High‐Resolution X‐Ray Spectroscopy of the Interstellar Medium. II. Neon and Iron Absorption Edges

We present high-resolution spectroscopy of the neon K-shell and iron L-shellinterstellar absorption edges in nine X-ray binaries using the High EnergyTransmission Grating Spectrometer (HETGS) onboard the Chandra X-rayObservatory. We found that the iron absorption is well fit by an experimentaldetermination of the cross-section for metallic iron, although with a slightwavelength shift of ~20 mA. The neon edge region is best fit by a model thatincludes the neutral neon edge and three Gaussian absorption lines. We identifythese lines as due to the 1s-2p transitions from Ne II, Ne III, and Ne IX. Aswe found in our oxygen edge study, the theoretical predictions for neutral andlow-ionization lines all require shifts of ~20 mA to match our data. Combinedwith our earlier oxygen edge study, we find that a best fit O/Ne ratio of5.4+/-1.6, consistent with standard interstellar abundances. Our best fit Fe/Neratio of 0.20+/-0.03 is significantly lower than the interstellar value. Weattribute this difference to iron depletion into dust grains in theinterstellar medium. We make the first measurement of the neon ionizationfraction in the ISM. We find Ne II/Ne I ~ 0.3 and Ne III/Ne I ~ 0.07. Thesevalues are larger than is expected given the measured ionization ofinterstellar helium. For Ne IX, our results confirm the detection of the hotionized interstellar medium of the Galaxy.Comment: 15 pages, 12 figures, accepted for publication in the Astrophysical Journal (Vol. 648, September 10 issue