High Energy Transmission Grating Research Articles

High-Resolution Spectroscopy of the X-Ray-photoionized Wind in Cygnus X-3 with the Chandra High-Energy Transmission Grating Spectrometer.

We present a preliminary analysis of the 1-10 keV spectrum of the massive X-ray binary Cygnus X-3, obtained with the high-energy transmission grating spectrometer on the Chandra X-Ray Observatory. The source reveals a richly detailed discrete emission spectrum, with clear signatures of photoionization-driven excitation. Among the spectroscopic novelties in the data are the first astrophysical detections of a number of He-like "triplets" (Si, S, Ar) with emission-line ratios characteristic of photoionization equilibrium, fully resolved narrow radiative recombination continua of Mg, Si, and S, the presence of the H-like Fe Balmer series, and a clear detection of an approximately 800 km s-1 large-scale velocity field as well as an approximately 1500 km s-1 FWHM Doppler broadening in the source. We briefly touch on the implications of these findings for the structure of the Wolf-Rayet wind.

ITAL]Chandra[/ITAL] Discovers a Very High Density X-Ray Plasma on the O Star ζ Orionis

We report on a Chandra line spectrum observation of the O supergiant ζ Orionis (O9.7 Ib). A 73.4 ks High-Energy Transmission Grating Spectrometer observation shows a wide range of ionization stages and line strengths over the wavelength range of 5-26 A. The observed emission lines indicate a range in temperature of 2-10 MK, which is consistent with earlier X-ray observations of ζ Ori. Many lines are spectrally resolved showing Doppler broadening of 900 ± 200 km s-1. The observed He-like ions (O VII, Ne IX, Mg XI, and Si XIII) provide information about the spatial distribution of the X-ray emission. Although the observations support a wind distribution of X-ray sources, we find three conflicting results. First, line diagnostics for Si XIII indicate that this line emission forms very close to the stellar surface, where the density is of order 1012 cm-3, but the velocity there is too small to produce the shock jump required for the observed ionization level. Second, the strong X-ray line profiles are symmetric and do not show any evidence of Doppler-blueshifted line centroids, which are expected to accompany an outwardly moving source in a high-density wind. Third, the observed velocity dispersions do not appear to correlate with the associated X-ray source radii velocities, contrary to expectations of wind-distributed source models. A composite source model involving wind shocks and some magnetic confinement of turbulent hot plasma in a highly nonsymmetric wind appears to be needed to explain the line diagnostic anomalies.

Resonant Absorption in the Active Galactic Nucleus Spectra Emerging from Photoionized Gas: Differences between Steep and Flat Ionizing Continua

We present photoionization models accounting for both photoelectric and resonant absorption. Resonance absorption lines from C, O, Ne, Mg, Si, S, and Fe between 0.1 and 10 keV are treated. In particular we consider the complex of almost 60 strong Fe L absorption lines around 1 keV. We calculate profiles, intensities, and equivalent widths of each line, considering both Doppler and natural broadening mechanisms. Doppler broadening includes a term accounting for turbulence of the gas along the line of sight. We computed spectra transmitted by gas illuminated by drastically different ionizing continua and compared them to spectra observed in flat X-ray spectrum, broad optical emission-line type 1 active galactic nuclei (AGNs), and steep X-ray spectrum, narrow optical emission-line type 1 AGNs. We show that the ~1 keV absorption feature observed in moderate-resolution X-ray spectra of several narrow-line Seyfert 1 galaxies can be explained by photoionization models, taking into account resonance absorption, without requiring relativistic outflowing velocities of the gas, if the physical properties of these absorbers are close to those found in flat X-ray spectrum Seyfert 1 galaxies. We finally present simulations of the spectra emerging from gas illuminated by both steep and flat ionizing continua, as seen by the AXAF high-energy transmission gratings and the baseline Constellation-X calorimeter. We discuss briefly the relevant physics that can be investigated with these instruments.

Simulating AXAF Grating Spectra of Accreting White Dwarfs

AbstractWe present simulated AXAF spectra of accreting white dwarfs, using parameters appropriate for magnetic cataclysmic variables. The very high spectral resolution that can be obtained with the High-Energy Transmission Grating of AXAF can resolve the keV X-ray emission lines that characterise the temperature, density and velocity profiles of the shock-heated emission regions of these systems. These simulations demonstrate that actual spectra will allow us to place constraints on the white-dwarf mass and the accretion rate of the systems. The high-resolution spectra also allow the measurement of the velocity of the accretion flow in regions close to the white-dwarf surface.

EXAFS and XANES: New Astrophysical Tools to Study the Solid State Structure of Interstellar Grains

I show that new information about the compositions and crystal structures of interstellar grains can be derived from measurements of extended X-ray absorption fine structure (EXAFS) above the absorption edges in the spectra of highly extincted and bright X-ray stars. It will be also possible to probe the long-range crystal structures of solid grains by analyzing extended X-ray absorption-edge profiles modified by X-ray absorption near edge structure (XANES). I show that recently developed high-resolution (E/ΔE ≈ 1000) X-ray spectrometers (e.g., the High-Energy Transmission Grating Spectrometer on the Advanced X-Ray Astrophysics Facility [AXAF] or the Reflection Grating Spectrometer on the X-Ray Multi-Mirror satellite [XMM]) will be able to measure these features.

Transmission grating spectroscopy and the Advanced X-Ray Astrophysics Facility

The use of transmission gratings with grazing-incidence telescopes in celestial x-ray astronomy is reviewed. The basic properties of transmission grating spectrometers and the use of "phased" gratings to enhance the diffraction efficiency are outlined. Special attention is given to the AXAF high-energy transmission grating (HETG) being fabricated at MIT. The HETG operates over the range 0.4 to 8 keV, gives resolving powers of 100 to 1000, effective areas of 10 to 300 cm², and a minimum detectable line flux of 1 to 10 x 10^-6 photons cm^-2 s^-1. The instrument consists of a single array with two types of membrane-supported grating facets: medium-energy gratings (0.6-μm period, 0.5-μm-thick silver) mounted behind the outer three AXAF mirrors, and high-energy gratings (0.2-μm period, 1.0-μm-thick gold) mounted behind the inner three mirrors. The materials and thicknesses are selected to maximize efficiency throughout the energy band. The facets are fabricated at MIT using a process involving x-ray lithography. AXAF will also carry a low-energy transmission grating (LETG) supplied by the Laboratory for Space Research at Utrecht. It uses mesh-supported grating facets of 1.0-μm period and is optimized for operation down to 80 eV. Gratings are most effective for the study of point sources, but they also give moderate resolution spectra of slightly extended sources and can be used to map the spatial distribution of line-emitting regions.