Detection of Highly Ionized O and Ne Absorption Lines in the X-ray Spectrum of 4u1820–303 in the Globular

We searched for absorption lines of highly ionized O and Ne in the energy spectra of two Low-mass Xray binaries, 4U1820–303 in the globular cluster NGC6624 and Cyg X-2, observed with the Chandra LETG, and detected O VII, O VIII and Ne IX absorption lines for 4U1820–303. The equivalent width of the O VII Kα line was 1.19 +0.47 −0.30 eV (90 % errors) and the significance was 6.5 σ. Absorption lines were not detected for Cyg X-2 with a 90 % upper limit on the equivalent width of 1.06 eV for O VII Kα. The intrinsic line width was not resolved and an upper limit corresponding to a velocity dispersion of b = 420 km s was obtained for the O VII Kα line of 4U1820–303. The ion column densities were estimated from the curve of growth analysis assuming several different values of b. The absorption lines observed in 4U1820–303 are likely due to hot interstellar medium, because O will be fully photo-ionized if the absorbing column is located close to the binary system. The velocity dispersion is restricted to b = 200 − 420 km s from consistency between O VII Kα and Kβ lines, Ne/O abundance ratio, and H column density. The average temperature and the O VII density are respectively estimated to be log(T [K]) = 6.2−6.3 and nOVII = (0.7−2.3)×10−6 cm. The difference of O VII column densities for the two sources may be connected to the enhancement of the soft X-ray background (SXB) towards the Galactic bulge region. Using the polytrope model of hot gas to account for the SXB we corrected for the density gradient and estimated the midplane O VII density at the solar neighborhood. The scale height of hot gas is then estimated using the AGN absorption lines. It is suggested that a significant portion of both the AGN absorption lines and the high-latitude SXB emission lines can be explained by the hot gas in our Galaxy. Subject headings: X-rays:individual(4U1820–303, NGC 6624, Cyg X-2) — X-rays:ISM