J. Madej

MODEL ATMOSPHERES AND X-RAY SPECTRA OF BURSTING NEUTRON STARS: HYDROGEN-HELIUM COMPTONIZED SPECTRA

Compton scattering plays a crucial role in determining the structure of the atmosphere of an X-ray burster and its theoretical spectrum. Our paper presents a description of the plane-parallel model atmosphere of a very hot neutron star and its theoretical flux spectrum of outgoing radiation. Our model equations take into account all bound-free and free-free monochromatic opacities relevant to hydrogen-helium chemical composition and take into account the effects of Compton scattering of radiation in thermal plasma with fully relativistic thermal velocities. We use Compton scattering terms in the equation of transfer, which precisely describe photon-electron energy and momentum exchange for photons with initial energies exceeding the electron rest mass of 511 keV. Model atmosphere equations are solved with the variable Eddington factors technique. The grid of H-He model atmospheres and flux spectra is computed on a dense mesh of 107 K ≤ Teff ≤ 3 × 107 K and a surface gravity of log g. In many cases, the assumed log g approached the critical gravity log gcr, i.e., the Eddington limit. We confirm that H-He spectra of X-ray bursters deviate from blackbody spectra and discuss their shapes. The table of color to effective temperature ratios shows that theoretical values of Tc/Teff do not exceed 1.9 in H-He atmospheres in hydrostatic and radiative equilibrium.

A catalog of stellar magnetic rotational phase curves

Magnetized stars usually exhibit periodic variations of the effective (longitudinal) magnetic field Be caused by their rotation. We present a catalog of magnetic rotational phase curves, Be vs. the rotational phase φ, and tables of their parameters for 136 stars on the main sequence and above it. Phase curves were obtained by the least squares fitting of sine wave or double wave functions to the available Be measurements, which were compiled from the existing literature. Most of the catalogued objects are chemically peculiar A and B type stars (127 stars). For some stars we also improved or determined periods of their rotation. We discuss the distribution of parameters describing magnetic rotational phase curves in our sample.

Model atmospheres and X-ray spectra of bursting neutron stars

Model atmosphere equations are derived which correspond to plane-parallel nongray atmospheres of very hot neutron stars in hydrostatic and radiative equilibrium. The nonlinear transfer equation used implements an exact photon redistribution function which precisely traces even those scattering events having large photon-electron energy exchange. The numerical results thus obtained encompass tables of the surface fluxes for 20 model atmospheres whose T(eff) ranges from 6 to 30 million K. The spectra of high gravity models are virtually identical with the blackbody spectrum shifted toward higher energies, while models approaching the Eddington limit develop a large low-energy hump. 39 refs.

Model atmospheres and X-ray spectra of bursting neutron stars II. Iron rich Comptonized Spectra

This paper presents the set of plane-parallel model atmosphere equations for a very hot neutron star (X-ray burst source). The model equations assume both hydrostatic and radiative equilibrium, and the equation of state of an ideal ga s in local thermodynamic equilibrium (LTE). The equation of radiative transfer includes terms describing Compton scattering of photons on free electrons in fully relativistic thermal motion, for photon energies approaching me c 2 . Model equations take into account many bound-free and free-free energy-dependent opacities of hydrogen, helium, and the iron ions, and also a dozen bound- bound opacities for the highest ions of iron. We solve model equations by partial linearisation and the technique of vari able Eddington factors. Large grid of H-He-Fe model atmospheres of X-ray burst sources has been computed for 10 7 ≤ Tef f ≤ 3×10 7 K, a wide range of surface gravity, and various iron abundances. We demonstrate that the spectra of X-ray bursters with iron present in the accreting matter differ significantly from pure H-He spectra (published in an earl ier paper), and also from blackbody spectra. Comptonized spectra with significant ir on abundance are generally closer to blackbody spectra than spectra of H-He atmospheres. The ratio of color to effective temperatures in our grid always remains in the range 1.2< Tc/Tef f < 1.85. The present grid of model atmospheres and theoretical X-ray spectra will be used to determine the effective temperatures, radii and M/R ratios of bursting neutron stars from observational data.

Secular variability of the longitudinal magnetic field of the Ap star γ Equ

We present an analysis of the secular variability of the longitudinal magnetic field Be in the roAp star y Equ (HD 201601). Measurements of the stellar magnetic field Be were mostly compiled from the literature, and we appended also our 33 new Be measurements which were obtained with the 1-m optical telescope of the Special Astrophysical Observatory (Russia). All the available data cover the time period of 58 yr, and include both phases of the maximum and minimum Be. We determined that the period of the long-term magnetic Be variations equals 91.1 ± 3.6 yr, with B e (max) = +577 ± 31 G and B e (min) = -1101 ± 31 G.

The effects of Compton scattering in X-ray spectra

The ways in which Compton scattering on free electrons influences the continuous spectrum and temperature structure of a hot stellar atmosphere, where most of its radiation is emitted in the 1-10 keV energy range, are discussed. The numerical results pertain to the continuum spectra of X-ray bursters. The formulation and development of the numerical code, which reproduces angle-averaged relativistic Compton scattering of thermal (unpolarized) X-rays in a plane-parallel atmosphere in hydrostatic and radiative equilibrium, are described. 24 refs.

