• Gatuzz, E.
• García, J.

Abstract

With the advent of X-ray space telescopes (Chandra, XMM-Newton, Suzaku) at the turn of the century, high-resolution spectroscopy has become a powerful technique for studying in amazing detail a large variety of astronomical sources [1]. While many of them are indeed exotic, e.g. galaxy clusters, supermassive black holes, and supernova remnants, others are somewhat more familiar such as the interstellar medium (ISM) of our galaxy. In any case, the iron K and L lines and edges have proven to be the most coveted spectral items considering their plasma diagnostic capabilities, and because Fe abundance variations provide measures of cosmic evolution, galactic gradients, and gas-to-grain fractions. However, the reliability of such plasma diagnostics is currently the subject of heated controversies, three of which are briefly reviewed in the present paper. Toccata - 3C/3D line ratio in Fe XVII. Emission L-shell lines (n = 3-2) of this species are dominant features in the spectra of hot (1-7 MK) bodies, but their diagnostic potential has been limited by a weaker-than-predicted resonance line. Although it was earlier assumed that the collisional data were faulty, a persistent discrepancy in the experimental and theoretical 3C/3D f-value ratios has given rise an entrenched polemic [2,3]. Nonlinear dynamical modeling and nonequilibrium plasma effects have also been invoked [4, 5]. Fugue - ISM Fe abundance. Modeling of the Fe L edge in the ISM absorption spectra towards low-mass X-ray binaries (LMXBs) would in principle allow the determination of the gas-to-solid abundance ratio. Fe is expected to be mostly in grains, but the composition and morphology of these compounds is still unclear [6,7]: is it in amorphous silicates or metallic form? We have analyzed high-resolution Chandra spectra of the available LMXBs to identify absorption features in the Fe L-edge region; in addition to metallic iron, we found O VII lines but no evidence of absorption features due to atomic iron. Coda - Fe abundance in black-hole accretion disks. Fe K emission lines reflected from the innermost regions are broadened and skewed by Doppler and gravitational effects, providing information about the black-hole spin. However, spectral models have been seriously questioned as supersolar Fe abundances are required, which is believed to be caused by their inapplicability at very high densities (> 1018 cm-3) where the atomic data are believed to be modified by plasma effects, e.g. continuum lowering in the photoionization cross sections [8].

Topics

• impedance spectroscopy
• compound
• cluster
• amorphous
• grain
• iron