital motion of the star, and are probably due to variations of the stellar wind intensity. Perturbations of the thermal structure of the atmosphere due to irradiation in various states of Cyg X-1 (including outburst) do not lead to the formation of a hot photosphere with an electron temperature exceeding the effective temperature. As a result, variations of the profiles of optical lines of HI, HeI, and heavy elements due to the orbital motion of the star and variations of the irradiating X-ray flux do not exceed 1% of the residual intensities. Allowing for deviations from LTE enhances the HI and HeI lines by factors of two to three and the MgII lines by a factor of nine, and is therefore required for a fully adequate analysis of the observational data. Analysis of the HI, HeI, and HeII lines profiles yielded the following set of parameters for theOstar at the observing epoch: Teff = 30 500±500 K, log g = 3.31±0.05, [He/H] = 0.42 ± 0.05. The observed HeI line profiles have emission components that are formed in the stellar wind and increase with the line intensity. The abundances of 11 elements in the atmospheres of V1357 Cyg and α Cam, which has a similar spectral type and luminosity class, are derived. The chemical composition of V1357 Cyg is characterized by a strong excess of helium, nitrogen, neon, and silicon, which is related to the binarity of the system.

Spectroscopic monitoring of V1357 Cyg = Cyg X-1 in 2002–2004

We discuss the results of optical spectroscopic monitoring of Cyg X-1 = HDE 226868/V1357 Cyg in 2002–2004. Our spectroscopy was carried out at the Terskol Observatory (Kabarda-Balkaria, Russia; the resolving power was R = 45 000 and 13 000) and at the Bohyunsan Optical Astronomy Observatory (BOAO, Korea, R = 30 000 and 44 000). Each spectrum covers most of the optical range. We obtained a total of 75 echelle spectra on 33 nights, during both “soft” and “hard” X-ray states of Cyg X-1. We study the influence of the X-rays on spectral-line profiles using RXTE/ASM X-ray data. We find that the X-ray flare of June 13, 2003 resulted in strong variations of the emission profiles of the Hα and Hellλ4686 Å lines within a night. This behavior is due to variations of the ionization state of the gas in the system. We also analyzed line-profile variations with orbital phase. A spectral atlas of Cyg X-1 was created, and the lines it contains identified. A total of 172 stellar lines and blends belonging to 12 chemical elements (H, He, C, N, O, Ne, Mg, Al, Si, S, Fe, Zn) were identified. The spectral classification of HDE 226868 as an ON star is confirmed.

UBVR colours of accretion disks and power-law sources of radiation with interstellar reddening

Abstract The (U–B), (B–V) and (V–R) colour indexes from accretion disks, and sources with power-law spectra, are calculated. They take into account interstellar extinction, in detail. Results are given on two-colour diagrams (U–B)-(B–V) and (B–V)−(V–R). Applications to Nova Vul 1988 and A0620–00 are discussed shortly.

Analysis of V616 Mon/A 0620-00 brightness variations during the outburst of 1975–1976

Abstract Analyzing the V616 Mon/A 0620-00 photoelectric observation data obtained during the decay phase of the 1975–1976 outburst has discriminated the brightness variations with the orbital period which is close to its present-day value. Besides, the period . Seems to have occured to one or another degree throughout the brightness decay interval. The decrease of brightness is mainly due to re - emission of a fading X - ray flux by accretion disk and by star. The light curve with is formed mainly due to X - ray flux re-emission by accretion disk and to mutual eclipses of the star and the disk. The possible variations of the orbital light curves with the brightness decay onset agree with the concept of a disk precessing with this period.

Polarization of intrinsic radiation of tidally distorted stars with electron-scattering atmospheres. I: The case of conservative scattering

The variable linear polarization of the intrinsic radiation emitted by tidally distorted stars was computed with approximation of pure electron or Rayleigh scattering in the atmospheres. A precise form of the equipotential surfaces within the binary system Roche model, which approximates the form of the star surface, was taken into account. Calculations were made using the Sobolev (1949) and Chandrasekhar (1950) solutions of transport equations for an optically thick medium without true absorption. The behaviour of the Stokes polarization parametersQ/I andU/I and the dependence of these on the parameters of the (Q×U) plane. The possibility of establishing the inclination anglei of the system and the degree of filling the Roche-lobe μ from these curves is discussed. The conclusion is drawn that the amplitude of polarization variations is small, not generally exceeding 0.4%; however, in a number of cases it remains at the observable level.

Interstellar matter structure along the line of sight to Cyg X-1

Abstract High-resolution spectra (R = 60 000) of Cyg X-1 = V1357 Cyg obtained with the NES echelle spectrograph of the Russian 6 meter telescope (3950–6690 Å) were used to study narrow interstellar absorption lines. We resolved the interstellar line blends by fitting them with Gaussian profiles. The main three absorption components, with the heliocentric radial velocities Vr = −1, −13 and −26 km s−1, were revealed. They correspond to three interstellar gas and dust complexes along the line of sight to the object. Thus, we get information on the distribution of interstellar matter along the way to Cyg X-1. The interstellar calcium abundance and ionization degree, averaged along the line of sight, were determined. A weak component with Vr = −43 km s−1 is revealed in the profiles of the strongest lines. We relate it to the approaching wall of the expanding interstellar envelope around the Cyg OB3 association (superbubble). This finding supports the view that Cyg X-1 was born in this stellar association and is still located in it.

Some Developments of the Weak Stellar Magnetic Field Determination Method for the Example of Cygnus X-1

Some developments of measurements of the weak stellar magnetic fields by the least square technique applied to spectropolarimetric data are proposed and used for the X-ray binary CygX-1 = HDE 226868 (the optical counterpart is an O 9.7 supergiant).

New RRa variable star near the X-ray Nova Oph 1993=V 2293 Oph and their distance evaluations

Abstract A new RRa variable star has been discovered in 2′.5 from X-ray Nova Oph 1993 = GRS 1716–249 = V 2293 Oph. Its period is 0.d4908409. The brightness varies in the range of 15.9 m –(17.5 m pg. The estimations of interstellar extinctions and distances to Nova Oph 1993 and the new RRa variable star have been received.

Parameters of the SS433 Accretion Disk from Photometry and Polarimetry

Photometric and polarimetric data for SS433 show the presence of a geometrically and optically thick slaved disk oriented normally to the jets. The observed matter is a hot, outflowing gas spreading mainly along the accretion disk and forming a photosphere at a distance of several times 1011 cm. The ratio of the effective diameter of the disk to the thickness is 2 to 3. The disk is surrounded by a corona of outflowing matter that forms a large gas cloud around the binary and that is elongated in the orbital plane.