X‐Ray Absorption Line Spectroscopy of the Galactic Hot Interstellar Medium

We present an X-ray absorption line spectroscopic study of the large-scalehot interstellar medium (HISM) in the Galaxy. We detect Ne IX K$_\alpha$absorption lines in the {\sl Chandra} grating spectra of seven Galacticlow-mass X-ray binaries. Three of these sources also show absorption of O VIIK$_\alpha$, O VII K$_\beta$, and/or O VIII K$_\alpha$. Both centroid and widthof the lines are consistent with a Galactic HISM origin of the absorption. Byjointly fitting the multiple lines, accounting for line saturation and assumingthe collisional ionization equilibrium, we estimate the average absorbing gastemperature as $\sim 2.4\pm0.3\times10^6$ K (90% confidence errors). We furthercharacterize the spatial density distribution of the gas as ${\rm6.4_{-1.7}^{+2.4} exp(-|z|/1.2_{-0.5}^{+1.0} kpc) \times10^{-3}{\rm cm^{-3}}} $(a disk morphology) or ${\rm 6.2_{-2.1}^{+3.8} [ 1 + (R/2.3_{-1.1}^{+1.6}kpc)^2]^{-1} \times10^{-3} {\rm cm^{-3}}}$ (a sphere morphology), where $z$ and$R$ are the distances from the Galactic plane and Galactic center (GC)respectively. Since nearly all the sight-lines with significant absorptionlines detected are somewhat toward GC and at low Galactic latitudes, theseresults could be severely biased. More observations toward off-GC sight-linesand at high latitudes are urgently needed to further the study. Nevertheless,the results demonstrate the excellent potential of X-ray absorption linespectroscopy in the study of the HISM.Comment: 29 pages, 9 figures, 5 tables, ApJ accepted, 2005 June 1, v625, 2 issu