Warm-hot gas in and around the Milky Way: Detection and implications of OVII absorption toward LMC X-3

X-ray absorption lines of highly-ionized species such as O VII at about zero redshift have been firmly detected in the spectra of several active galactic nuclei. However, the location of the absorbing gas remains a subject of debate. To separate the Galactic and extragalactic contributions to the absorption, we have obtained Chandra LETG-HRC and Far Ultraviolet Spectroscopic Explorer observations of the black hole X-ray binary LMC X–3. We clearly detect the O VII Kα absorption line with an equivalent width of 20(14, 26) mÅ (90% confidence range). The Ne IX Kα absorption line is also detected, albeit marginally. A joint analysis of these lines, together with the non-detection of the O VII Kβ and O VIII Kα lines, gives the temperature, velocity dispersion, and hot oxygen column density as 1.3(0.7, 1.8)× 10 K, 79(62, 132) km s, and 1.9(1.2, 3.2)×10 cm, assuming a collisional ionization equilibrium of the X-ray-absorbing gas and a Galactic interstellar Ne/O number ratio of 0.18. The X-ray data allow us to place a 95% confidence lower limit to the Ne/O ratio as 0.14, but the upper limit is not meaningfully constrained. The O VII line centroid and its relative shift from the Galactic O I Kα absorption line, detected in the same observations, are inconsistent with the systemic velocity of LMC X–3 (+310 km s). The far-UV spectrum shows O VI absorption at Galactic velocities, but no O VI absorption is detected at the LMC velocity at > 3σ significance. The measured Galactic O VI column density is higher than the value predicted from the O VIIbearing gas, indicating multi-phase absorption. Both the nonthermal broadening and the decreasing scale height with the increasing ionization state further suggest an origin of the highly-ionized gas in a supernova-driven galactic fountain. In addition, we estimate the warm and hot electron column densities from our detected O II Kα line in the LMC X–3 X-ray spectra and from the dispersion measure of a pulsar in the LMC vicinity. We then infer the O/H ratio of the gas to be & 8 × 10, consistent with the chemically-enriched galactic fountain scenario. We conclude that the Galactic hot interstellar medium should in general substantially contribute to zero-redshift X-ray absorption lines in extragalactic sources. Subject headings: galaxies: intergalactic medium— ISM: kinematics and dynamics — stars: individual (LMC X–3) — ultraviolet: ISM — X-Rays: ISM