We describe the latest release of AtomDB, version 2.0.2, a database of atomic data and a plasma modeling code with a focus on X-ray astronomy. This release includes several major updates to the fundamental atomic structure and process data held within AtomDB, incorporating new ionization balance data, state-selective recombination data, and updated collisional excitation data for many ions, including the iron L-shell ions from Fe+16 to Fe+23 and all of the hydrogen- and helium-like sequences. We also describe some of the effects that these changes have on calculated emission and diagnostic line ratios, such as changes in the temperature implied by the He-like G-ratios of up to a factor of two.

https://iopscience.iop.org/article/10.1088/0004-637X/756/2/128/pdf

Updated Atomic Data and Calculations for X-Ray Spectroscopy

We describe the latest release of AtomDB, version 2.0.2, a database of atomic data and a plasma modeling code with a focus on X-ray astronomy. This release includes several major updates to the fundamental atomic structure and process data held within AtomDB, incorporating new ionization balance data, state-selective recombination data, and updated collisional excitation data for many ions, including the iron L-shell ions from Fe$^{+16}$ to Fe$^{+23}$ and all of the hydrogen- and helium-like sequences. We also describe some of the effects that these changes have on calculated emission and diagnostic line ratios, such as changes in the temperature implied by the He-like G-ratios of up to a factor of 2.

Updated Atomic Data and Calculations for X-ray Spectroscopy

We present X-ray spectral analysis of the accreting young star TW Hydrae from a 489 ks observation using the Chandra High Energy Transmission Grating. The spectrum provides a rich set of diagnostics for electron temperature T{sub e} , electron density N{sub e} , hydrogen column density N{sub H} , relative elemental abundances, and velocities, and reveals its source in three distinct regions of the stellar atmosphere: the stellar corona, the accretion shock, and a very large extended volume of warm postshock plasma. The presence of Mg XII, Si XIII, and Si XIV emission lines in the spectrum requires coronal structures at {approx}10 MK. Lower temperature lines (e.g., from O VIII, Ne IX, and Mg XI) formed at 2.5 MK appear more consistent with emission from an accretion shock. He-like Ne IX line ratio diagnostics indicate that T{sub e} = 2.50 +- 0.25 MK and N{sub e} = 3.0 +- 0.2 x 10{sup 12} cm{sup -3} in the shock. These values agree well with standard magnetic accretion models. However, the Chandra observations significantly diverge from current model predictions for the postshock plasma. This gas is expected to cool radiatively, producing O VII as it flows into an increasingly dense stellar atmosphere.more » Surprisingly, O VII indicates N{sub e} = 5.7{sup +4.4}{sub -1.2} x 10{sup 11} cm{sup -3}, 5 times lower than N{sub e} in the accretion shock itself and {approx}7 times lower than the model prediction. We estimate that the postshock region producing O VII has roughly 300 times larger volume and 30 times more emitting mass than the shock itself. Apparently, the shocked plasma heats the surrounding stellar atmosphere to soft X-ray emitting temperatures and supplies this material to nearby large magnetic structures-which may be closed magnetic loops or open magnetic field leading to mass outflow. Our model explains the soft X-ray excess found in many accreting systems as well as the failure to observe high N{sub e} signatures in some stars. Such accretion-fed coronae may be ubiquitous in the atmospheres of accreting young stars.« less

http://iopscience.iop.org/article/10.1088/0004-637X/710/2/1835/pdf

A deep chandra x-ray spectrum of the accreting young star tw hydrae

CHIANTI—An Atomic Database for Emission Lines. XVI. Version 10, Further Extensions

We review X-ray plasma diagnostics based on the line ratios of He-like ions. Triplet/singlet line intensities can be used to determine electronic temperature and density, and were first developed for the study of the solar corona. Since the launches of the X-ray satellites Chandra and XMM-Newton, these diagnostics have been extended and used (from C v to Si xiii) for a wide variety of astrophysical plasmas such as stellar coronae, supernova remnants, solar system objects, active galactic nuclei, and X-ray binaries. Moreover, the intensities of He-like ions can be used to determine the ionization process(es) at work, as well as the distance between the X-ray plasma and the UV emission source for example in hot stars. In the near future thanks to the next generation of X-ray satellites (e.g., Astro-H and IXO), higher-Z He-like lines (e.g., iron) will be resolved, allowing plasmas with higher temperatures and densities to be probed. Moreover, the so-called satellite lines that are formed closed to parent He-like lines, will provide additional valuable diagnostics to determine electronic temperature, ionic fraction, departure from ionization equilibrium and/or from Maxwellian electron distribution.

