Chapter Four - Atomic Data Needs for Understanding X-ray Astrophysical Plasmas

The Advanced X-ray Astrophysics Facility (AXAF) is a major NASA space observatory and is scheduled for launch in 1998. AXAF will perform high spatial and spectral resolution observations of celestial sources in the soft x-ray band 0.1 - 10 keV. The high resolution camera (HRC) is one of two focal plane instruments being developed for the AXAF. The HRC will be capable of observing point and extended sources with high sensitivity and high spatial resolution and will be used to record the high resolution spectra produced by an objective transmission grating. The HRC is based on microchannel plates (MCPs). We describe the design and development of the HRC, its expected performance, and some of its observational goals. The HRC consists of two separate detectors, HRC-I (imaging) and HRC-S (spectroscopy). HRC-I is used for imaging and has a field of view of 31 arc min by 31 arc min and a spatial resolution of less than 25 micrometers (equivalent to less than 0.5 arc sec). HRC- S is optimized to readout the spectrum of AXAF's low energy transmission grating (LETG) and this combination will achieve resolving powers in excess of 1000 at low energies and cover a wavelength range of 4 to 140 angstroms.

The Advanced X-ray Astrophysics Facility (AXAF) is scheduled for launch in summer/fall 1998. One of its two focal plane instruments is the high resolution camera (HRC). The HRC consists of two detectors; an imaging detector (HRC-I) and a detector (HRC-S) for the spectroscopic read-out of the low energy transmission grating (LETG). Both detectors are comprised of a chevron pair of microchannel plates with a crossed grid charge detector (CGCD) and a UV/ion shield (UVIS). Each UVIS is mounted as a free standing window in front of the MCPs. The HRC-I UVIS is 10 cm multiplied by 10 cm and consists of 5000 angstrom polyimide, one side of which is coated with 700 angstrom aluminum. The other side is coated with 200 angstroms of carbon. The HRC-S UVIS consists of three 3 cm multiplied by 10 cm segments. The thickness of the polyimide film (2000 - 2500 angstrom) and of the aluminum coating (300 - 2000 angstrom) of each segment has been varied to optimize the shield's performance with the LETG. In this paper, x-ray transmission models are presented. Results of laboratory x-ray transmission measurements of the flight HRC-I UVIS at various energies in the range of 0.1 to 1.5 keV, as well as results of x-ray transmission measurements of a flight UVIS-I witness sample, are discussed. Results of UV transmission measurements of a flight UVIS-I witness sample also are presented.

The Chandra X-ray Observatory was successfully launched on July 23, 1999, and subsequently began an intensive calibration phase. We present preliminary results from in- flight calibration of the low energy response of the High Resolution Camera Spectroscopic readout (HRC-S) combined with the Low Energy Transmission Grating (LETG) aboard Chandra. These instruments comprise the Low Energy Transmission Grating Spectrometer (LETGS). For this calibration study, we employ a pure hydrogen non-LTE white dwarf emission model (T<SUB>eff</SUB> equals 25000 K and log g equals 9.0) for comparison with the Chandra observations of Sirius B. Pre-flight calibration of the LETGS effective area was conducted only at wavelengths shortward of 45 angstroms (E &gt; 0.277 keV). Our Sirius B analysis shows that the HRC-S quantum efficiency (QE) model assumed for longer wavelengths overestimates the effective area on average by a factor of 1.6. We derive a correction to the low energy HRC-S QE model to match the predicted and observed Sirius B spectra over the wavelength range of 45 - 185 angstroms. We make an independent test of our results by comparing a Chandra LETGS observation of HZ 43 with pure hydrogen model atmosphere predictions and find good agreement.

Trends and focii of interest in atomic modelling and data are identified in connection with recent observations and experiments in fusion and astrophysics. In the fusion domain, spectral observations are included of core, beam penetrated and divertor plasma. The helium beam experiments at JET and the studies with very heavy species at ASDEX and JET are noted. In the astrophysics domain, illustrations are given from the SOHO and CHANDRA spacecraft which span from the solar upper atmosphere, through soft x-rays from comets to supernovae remnants. It is shown that non-Maxwellian, dynamic and possibly optically thick regimes must be considered.The generalized collisional-radiative model properly describes the collisional regime of most astrophysical and laboratory fusion plasmas and yields self-consistent derived data for spectral emission, power balance and ionization state studies. The tuning of this method to routine analysis of the spectral observations is described. A forward look is taken as to how such atomic modelling, and the atomic data which underpin it, ought to evolve to deal with the extended conditions and novel environments of the illustrations. It is noted that atomic physics influences most aspects of fusion and astrophysical plasma behaviour but the effectiveness of analysis depends on the quality of the bi-directional pathway from fundamental data production through atomic/plasma model development to the confrontation with experiment. The principal atomic data capability at JET, and other fusion and astrophysical laboratories, is supplied via the Atomic Data and Analysis Structure (ADAS) Project. The close ties between the various experiments and ADAS have helped in this path of communication.

