Petr Hadrava

THE ULTRAVIOLET SPECTRUM AND PHYSICAL PROPERTIES OF THE MASS DONOR STAR IN HD 226868 = Cygnus X-1 ∗

We present an examination of high-resolution, ultraviolet (UV) spectroscopy from Hubble Space Telescope of the photospheric spectrum of the O-supergiant in the massive X-ray binary HD 226868 = Cyg X-1. We analyzed this and ground-based optical spectra to determine the effective temperature and gravity of the O9.7 Iab supergiant. Using non-LTE, line-blanketed, plane-parallel models from the TLUSTY grid, we obtain T eff = 28.0 ± 2.5 kK and log g 3.00 ± 0.25, both lower than in previous studies. The optical spectrum is best fit with models that have enriched He and N abundances. We fit the model spectral energy distribution for this temperature and gravity to the UV, optical, and infrared (IR) fluxes to determine the angular size and extinction toward the binary. The angular size then yields relations for the stellar radius and luminosity as a function of distance. By assuming that the supergiant rotates synchronously with the orbit, we can use the radius-distance relation to find mass estimates for both the supergiant and black hole (BH) as a function of the distance and the ratio of stellar to Roche radius. Fits of the orbital light curve yield an additional constraint that limits the solutions in the mass plane. Our results indicate masses of 23+8 –6 M ☉ for the supergiant and 11+5 –3 M ☉ for the BH.

Stellar wind variations during the X-ray high and low states of Cygnus X-1

We present results from Hubble Space Telescope ultraviolet spectroscopy of the massive X-ray and black hole binary system, HD 226868 = Cyg X-1. The spectra were obtained at both orbital conjunction phases in 2002 and 2003, when the system was in the X-ray high/soft state. The UV stellar wind lines suffer large reductions in absorption strength when the black hole is in the foreground due to the X-ray ionization of the wind ions. We constructed model UV wind line profiles assuming that X-ray ionization occurs everywhere in the wind except the zone where the supergiant blocks the X-ray flux. The good match between the observed and model profiles indicates that the wind ionization extends to near the hemisphere of the supergiant facing the X-ray source. We also present contemporaneous spectroscopy of the Hα emission that forms in the high-density gas at the base of the supergiants wind and the He II λ4686 emission that originates in the dense, focused wind gas between the stars. The Hα emission strength is generally lower in the high/soft state than in the low/hard state, but the He II λ4686 emission is relatively constant between X-ray states. The results suggest that mass transfer in Cyg X-1 is dominated by the focused wind flow that peaks along the axis joining the stars, and that the stellar wind contribution from the remainder of the hemisphere facing the X-ray source is shut down by X-ray photoionization effects (in both X-ray states).

Disentangling telluric lines in stellar spectra

The use of a method of spectra disentangling for telluric lines is explained in detail, with a particular emphasis on high-precision radial-velocity measurements for the search for extrasolar planets. New improvements to the method are introduced.

Really focused stellar winds in X-ray binaries

Aims. We investigate the anisotropy of stellar winds in binaries to improve the models of accretion in high-mass X-ray binaries. Methods. We model numerically the stellar wind from a supergiant component of a binary in radial and three-dimensional radiation hydrodynamic approximation taking into account the Roche potential, Coriolis force, and radiative pressure in the continuum and spectral lines. Results. The Coriolis force influences substantially the mass loss and thus also the accretion rate. The focusing of the stellar wind by the gravitational field of the compact companion leads to the formation of a gaseous tail behind the companion.

Notes on disentangling of spectra II. Intrinsic line-profile variability due to Cepheid pulsations ,

Context. The determination of pulsation velocities from observed spectra of Cepheids is needed for the Baade-Wesselink calibration of these primary distance markers. Aims. The applicability of the Fourier-disentangling technique for the determination of pulsation velocities of Cepheids and other pulsating stars is studied. Methods. The KOREL-code was modified to enable fitting of free parameters of a prescribed line-profile broadening function corresponding to the radial pulsations of the stellar atmosphere. It was applied to spectra of δ Cep in the H-alpha region observed with the Ondy 2-m telescope. Results. The telluric lines were removed using template-constrained disentangling, phase-locked variations of line-strengths were measured and the curves of pulsational velocities obtained for several spectral lines. It is shown that the amplitude and phase of the velocities and line-strength variations depend on the depth of line formation and the excitation potential. Conclusions. The disentangling of pulsations in the Cepheid spectra may be used for distance determination.

