In:
The Astrophysical Journal, American Astronomical Society, Vol. 965, No. 2 ( 2024-04-01), p. 172-
Abstract:
We present a high-resolution spectral study of Fe L -shell extinction by the diffuse interstellar medium (ISM) in the direction of the X-ray binaries Cygnus X-1 and GX 339–4, using the XMM-Newton reflection grating spectrometer. The majority of interstellar Fe is suspected to condense into dust grains in the diffuse ISM, but the compounds formed from this process are unknown. Here, we use the laboratory cross sections from Kortright & Kim (2000) and Lee et al. (2005) to model the absorption and scattering profiles of metallic Fe, and the crystalline compounds fayalite (Fe 2 SiO 4 ), ferrous sulfate (FeSO 4 ), hematite ( α -Fe 2 O 3 ), and lepidocrocite ( γ -FeOOH), which have oxidation states ranging from Fe 0 to Fe 3+ . We find that the observed Fe L -shell features are systematically offset in energy from the laboratory measurements. An examination of over two dozen published measurements of Fe L -shell absorption finds a 1–2 eV scatter in energy positions of the L -shell features. Motivated by this, we fit for the best energy-scale shift simultaneously with the fine structure of the Fe L -shell extinction cross sections. Hematite and lepidocrocite provide the best fits (≈ + 1.1 eV shift), followed by fayalite (≈ + 1.8 eV shift). However, fayalite is disfavored, based on the implied abundances and knowledge of ISM silicates gained by infrared astronomical observations and meteoritic studies. We conclude that iron oxides in the Fe 3+ oxidation state are good candidates for Fe-bearing dust. To verify this, new absolute photoabsorption measurements are needed on an energy scale accurate to better than 0.2 eV.
Type of Medium:
Online Resource
ISSN:
0004-637X
,
1538-4357
DOI:
10.3847/1538-4357/ad2939
Language:
Unknown
Publisher:
American Astronomical Society
Publication Date:
2024
detail.hit.zdb_id:
2960-9
detail.hit.zdb_id:
1473835-1
SSG:
16,12