We present a detailed study of the ESO 3060170 galaxy group, combining Chandra, XMM-Newton, and optical observations. The system is found to be a fossil galaxy group. The group X-ray emission is composed of a central, dense, cool core (10 kpc in radius) and an isothermal medium beyond the central 10 kpc. The region between 10 and 50 kpc (the cooling radius) has the same temperature as the gas from 50 to 400 kpc, although the gas cooling time between 10 and 50 kpc (2-6 Gyr) is shorter than the Hubble time. Thus, the ESO 3060170 group does not have a group-sized cooling core. We suggest that the group cooling core may have been heated by a central active galactic nucleus (AGN) outburst in the past and that the small, dense, cool core is the truncated relic of a previous cooling core. The Chandra observations also reveal a variety of X-ray features in the central region, including a finger, an edgelike feature, and a small tail, all aligned along a north-south axis, as are the galaxy light and group galaxy distribution. The proposed AGN outburst may cause gas to slosh around the center and produce these asymmetric features. The observed flat temperature profile to rvir is not consistent with the predicted temperature profile in recent numerical simulations. We compare the entropy profile of the ESO 3060170 group with those of three other groups and find a flatter relation than that predicted by simulations involving only shock heating, S ∝ r~0.85. This is direct evidence of the importance of nongravitational processes in group centers. We derive the mass profiles within rvir and find that the ESO 3060170 group is the most massive fossil group known [ × 1014 M☉]. The M/L ratio of the system, ~150 at 0.3rvir, is normal.

We present the in-flight effective area calibration of the Low Energy Transmission Grating Spectrometer (LETGS), which comprises the High Resolution Camera Spectroscopic readout (HRC-S) and the Low Energy Transmission Grating (LETG) aboard the Chandra X-ray Observatory. Previous studies of the LETGS effective area calibration have focused on specific energy regimes: 1) the low-energy calibration for which we compared observations of Sirius B and HZ 43 with pure hydrogen non-LTE white dwarf emission models; and 2) the mid-energy calibration for which we compared observations of the active galactic nuclei PKS 2155-304 and 3C 273 with simple power-law models of their seemingly featureless continua. The residuals of the model comparisons were taken to be true residuals in the HRC-S quantum efficiency (QE) model. Additional in-flight observations of celestial sources with well-understood X-ray spectra have served to verify and fine-tune the calibration. Thus, from these studies we have derived corrections to the HRC-S QE to match the predicted and observed spectra over the full practical energy range of the LETGS. Furthermore, from pre-flight laboratory flatfield data we have constructed an HRC-S quantum efficiency uniformity (QEU) model. Application of the QEU to our semi-empirical in-flight HRC-S QE has resulted in an improved HRC-S on-axis QE. Implementation of the HRC-S QEU with the on-axis QE now allows for the computation of effective area for any reasonable Chandra/LETGS pointing.

Over the past 16 years, NASA's Chandra X-ray Observatory has provided an unparalleled means for exploring the high energy universe with its half-arcsecond angular resolution. Chandra studies have deepened our understanding of galaxy clusters, active galactic nuclei, galaxies, supernova remnants, planets, and solar system objects addressing most, if not all, areas of current interest in astronomy and astrophysics. As we look beyond Chandra, it is clear that comparable or even better angular resolution with greatly increased photon throughput is essential to address even more demanding science questions, such as the formation and subsequent growth of black hole seeds at very high redshift; the emergence of the first galaxy groups; and details of feedback over a large range of scales from galaxies to galaxy clusters. Recently, NASA Marshall Space Flight Center, together with the Smithsonian Astrophysical Observatory, has initiated a concept study for such a mission now named the X-ray Surveyor. This concept study starts with a baseline payload consisting of a high resolution X-ray telescope and an instrument set which may include an X-ray calorimeter, a wide-field imager and a dispersive grating spectrometer and readout. The telescope would consist of highly nested thin shells, for which a number of technical approaches are currently under development, including adjustable X-ray optics, differential deposition, and modern polishing techniques applied to a variety of substrates. In many areas, the mission requirements would be no more stringent than those of Chandra, and the study takes advantage of similar studies for other large area missions carried out over the past two decades. Initial assessments indicate that such an X-ray mission is scientifically compelling, technically feasible, and worthy of a high prioritization by the next American National Academy of Sciences Decadal Survey for Astronomy and Astrophysics.

