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

Abstract

We present high-resolution spectroscopy of the neon K-shell and iron L-shell interstellar absorption edges in nine X-ray binaries using the High Energy Transmission Grating Spectrometer (HETGS) onboard the Chandra X-ray Observatory. We found that the iron absorption is well fit by an experimental determination of the cross-section for metallic iron, although with a slight wavelength shift of ≈20 mÅ. The neon edge region is best fit by a model that includes the neutral neon edge and three Gaussian absorption lines. We identify these lines as due to the 1s-2p transitions from Ne II, Ne III, and Ne IX. As we found in our oxygen edge study, the theoretical predictions for neutral and lowionization lines all require shifts of ≈20 mÅ to match our data. Combined with our earlier oxygen edge study, we find that a best fit O/Ne ratio of 5.4±1.6, consistent with standard interstellar abundances. Our best fit Fe/Ne ratio of 0.20±0.03 is significantly lower than the interstellar value. We attribute this difference to iron depletion into dust grains in the interstellar medium. We make the first measurement of the neon ionization fraction in the ISM. We find Ne II/Ne I≈0.3 and Ne III/Ne I≈0.07. These values are larger than is expected given the measured ionization of interstellar helium. For Ne IX, our results confirm the detection of the hot ionized interstellar medium of the Galaxy.

Dipping in CygnusX-2 in a multi-wavelength campaign due to absorption of extended ADC emission

“…Fe L edge (above) of (2.5 ± 0.3) × 10 21 compares very well with (2.64 +1.03 −0.34 )×10 21 atom cm −2 obtained by Juett et al (2006). Thus this also suggests that no change has taken place.…”

“…We can use the HETGS data to investigate photoelectric absorption during persistent emission and during dips. We detect absorption edges for Mg K, Ne K, Fe L, and O K at 9.48 Å (1308 eV), 14.29 Å (867 eV), 17.51 Å (708 eV), and 22.89 Å (542 eV) (Juett et al 2004(Juett et al , 2006. Since the Chandra data were taken in CC graded mode, we cannot model the spectral continuum as we did for the pn-spectra, but need to defer to a local continuum modelling.…”

“…The difference between Ne and Fe values may indicate an underabundance of Fe, i.e. a low Fe/Ne ratio as suggested by Juett et al (2006). If we were to assume that the larger value in the present work from the Ne edge of (3.40 ± 0.96) × 10 21 atom cm −2 and the value of Juett et al (2004) from the oxygen edge are actually different, the implication could be that oxygen is depleted; however, Juett et al (2004) found that the O/Ne ratio in several sources was consistent with interstellar abundances; i.e.…”

High-resolution X-ray spectroscopy of the interstellar medium

“…We also show the abundances obtained by Juett et al (2006) towards the LMXB 4U 1820−303, which like GS 1826−238 is close to the Galactic center. Even more interesting is the comparison with the abundances estimated by Kaastra et al (2009) in the direction of the Crab nebula and those estimated by Yao et al (2009) towards Cyg X−2, which are two different lines of sight in our Galaxy (see the map in Fig.…”

“…9 and 10), and the column density of the warm gas is finally reduced to about 5% of the full gas column (see Table 4). Juett et al (2006). (e) Estimated through a local fit in the 27-33 Å range.…”