Nancy S. Brickhouse

Context. Collisionally ionized plasmas produce X-ray spectra that depend on electron energy and atomic processes, requiring accurate atomic rate coefficients for modeling. While most data are theoretical, experimental measurements – such as those for Fe XXV – are critical for providing important benchmarks and quantifying uncertainties, thereby improving astrophysical spectral analysis, especially with high-resolution observatories like XRISM. Aims. This study aimed to experimentally measure the electron impact ionization (EII) cross section of Fe XXV, a key ion in astrophysical plasma diagnostics, and to evaluate the impact of these measurements on the interpretation of X-ray spectra from high-temperature plasma environments. Methods. The EII cross sections were measured using X-ray spectroscopy with an electron beam ion trap (EBIT) at various electron beam energies. The intensity ratios of the w resonance line in Fe XXV and the Lyα_1,2 lines in Fe XXVI were used to derive the cross sections. Level-resolved population calculations were performed using the NOMAD collisional-radiative code to account for processes affecting the emission ratios and ionization balance in the non-Maxwellian EBIT plasma. Results. The measured EII cross sections show good agreement with relativistic convergent close-coupling calculations. The uncertainties in the measurements, ranging from 8% to 12%, are discussed in terms of their impact on the accuracy of temperature diagnostics for astrophysical plasma environments.

Electron beam ion traps (EBITs) are compact devices optimized for producing ions in high charge states for spectroscopic studies or as extracted beams. Key characteristics, such as current density and electron-ion overlap, govern ionization and excitation rates. Using visible and X-ray imaging of emissions from highly charged ions, the spatial distributions of the electron beam and ion cloud in the Smithsonian Astrophysical Observatory (SAO) EBIT were measured, enabling the determination of the effective electron density. The nominal electron beam full width at half maximum (FWHM) was determined to be 92.0 ± 9.7 μm, while the ion cloud FWHM was 410.5 ± 16.5 μm, indicating an effective electron density roughly an order of magnitude lower than determined geometrically. The effects of magnetic fields on the electron beam size were also investigated, demonstrating sensitivity to the focusing magnet and bucking coil currents. These findings emphasize the need for simultaneous measurement of the effective electron density to improve the accuracy of density-sensitive studies in EBIT systems.

Charge-exchange recombination with neutral atoms significantly influences the ionization balance in electron beam ion traps (EBIT) because its cross section is relatively large compared to cross sections of electron collision induced processes. Modeling the highly charged ion cloud requires the estimate of operating parameters, such as electron beam energy and density, the density of neutral atoms, and the relative velocities of collision partners. Uncertainty in the charge-exchange cross section can dominate the overall uncertainty in EBIT experiments, especially when it compounds with the uncertainties of experimental parameters that are difficult to determine. We present measured and simulated spectra of few-electron Fe ions, where we used a single charge-exchange factor to reduce the number of free parameters in the model. The deduction of the charge-exchange factor from the ratio of Li-like and He-like features allows for predicting the intensity of H-like lines in the spectra.

In this report we describe the design and operation of the electron beam ion trap (EBIT) at the Smithsonian Astrophysical Observatory (SAO). We also provide an overview of recent upgrades that have led to improved system stability and greater user control, increasing the scope of possible experiments. Observations of X-ray emission from background elements were made after the system upgrades. The evolution of the spectrum, produced at beam energies ranging from 1285 eV to 3095 eV, allowed us to identify emission from multiple charge states and from key processes, such as dielectronic recombination, in Ba and Si ions. Emission from these background elements was easily removed by periodically dumping the trap every 2 s or less.