We present a non-LTE (NLTE) model atmosphere analysis of Chandra High Resolution Camera (HRC-S) and Low Energy Transmission Grating (LETG) and XMM-Newton Reflection Grating Spectrometer (RGS) spectroscopy of the prototypical supersoft source CAL 83 in the Large Magellanic Cloud. Taken with a 16 month interval, the Chandra and XMM-Newton spectra are very similar. They reveal a very rich absorption-line spectrum from the hot white dwarf photosphere but no spectral signatures of a wind. We also report a third X-ray off-state during a later Chandra observation, demonstrating the recurrent nature of CAL 83. Moreover, we found evidence of short-timescale variability in the soft X-ray spectrum. We completed the analysis of the LETG and RGS spectra of CAL 83 with new NLTE line-blanketed model atmospheres that explicitly include 74 ions of the 11 most abundant species. We successfully matched the Chandra and XMM-Newton spectra assuming a model composition with LMC metallicity. We derived the basic stellar parameters of the hot white dwarf, but the current state of atomic data in the soft X-ray domain precludes a detailed chemical analysis. We have obtained the first direct spectroscopic evidence that the white dwarf is massive (MWD 1 M☉). The short timescale of the X-ray off-states is consistent with a high white dwarf mass. Our analysis thus provides direct support for supersoft sources as likely progenitors of Type Ia supernovae (SNe Ia).

The response of radiation protection dosemeters in terms of the phantom-related operational quantities Hp(10) and H'(10.0 degrees) was measured for personal and area monitoring systems in mixed high-energy electron and photon radiation fields with energies up to 7 MeV. Using mixed radiation fields composed of different fractions of charged particle and photon fluence, three conditions were produced at the point of measurement: charged particle equilibrium (CPE) (a), a lack (b) and an excess (c) of charged particles relative to the conditions of CPE. Personal and area dosemeters of different types were investigated under conditions (a)-(c). A large variability of the response of the different dosemeter types was observed. The results are presented and discussed.

We present the results of the first X-ray gratings spectroscopy observations of a planetary nebula (PN)—the X-ray-bright, young BD +30°3639. We observed BD +30°3639 for a total of ~300 ks with the Chandra X-ray Observatory's Low Energy Transmission Gratings in combination with its Advanced CCD Imaging Spectrometer (LETG/ACIS-S). The LETG/ACIS-S spectrum of BD +30°3639 is dominated by H-like resonance lines of O VIII and C VI and the He-like triplet line complexes of Ne IX and O VII. Other H-like resonance lines, such as N VII, and lines of highly-ionized Fe are weak or absent. Continuum emission is evident over the range 6-18 A. Spectral modeling indicates the presence of a range of plasma temperatures from Tx ~ 1.7 × 106 K to 2.9 × 106 K and an intervening absorbing column NH ~ 2.4 × 1021 cm–2. The same modeling conclusively demonstrates that C and Ne are highly enhanced, with abundance ratios of C/O ~ 15-45 and Ne/O ~ 3.3-5.0 (90% confidence ranges, relative to the solar ratios), while N and Fe are depleted, with abundances N/O ~ 0.0-1.0 and Fe/O ~ 0.1-0.4, respectively. The intrinsic luminosity of the X-ray source determined from the modeling and the measured flux (FX = 4.1 × 10–13 ergs cm–2 s–1) is LX ~ 8.6 × 1032 erg s–1 (assuming D = 1.2 kpc). These gratings spectroscopy results are generally consistent with earlier results obtained from X-ray CCD imaging spectroscopy of BD +30°3639, but are far more precise. Hence, the Chandra/LETG-S results for BD +30°3639 place severe new constraints on models of PN wind-wind interactions in which X-ray emitting gas within PNs is generated via shocks and the plasma temperature is moderated by effects such as heat conduction or rapid evolution of the fast wind. The tight constraints placed on the (nonsolar) abundances directly implicate the present-day central star—hence, ultimately, the intershell region of the progenitor asymptotic giant branch star—as the origin of the shocked plasma now emitting in X-rays.

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.

