Limits on Hot Galactic Halo Gas from X-Ray Absorption Lines

Although the existence of large-scale hot gaseous halos around massive disk galaxies has been theorized for a long time, there is yet very little observational evidence. We report the Chandra and XMM-Newton grating spectral detection of O VII and Ne IX Kα absorption lines along the sight line of 4U 1957+11. The line absorption is consistent with the interstellar medium in origin. Attributing these line absorptions to the hot gas associated with the Galactic disk, we search for the gaseous halo around the Milky Way by comparing this sight line with more distant ones (toward the X-ray binary LMC X-3 and the active galactic nucleus Mrk 421). We find that all the line absorptions along the LMC X-3 and Mrk 421 sight lines are attributable to the hot gas in a thick Galactic disk, as traced by the absorption lines in the spectra of 4U 1957+11 after a Galactic latitude-dependent correction. We constrain the O VII column density through the halo to be NO VII < 5 × 1015 cm−2 (95% confidence limit) and conclude that the hot gas contribution to the metal line absorptions, if existing, is negligible.

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