Arcus is a high-resolution soft X-ray and far-ultraviolet spectroscopy mission submitted to the National Aeronautics and Space Administration’s inaugural Astrophysics Probe solicitation. Arcus makes simultaneous observations in these two critical wavelength regimes to address a broad range of science questions highlighted by the 2020 Astronomy and Astrophysics Decadal Survey, from the temperature and composition of the missing baryons in the intergalactic medium to the evolution of stars and their influence on orbiting planets. We present the science motivation for and performance of the Arcus ultraviolet spectrograph (UVS). UVS comprises a 60-cm, off-axis Cassegrain telescope feeding an imaging spectrograph operating over the 970- to 1580-Å bandpass. The instrument employs two interchangeable diffraction gratings to provide medium-resolution spectroscopy (R>20,000 in two grating modes centered at ∼1110 and 1390 Å). The spectra are recorded on an open-face, photon-counting microchannel plate detector. The instrument design achieves an end-to-end sensitivity >10 times that of the Far-Ultraviolet Spectroscopic Explorer over the key 1020- to 1150-Å range and offers arcsecond-level angular resolution spectral imaging over a 6-arcminute-long slit for observations of extended sources. We describe the example science investigations for far-ultraviolet spectroscopy on Arcus, the resultant instrument design and predicted performance, and simulated data from potential General Observer programs with Arcus.

Accurate atomic data and plasma models are essential for interpreting the upcoming high-quality spectra from missions like XRISM and Athena. Estimating physical quantities, like temperature, abundance, turbulence, and the resonance scattering factor, is highly dependent on the underlying atomic data. We use the AtomDB tool variableapec to estimate the impact of atomic data uncertainties in Einstein A coefficients, collisional rate coefficients, and the ionization and recombination rates of H-, He-, and Li-like iron in modeling the spectrum of Perseus observed by Hitomi. The best-fit temperatures, abundances, resonance scattering factors, and turbulence parameters including atomic data uncertainties vary by approximately 17%, 35%, 30%, and 3%, respectively, from the best-fit temperatures, abundances, RS factors, and turbulence parameters estimated without atomic data uncertainties. These indicate that approximately 32%, 35%, and 25% of the best-fit temperatures, abundances, and resonance scattering factors when including uncertainties lie outside the 3σ error regions of their corresponding best-fit values computed with zero atomic data errors. Expanding the energy range to 1.8–20.0 keV shows less variability, with 26% of the abundances and 22% of the resonance scattering factors lying outside the 3σ errors of the best-fit values. We also studied correlations between physical parameters and atomic rate uncertainties to identify key atomic quantities requiring precise lab measurements. We report negative correlations between the best-fit temperatures and the z (1s2s ³S₁ → 1s²) collisional rate coefficients, abundances and y (1s2p ³P₁ → 1s²) collisional rate coefficients, and abundances and z collisional rate coefficients, and a positive correlation between the resonance scattering factors and the w (1s2p ¹P₁ → 1s²) collisional rate coefficients.

The detailed design and operation of the Smithsonian Astrophysical Observatory's EBIT are described for the first time, including recent design upgrades that have led to improved system stability and greater user control, increasing the scope of possible experiments. Measurements of emission from highly charged Ar were taken to determine the spatial distribution of the ion cloud and electron beam. An optical setup consisting of two lenses, a narrow band filter, and a CCD camera was used to image visible light, while an X-ray pinhole and CCD camera were used to image X-rays. Measurements were used to estimate an effective electron density of 1.77 x 10$^{10}$ cm$^{-3}$. Additionally, observations of X-ray emission from background EBIT gases were measured with a Silicon Lithium detector. Measurements indicate the presence of Ba and Si, which are both easily removed by dumping the trap every 2 s or less.

In this paper, we present the simple, one-step, self-consistent, and fast resonance scattering model rsapec based on the AtomDB database. This model can be used as an alternative to the commonly used APEC model for fitting X-ray spectra with optically thick lines. The current model is intended, in general, for verifying the presence of the effect and for spectral modeling of galaxy clusters and elliptical galaxies under applicable assumptions. We test rsapec to derive the line suppression in the elliptical galaxy NGC 4636 and the Perseus cluster of galaxies and obtain resonance suppressions of ∼1.24 and ∼1.30, respectively.

In this paper, we present a simple, one-step, self-consistent, and fast resonance scattering model rsapec based on the AtomDB database. This model can be used as an alternative to the commonly used APEC model for fitting such X-ray spectra with optically thick lines. The current model is intended, in general, for verifying the presence of the effect and for spectral modeling of galaxy clusters and elliptical galaxies under applicable assumptions. We test rsapec to derive the line suppression in the elliptical galaxy NGC 4636 and the Perseus cluster of galaxies and obtain resonance suppression of ~ 1.24 and ~ 1.30, respectively.