General Relativistic Effects on the Spectrum Reflected by Accretion Disks around Black Holes

Making the usual assumption that the relatively cold matter within the central engine of an active galactic nucleus (or galactic black hole candidate) is in the form of a relativistic accretion disk, we compute the composite spectrum of the original disk plus a primary X-ray power-law source illuminating it from above, as well as the reflected emission from the disk. All special and general relativistic effects on both infalling photons and outgoing photons are considered in a Schwarzschild geometry. The strength, shape, and broadening of the reflected spectrum depend on the direction of the X-ray source relative to the disk and the observers viewing angle. The reflected photons extract energy and angular momentum from the relativistically rotating accretion disk and are beamed in the direction of the disk velocity. The reflection hump could essentially disappear if viewed far from the symmetry axis because the X-ray photons are affected by gravity both approaching and leaving the disk. This may produce a difference between X-ray spectra for Seyfert 1 and Seyfert 2 galaxies. For a given observation angle, the reflection hump is most sensitive to the inclination of the source relative to the accretion disk. Thus the spectral shape may also shed light on the location of the primary X-ray source, which is probably either in a jet or in a corona; however, additional computations involving distributed sources will be necessary before detailed comparisons with observations are feasible.

Notes on the disentangling of spectra I. Enhancement in precision

Context. The technique of disentangling has been applied to numerous high-precision studies of spectroscopic binaries and multiple stars. Although, its possibilities have not yet been fully understood and exploited. Aims. Theoretical background aspects of the method, its latest improvements and hints for its use in practice are explained in this series of papers. Methods. In this first paper of the series, we discuss spectral-resolution limitations due to a discrete representation of the observed spectra and introduce a new method how to achieve a precision higher than the step of input-data binning. Results. Based on this principle, the latest version of the KOREL code for Fourier disentangling achieves an increase in precision for an order of magnitude.

Optical spectroscopic observations of Cyg X-1=HDE 226868

We present the results of the spectroscopic observations of HDE 226868, the optical counterpart to the black hole X-ray binary Cyg X-1, from 2001 to 2006. We analyze the variabilities of the two components in the complex Hα line: one P-Cygni shaped component which follows the motion of the supergiant and another emission component moving with an antiphase orbital motion relative to the supergiant, which is attributed to a focused stellar wind. The results of KOREL disentangling of our spectra indicate that the focused stellar wind is responsible for the major part of the variability of the Hα emission line. The emission of the supergiant component had a small difference between the low/hard and high/soft states, while the focused-wind component became strong in the low/hard state and weak in the high/soft state. The wind is nearly undisturbed by the X-ray photoionization during the low/hard state. However, during the high/soft state, the X-rays from the compact object could decelerate the line-driven wind and result in a high-mass accretion rate, due to the effect of the X-ray photoionization. The X-ray illuminating could also change the temperature profile of the stellar wind and increase its temperature, and thus decrease the Hα emissivity of the wind, which could explain the Hα variabilities of Cyg X-1 during different X-ray states.

Stellar wind in state transitions of high-mass X-ray binaries

Aims: We have developed a new code for the three-dimensional time-dependent raditation hydrodynamic simulation of the stellar wind in interacting binaries to improve models of accretion in high-mass X-ray binaries and to quantitatively clarify the observed variability of these objects. We used the code to test the influence of various parameters on the structure and properties of circumstellar matter. Methods: Our code takes into account acceleration of the wind due to the Roche effective potential, Coriolis force, gas pressure, and (CAK-) radiative pressure in the lines and continuum of the supergiant radiation field that is modulated by its gravity darkening and by the photo-ionization caused by X-ray radiation from the compact companion. The parameters of Cygnus X-1 were used to test the properties of our model. Results: Both two- and three-dimensional numerical simulations show that the Coriolis force substantially influences the mass loss and consequently the accretion rate onto the compact companion. The gravitational field of the compact companion focuses the stellar wind, which leads to the formation of a curved cone-like gaseous tail behind the companion. The changes of X-ray photo-ionization of the wind material during X-ray spectral-state transitions significantly influence the wind structure and offer an explanation of the variability of Cygnus X-1 in optical observations (the H

Properties and nature of Be stars. XXIII. Long-term variations and physical properties of x Dra

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