On the relevance of Compton scattering for the soft X-ray spectra of hot DA white dwarfs

Aims. We re-examine the effects of Compton scattering on the emergent spectra of hot DA white dwarfs in the soft X-ray range. Earlier studies have implied that sensitive X-ray observations at wavelengths λ< 50 A might be capable of probing the flux deficits predicted by the redistribution of electron-scattered X-ray photons toward longer wavelengths. Methods. We adopt two independent numerical approaches to the inclusion of Compton scattering in the computation of pure hydrogen atmospheres in hydrostatic equilibrium. One employs the Kompaneets diffusion approximation formalism, while the other uses the cross-sections and redistribution functions of Guilbert. Models and emergent spectra are computed for stellar parameters representative of HZ 43 and Sirius B, and for models with an effective temperature Teff = 100 000 K. Results. The differences between emergent spectra computed for Compton and Thomson scattering cases are completely negligible in the case of both HZ 43 and Sirius B models, and are also negligible for all practical purposes for models with temperatures as high as Teff = 100 000 K. Models of the soft X-ray flux from these stars are instead dominated by uncertainties in their fundamental parameters.

Iron lines in model disk spectra of Galactic black hole binaries

Context. We present angle-dependent, broad-band intensity spectra from accretion disks around black holes of 10 M� . In our computations disks are assumed to be slim, which means that the radial advection is taken into account while computing the effective temperature of the disk. Aims. We attempt to reconstruct continuum and line spectra of X-ray binaries in soft state, i.e. dominated by the disk component of multitemperature shape. We follow how the iron-line complex depends on the external irradiation, an accretion rate, and a black hole spin. Methods. Full radiative transfer is solved including effects of Compton scattering, free-free and all important bound-free transitions of 10 main elements. We assume the LTE equation of state. Moreover, we include here the fundamental series of iron lines from helium-like and hydrogen-like ions, and fluorescent Kα and Kβ lines from low ionized iron. We consider two cases: nonrotating black hole, and black hole rotating with almost maximum spin a = 0.98, and obtain spectra for five accretion disks from hard X-rays to the infrared. Results. In nonirradiated disks, resonance lines from He-like and H-like iron appear mostly in absorption. Such disk spectra exhibit limb darkening in the whole energy range. External irradiation causes that iron resonance lines appear in emission. Furthermore, depending on disk effective temperature, fluorescent iron Kα and Kβ lines are present in disk emitting spectra. All models with irradiation exhibit limb brightening in their X-ray reflected continua. Conclusions. We show that the disk around stellar black hole itself is hot enough to produce strong-absorption resonance lines of iron. Emission lines can only be observed if heating by external X-rays dominates thermal processess in a hot disk atmosphere. Irradiated disks are usually brighter in X-ray continuum when seen edge on, and fainter when seen face on.

PRESSURE SHIFT AND GRAVITATIONAL REDSHIFT OF BALMER LINES IN WHITE DWARFS: REDISCUSSION*

The Stark-induced shift and asymmetry, the so-called pressure shift (PS) of Hα and Hβ Balmer lines in spectra of DA white dwarfs (WDs), have been examined in detail as masking effects in measurements of the gravitational redshift in WDs. The results are compared with our earlier ones from a quarter of a century ago. In these earlier papers, the standard, symmetrical Stark line profiles, as a dominant constituent of the Balmer line profiles but shifted as a whole by the PS effect, were applied to all spectrally active layers of the WD atmosphere. At present, in each of the WD layers, the Stark line profiles (especially of Hβ) are inherently asymmetrical and shifted due to the effects of strong inhomogeneity of the perturbing fields in plasma. To calculate the Stark line profiles in successive layers of the WD atmosphere we used the modified Full Computer Simulation Method, able to take adequately into account the complexity of local elementary quantum processes in plasma. In the case of the Hα line, the present value of Stark-induced shift of the synthetic Hα line profile is about half the previous one and it is negligible in comparison with the gravitational redshift. In the case of the Hβ line, the present value of Stark-induced shift of the synthetic Hβ line profile is about twice the previous one. The source of this extra shift is the asymmetry of Hβ peaks.

X-ray-binary spectra in the lamp post model

[Abridged] Context. The high-energy radiation from black-hole binaries may be due to the reprocessing of a lamp located on the black hole axis, emitting X-rays. The observed spectrum is made of 3 components: the direct spectrum; the thermal bump; and the reflected spectrum made of the Compton hump and the iron-line complex. Aims. We aim at computing accurately the complete reprocessed spectrum (thermal bump + reflected) of black-hole binaries over the entire X-ray band. We also determine the strength of the direct component. Our choice of parameters is adapted to a source showing an important thermal component. Methods. We compute in full GR the illumination of a thin disk by a lamp along the rotation axis. We use the ATM21 radiative transfer code to compute the spectrum emitted along the disk. We ray trace this local spectrum to determine the reprocessed spectrum as observed at infinity. We discuss the dependence of the local and ray-traced spectra on the emission angle and spin. Results. We show the importance of the angle dependence of the total disk specific intensity spectrum emitted by the illuminated atmosphere when the thermal disk emission if fully taken into account. High spin implies high temperature in the inner regions, so the emitted thermal disk spectrum covers the iron-line complex. As a result we locally observe absorption lines produced in the hot disk atmosphere. Absorption lines are narrow and disappear after ray tracing the local spectrum. Conclusions. Our results mainly highlight the importance of considering the angle dependence of the local spectrum when computing reprocessed spectra, as was already found in a recent study. The main new result of our work is to show the importance of computing the thermal bump of the spectrum, as this feature can change considerably the observed iron-line complex.