We model an accretion disk atmosphere and corona photoionized by a central X-ray continuum source. We calculate the opacity and one-dimensional radiation transfer for an array of disk radii, to obtain the two-dimensional structure of the disk and its X-ray recombination emission. The atmospheric structure is extremely insensitive to the viscosity α. We find a feedback mechanism between the disk structure and the central illumination, which expands the disk and increases the solid angle subtended by the atmosphere. We apply the model to the disk of a neutron star X-ray binary. The model is in agreement with the ∼ 12 • disk half-angle measured from optical light curves. We map the temperature, density, and ionization structure of the disk, and we simulate high resolution spectra expected from the Chandra and XMM-Newton grating spectrometers. X-ray emission lines from the disk atmosphere are detectable, especially for high-inclination binary systems. The grating observations of two classes of X-ray binary systems already reveal important spectral similarities with our models. The model spectrum is dominated by double-peaked lines of H-like and He-like ions, plus weak Fe L. The line flux is proportional to the luminosity and is dominated by the outer radii. Species with a broad range of ionization levels coexist at each radius: from Fe XXVI in the hot corona, to C VI at the base of the atmosphere. The line spectrum is very sensitive to the temperature, ionization, and emission measure of each atmospheric layer, and it probes the heating mechanisms in the disk. We assume a hydrostatic disk dominated by gas pressure, in thermal balance, and in ionization equilibrium. As boundary conditions, we take a Compton-temperature corona and an underlying Shakura-Sunyaev disk. The choice of thermally stable solutions strongly affects the spectrum, since a thermal instability is present in the regime where X-ray recombination emission is most intense.

We present spin-resolved X-ray data of the neutron star binary Her X-1 taken using the EPIC detectors on XMM-Newton. The data were taken at three distinct epochs through the 35-d precession period. The energy-dependent light curves of this source vary significantly from epoch to epoch. It is known that the relative phasing of the soft ( 1 keV) and hard ( 2 keV) X-rays varies. Here, we find that the phase shift between the soft and hard bands during the main-on state is considerably different from that observed in the past. Further, it continues to change significantly during the other two observations. This suggests that we are observing, for the first time, a substantial and continuous variation in the tilt of the disc, as it is expected if the accretion disc is precessing in the system. Analysis of the spin resolved data confirms that the equivalent width variation of the fluorescence Fe K line at ∼6.4 keV follows that of the soft X-ray emission in the main-on state, thus suggesting a common origin for Fe K line and thermal component. The Fe K line is considerably broader when the source is brightest.

We present the results of a series of XMM–Newton European Photon Imaging Camera (EPIC) and Optical Monitor observations of Her X‐1, spread over a wide range of the 35‐d precession period. We confirm that the spin modulation of the neutron star is weak or absent in the low state – in marked contrast to the main or short‐on states. During the states of higher intensity, we observe a substructure in the broad soft X‐ray modulation below ∼1 keV, revealing the presence of separate peaks which reflect the structure seen at higher energies. The strong fluorescence emission line at ∼6.4 keV is detected in all observations (apart from one taken in the middle of eclipse), with higher line energy, width and normalization during the main‐on state. In addition, we report the detection of a second line near 7 keV in 10 of the 15 observations taken during the low‐intensity states of the system. This feature is rather weak and not significantly detected during the main‐on state, when the strong continuum emission dominates the X‐ray spectrum. Spin‐resolved spectroscopy just after the rise to the main‐on state shows that the variation of the Fe Kα at 6.4 keV is correlated with the soft X‐ray emission. This confirms our past finding based on the XMM–Newton observations made further into the main‐on state, and indicates the common origin for the thermal component and the Fe Kα line detected at these phases. We also find that the normalization of the 6.4‐keV line during the low state is correlated with the binary orbital phase, having a broad maximum centred near φorbit∼ 0.5. We discuss these observations in the context of previous observations, investigate the origin of the soft and hard X‐rays and consider the emission site of the 6.4‐keV and 7‐keV emission lines.

We analyze the high-resolution X-ray spectrum of Hercules X-1, an intermediate-mass X-ray binary, which was observed with the XMM-Newton Reflection Grating Spectrometer. We measure the elemental abundance ratios by use of spectral models, and we detect material processed through the CNO-cycle. The CNO abundances, and in particular the ratio N/O > 4.0 times solar, provide stringent constraints on the evolution of the binary system. The low and short-on flux states of Her X-1 exhibit narrow line emission from C VI, N VI, N VII, O VII, O VIII, Ne IX, and Ne X ions. The spectra show signatures of photoionization. We measure the electron temperature, quantify photoexcitation in the Heα lines, and set limits on the location and density of the gas. The recombination lines may originate in the accretion disk atmosphere and corona, or on the X-ray illuminated face of the mass donor (HZ Her). The spectral variation over the course of the 35 d period provides additional evidence for the precession of the disk. During the main-on state, the narrow line emission is absent, but we detect excesses of emission at ∼10-15Å, and also near the O VII intercombination line wavelength.