Detection of Highly Ionized O and Ne Absorption Lines in the X‐Ray Spectrum of 4U 1820−303 in the Globular Cluster NGC 6624

We searched for absorption lines of highly ionized O and Ne in the energyspectra of two Low-mass X-ray binaries, 4U1820-303 in the globular clusterNGC6624 and Cyg X-2, observed with the Chandra LETG, and detected O VII, O VIIIand Ne IX absorption lines for 4U1820-303. The equivalent width of the O VII Kalpha line was 1.19 +0.47/-0.30 eV (90 % errors) and the significance was 6.5sigma. Absorption lines were not detected for Cyg X-2 with a 90 % upper limiton the equivalent width of 1.06 eV for O VII K alpha. The absorption linesobserved in 4U1820-303 are likely due to hot interstellar medium, because Owill be fully photo-ionized if the absorbing column is located close to thebinary system. The velocity dispersion is restricted to b = 200 - 420 km/s fromconsistency between O VII K alpha and K beta lines, Ne/O abundance ratio, and Hcolumn density. The average temperature and the O VII density are respectivelyestimated to be log(T[K]) = 6.2 - 6.3 and n(OVII) = (0.7 - 2.3) x 10^{-6}cm^{-3}. The difference of O VII column densities for the two sources may beconnected to the enhancement of the soft X-ray background (SXB) towards theGalactic bulge region. Using the polytrope model of hot gas to account for theSXB we corrected for the density gradient and estimated the midplane O VIIdensity at the solar neighborhood. The scale height of hot gas is thenestimated using the AGN absorption lines. It is suggested that a significantportion of both the AGN absorption lines and the high-latitude SXB emissionlines can be explained by the hot gas in our Galaxy.Comment: Accepted for publication in ApJ. 7 pages, 9 eps figure