He-like Ions as Practical Astrophysical Plasma Diagnostics: From Stellar Coronae to Active Galactic Nuclei

New laboratory measurements using an electron beam ion trap and an X-ray microcalorimeter are presented for the n = 3 to n = 2 Fe XVII emission lines in the 15-17 A range, along with new theoretical predictions for a variety of electron energy distributions. This work improves upon our earlier work on these lines by providing measurements at more electron-impact energies (seven values from 846 to 1185 eV), performing an in situ determination of the X-ray window transmission, taking steps to minimize the ion impurity concentrations, correcting the electron energies for space charge shifts, and estimating the residual electron energy uncertainties. The results for the 3C/3D and 3s/3C line ratios are generally in agreement with the closest theory to within 10%, and in agreement with previous measurements from an independent group to within 20%. Better consistency between the two experimental groups is obtained at the lowest electron energies by using theory to interpolate, taking into account the significantly different electron energy distributions. Evidence for resonance collision effects in the spectra is discussed. Renormalized values for the absolute cross sections of the 3C and 3D lines are obtained by combining previously published results and shown to be in agreement with the predictions of converged R-matrix theory. This work establishes consistency between results from independent laboratories and improves the reliability of these lines for astrophysical diagnostics. Factors that should be taken into account for accurate diagnostics are discussed, including electron energy distribution, polarization, absorption/scattering, and line blends.

Fe XVII X-ray Line Ratios for Accurate Astrophysical Plasma Diagnostics

New laboratory measurements using an Electron Beam Ion Trap (EBIT) and an x-ray microcalorimeter are presented for the n=3 to n=2 Fe XVII emission lines in the 15 A to 17 A range, along with new theoretical predictions for a variety of electron energy distributions. This work improves upon our earlier work on these lines by providing measurements at more electron impact energies (seven values from 846 to 1185 eV), performing an in situ determination of the x-ray window transmission, taking steps to minimize the ion impurity concentrations, correcting the electron energies for space charge shifts, and estimating the residual electron energy uncertainties. The results for the 3C/3D and 3s/3C line ratios are generally in agreement with the closest theory to within 10%, and in agreement with previous measurements from an independent group to within 20%. Better consistency between the two experimental groups is obtained at the lowest electron energies by using theory to interpolate, taking into account the significantly different electron energy distributions. Evidence for resonance collision effects in the spectra is discussed. Renormalized values for the absolute cross sections of the 3C and 3D lines are obtained by combining previously published results, and shown to be in agreement with the predictions of converged R-matrix theory. This work establishes consistency between results from independent laboratories and improves the reliability of these lines for astrophysical diagnostics. Factors that should be taken into account for accurate diagnostics are discussed, including electron energy distribution, polarization, absorption/scattering, and line blends.

We describe the effect that new atomic calculations, including fully-relativistic R-matrix calculations of collisional excitation rates and level-specific dielectronic and radiative recombination rates, have on line ratios from the astrophysically significant ion Ne IX. The new excitation rates systematically change some predicted Ne IX line ratios by 25% at temperatures at or below the temperature of maximum emissivity (4 × 10 6 K), while the new recombination rates lead to systematic changes at higher temperatures. The new line ratios are shown to agree with observations of Capella and � 2 CrB significantly better than older line ratios, showing that 25-30% accuracy in atomic rates is inadequate for high-resolution X-ray observations from existing spectrometers. Subject headings: atomic data, line: formation, X-rays: stars

A NEW CALCULATION OF Ne IX LINE DIAGNOSTICS

ERRATUM: A NEW CALCULATION OF Ne IX LINE DIAGNOSTICS (2009, ApJ, 700, 679)

We describe the effect that new atomic calculations, including fully-relativistic R-matrix calculations of collisional excitation rates and level-specific dielectronic and radiative recombination rates, have on line ratios from the astrophysically significant ion Ne IX. The new excitation rates systematically change some predicted Ne IX line ratios by 25% at temperatures at or below the temperature of maximum emissivity (4x10^6 K), while the new recombination rates lead to systematic changes at higher temperatures. The new line ratios are shown to agree with observations of Capella and sigma^2 CrB significantly better than older line ratios, showing that 25-30% accuracy in atomic rates is inadequate for high-resolution X-ray observations from existing spectrometers.

Guo-Xin Chen

Papers

Fe XVII X-ray Line Ratios for Accurate Astrophysical Plasma Diagnostics

Fe XVII X-ray Line Ratios for Accurate Astrophysical Plasma Diagnostics

A NEW CALCULATION OF Ne IX LINE DIAGNOSTICS

ERRATUM: A NEW CALCULATION OF Ne IX LINE DIAGNOSTICS (2009, ApJ, 700, 679)

A New Calculation of Ne IX Line Diagnostics