The Effects of Low‐Temperature Dielectronic Recombination on the Relative Populations of the Fe M‐Shell States

Abstract

We examine the effects of low-temperature, or Án ¼ 0, dielectronic recombination (DR) on the ionization balance of the Fe M shell (Fe ix-Fe xvi). Since Án ¼ 0 rates are not available for these ions, we have derived estimates based on the existing rates for the first four ionization states of the CNO sequence and newly calculated rates for L-shell ions of third-row elements and Fe. For a range of ionization parameter and column density applicable to the intrinsic absorbers detected in ASCA, Chandra, and XMM-Newton observations of Seyfert galaxies, we generated two grids of photoionization models, with and without DR. The results show that the ionization parameter at which the population of an Fe M-shell ion peaks can increase in some cases by a factor of more than 2 when these rates are included. More importantly, there are dramatic changes in the range in ionization parameter over which individual M-shell ions contain significant fractions of the total Fe (e.g., >10%) in the plasma. These results may explain the mismatch between the range of Fe ionization states detected in the X-ray spectra of Seyfert galaxies, identified by the energies of the M-shell unresolved transition array, and those predicted by photoionization models of the X-ray absorbers that reproduce lines of second-and third-row elements. The results suggest that care should be taken in using third-and fourth-row ions to constrain the physical conditions in photoionized X-ray plasmas until accurate DR rates are available. This underscores the importance of atomic physics in interpreting astronomical spectroscopy.

Dielectronic recombination into Mg-like ions

“…Many theoretical and experimental studies on DR have been done for K-shell and L-shell ions, but not so many for M-shell ions. Recently Netzer [1] and Kraemer et al [2] suggested importance of M-shell Fe ions and the data needs for astrophysical plasmas such as photoionized plasma of active galactic nuclei. The solar observational satellite HINODE was launched in 2006 and has EUV Imaging Spectrometer (EIS) to observe the sun with EUV spectral lines of Fe M-shell ions [3].Plasma diagnostics by using spectral lines of Fe ions will provide information on the active region and the transition region of the sun to study the coronal heating problem.…”

Dielectronic Recombination Rate Coefficients of Na‐like Ions from Mg ii to Zn xx Forming Mg‐like Systems

“…Much less attention has been given to M-shell ions. Recently, Kraemer et al (2004) investigated the effects of ÁN ¼ 0 DR on the relative populations of Fe M-shell ions in photoionized plasmas. They found that the derived ion column densities can change by up to a factor of few when such DR rate coefficients are included.…”

“…They found that the derived ion column densities can change by up to a factor of few when such DR rate coefficients are included. Without reliable calculations of low-temperature DR for these ions, Kraemer et al (2004) estimated the rate coefficients by scaling the results of L-shell ions given in Gu (2003a). Such scaling, however, is questionable and is only expected to be accurate to within a factor of few.…”

Ionization and Recombination with Electrons: Laboratory Measurements and Observational Consequences

“…However, these models over-predict the average Fe charge stage. This is believe to be due to the theoretical underestimate of the relevant low temperature DR rate coefficients for Mshell Fe [18,19].…”