Mario A. Jimenez-Garate

The Structure and X-Ray Recombination Emission of a Centrally Illuminated Accretion Disk Atmosphere and Corona

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 twodimensional structure of the disk and its X-ray recombination emission. The atmospheric structure is extremely insensitive to the viscosity � . 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. Subject headings: accretion, accretion disks — atomic processes — instabilities — line: formation — X-rays: binaries On-line material: color figures

High-Resolution X-Ray Spectroscopy of Hercules X-1 with the XMM-Newton Reflection Grating Spectrometer: CNO Element Abundance Measurements and Density Diagnostics of a Photoionized Plasma

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 day 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 ~10-15 A and also near the O VII intercombination line wavelength.

Discrete X-Ray Signatures of a Photoionized Plasma above the Accretion Disk of the Neutron Star EXO 0748–676

During the disk-mediated accretion phase, the high-resolution X-ray spectrum of the low-mass X-ray binary system EXO 0748-676 reveals a photoionized plasma that is orbiting the neutron star. Our observations with the Chandra High Energy Transmission Grating Spectrometer constrain the structure of the upper layers of the accretion disk by means of the recombination emission lines from the H-like and He-like ions of O, Ne, and Mg, which have a mean velocity broadening σv ~ 750 ± 120 km s-1. The Mg XI emission region has density ne 1012 cm-3 and is located within 7 × 109 cm < r < 6 × 1010 cm of the neutron star, while the temperature of the Ne X region is kT 20 eV. These lines favor a vertically stratified distribution of ions in the disk. The spectra show that the line region is spatially extended and unabsorbed, while the continuum region is compact and heavily absorbed. The absorber has variable column density and is composed of both neutral and ionized gas, which can explain the stochastic and periodic X-ray intensity dips, the X-ray continuum evolution, and the O VII and Mg XI K-shell absorption edges. The absorber is located 8°-15° above the disk midplane, inclusive of two bulges near the disk edge. This outer disk gas may participate in the outflow of ionized plasma that was previously identified in XMM-Newton grating spectra obtained during type I bursts. The thickened photoionized region above the disk can be produced by heating from the neutron star X-rays and by the impact of the accretion stream.

XMM-Newton EPIC observations of Her X-1

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.

XMM–Newton EPIC and Optical Monitor observations of Her X-1 over the 35-d beat period

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 ke V, 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 φ oribit ∼ 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.

Development and production of hard X-ray multilayer optics for HEFT

The High Energy Focusing Telescope (HEFT) will observe a wide range of objects including young supernova remnants, active galactic nuclei, and galaxy clusters at energies between 20 and 70 keV. Large collecting areas are achieved by tightly nesting layers of grazing incidence mirrors in a conic approximation Wolter-I design. The segmented mirrors that form these layers are made of thermally formed glass substrates coated with depth-graded multilayer films for enhanced reflectivity. The mirrors are assembled using an over-constraint method that forces the overall shape of the nominally cylindrical substrates to the appropriate conic form. We will present performance data on the HEFT optics and report the current status of the assembly production.

Thermal forming of glass microsheets for x-ray telescope mirror segments.

We describe a technology to mass-produce ultrathin mirror substrates for x-ray telescopes of near Wolter-I geometry. Thermal glass forming is a low-cost method to produce high-throughput, spaceborne x-ray mirrors for the 0.1-200-keV energy band. These substrates can provide the collecting area envisioned for future x-ray observatories. The glass microsheets are shaped into mirror segments at high temperature by use of a guiding mandrel, without polishing. We determine the physical properties and mechanisms that elucidate the formation process and that are crucial to improve surface quality. We develop a viscodynamic model for the glass strain as the forming proceeds to find the conditions for repeatability. Thermal forming preserves the x-ray reflectance and scattering properties of the raw glass. The imaging resolution is driven by a large wavelength figure. We discuss the sources of figure errors, and we calculate the relaxation time of surface ripples.

Development of Thermally Formed Glass Optics for Astronomical Hard X-ray Telescopes

The next major observational advance in hard X-ray/soft gamma-ray astrophysics will come with the implementation of telescopes capable of focusing 10-200 keV radiation. Focusing allows high signal-to-noise imaging and spectroscopic observations of many sources in this band for the first time. The recent development of depth-graded multilayer coatings has made the design of telescopes for this bandpass practical, however the ability to manufacture inexpensive substrates with appropriate surface quality and figure to achieve sub-arcminute performance has remained an elusive goal. In this paper, we report on new, thermally-formed glass micro-sheet optics capable of meeting the requirements of the next-generation of astronomical hard X-ray telescopes.

X-Ray Line Emission from Evaporating and Condensing Accretion Disk Atmospheres

We model the X-rays reprocessed by an accretion disk in a fiducial low-mass X-ray binary system with a neutron star primary. An atmosphere, or the intermediate region between the optically thick disk and a Compton temperature corona, is photoionized by the neutron star continuum. X-ray lines from the recombination of electrons with ions dominate the atmosphere emission and should be observable with the Chandra and XMM-Newton high-resolution spectrometers. The self-consistent disk geometry agrees well with optical observations of these systems, with the atmosphere shielding the companion from the neutron star. At a critical depth range, the disk gas has one thermally unstable and two stable solutions. A clear difference between the model spectra exists between evaporating and condensing disk atmospheres. This difference should be observable in high-inclination X-ray binaries, or whenever the central continuum is blocked by absorbing material and the extended disk emission is not.

Hard x-ray optics for the HEFT balloon-borne payload: prototype design and status

We report on the current status and performance of prototype hard x-ray optics we are producing for use on the high energy focusing telescope (HEFT) experiment. The baseline substrates are thermally formed glass mirrors that are overcoated with multilayers to provide good performance throughout the 20-80 keV bandpass. Progress made in the thermal forming process as well as in the multilayer performance has allowed production of optics that meet or exceed all HEFT requirements. We present metrology on the substrates and result from x-ray characterization. A novel mounting scheme for the individual telescope shells is currently being tested. If successful the mounting technique will produce a monolithic, extremely stiff and robust optic.