RT Cru belongs to the rare class of hard X-ray emitting symbiotics, whose origin is not yet fully understood. In this work, we have conducted a detailed spectroscopic analysis of X-ray emission from RT Cru based on observations taken by the Chandra Observatory using the Low Energy Transmission Grating (LETG) on the High-Resolution Camera Spectrometer (HRC-S) in 2015 and the High Energy Transmission Grating (HETG) on the Advanced CCD Imaging Spectrometer S-array (ACIS-S) in 2005. Our thermal plasma modelling of the time-averaged HRC-S/LETG spectrum suggests a mean temperature of kT ∼ 1.3 keV, whereas kT ∼ 9.6 keV according to the time-averaged ACIS-S/HETG. The soft thermal plasma emission component (∼1.3 keV) found in the HRC-S is heavily obscured by dense materials (&amp;gt;5 × 1023 cm−2). The aperiodic variability seen in its light curves could be due to changes in either absorbing material covering the hard X-ray source or intrinsic emission mechanism in the inner layers of the accretion disc. To understand the variability, we extracted the spectra in the ‘low/hard’ and ‘high/soft’ spectral states, which indicated higher plasma temperatures in the low/hard states of both the ACIS-S and HRC-S. The source also has a fluorescent iron emission line at 6.4 keV, likely emitted from reflection off an accretion disc or dense absorber, which was twice as bright in the HRC-S epoch compared to the ACIS-S. The soft thermal component identified in the HRC-S might be an indication of a jet that deserves further evaluations using high-resolution imaging observations.

Context. The physical interpretation of scattering line polarization offers a novel diagnostic window for exploring the thermal and magnetic structure of the quiet regions of the solar atmosphere. Aims. We evaluate the impact of isotropic collisions with neutral hydrogen atoms on the scattering polarization signals of the 13 lines of multiplet 42 of Ti i and on those of the K line and of the IR triplet of Ca ii, with emphasis on the collisional transfer rates between nearby J-levels. Methods. We calculate the linear polarization produced by scattering processes in a plane-parallel layer illuminated by the radiation field from the underlying solar photosphere. We consider realistic multilevel models and solve the statistical equilibrium equations for the multipolar components of the atomic density matrix. Results. We give suitable formulae for calculating the collisional rates as a function of temperature and hydrogen number density. We confirm that the lower levels of the 13 lines of multiplet 42 of Ti i are completely depolarized by elastic collisions. Upperlevel depolarization caused by the collisional transfer rates between nearby J-levels turns out to have an unnoticeable impact on the emergent linear polarization amplitudes, except for the λ4536 and λ4544.7 lines. Concerning the Ca ii lines, we show that the collisional rates play no role in the polarization of the upper level of the K line, while they have a rather small depolarizing effect on the atomic polarization of the metastable lower levels of the Ca ii IR triplet. Conclusions. Although the collisional transfer rates seem to play a minor role for most of the lines we considered in this paper, except, for example, for the magnetically insensitive λ4536 line of Ti i, they might be important for other atomic or molecular systems with closer J-levels (e.g., hyperfine structured multiplets and/or molecules). Therefore, future research in this direction will be worthwhile.

The comet C/LINEAR 1999 S4 was observed by the ACIS-S instrument on the Chandra X-ray Observatory (CXO) in July 2000, and the resultant spectrum was by far the clearest cometary X-ray spectrum to date. It is the aim of this paper to see if the spectrum can be reproduced directly from atomic data using the ADAS (the Atomic Data and Analysis Structure) system to try and reproduce the conditions of the comet's interaction with the solar wind. This is accomplished by calculating the atomic rate coefficients for a simplified cometary plasma over a range of likely temperatures and combining the resultant predicted emission line spectrum with a pure bremsstrahlung continuum. The outcome is a model spectrum which can be compared to that Chandra observation.

The goal of this study is to determine and produce the thermal properties of solid-state phase-change materials appropriate for solar system space heating storage (transition temperatures in the 40 to 60°C range). A major effort is directed toward improving the overall heat storage characteristics of solid-state phase-change materials by increasing the materials’ thermal conductivities. The solid-state phase change materials focused on in this study are neopentyl glycol and pentaglycerine. The results from testing various materials are reported as thermophysical property values. The results from a constant heat flux, thermal storage charging experiment are reported for both the solid-state materials and the enhanced conductivity materials. The storage system modeled is a tube bank with hot fluid inside the tubes transferring heat to the solid-solid phase-change material outside the tubes.