ON THE ORIGIN OF HIGHLY IONIZED X-RAY ABSORBERS DETECTED IN THE GALACTIC X-RAY BINARIES

Abstract

X-ray observations of the Galactic X-ray binaries (XRB) revealed numerous highly ionized metal absorption lines. However, it is unclear whether such lines are produced by the hot interstellar medium (ISM) or the circumstellar medium (CSM) intrinsic to the binaries. Here we present a Chandra X-ray absorption line study of 28 observations on 12 X-ray binaries, with a focus on the NeIX and FeXVII lines. We report the first detections of these lines in a significant amount of observations. We do not find significant dependence of the line equivalent width on the distance of the XRBs, but we do see weak dependence on the source X-ray luminosity. We also find two out of twelve selected targets show strong temporal variation of the NeIX absorbers. While the line ratio between the two ion species suggests a temperature consistent with the previous predictions of the ISM, comparing with two theoretical models of the ISM shows the observed column densities are significantly higher than predictions. On the other hand, photoionzation by the XRBs provides reasonably good fit to the data. Our findings suggest that a significant fraction of these X-ray absorbers may originate in the hot gas intrinsic to the X-ray binaries, and that the ISM makes small, if not negligible, contribution. We briefly discuss the implications to the study of the Milky Way hot gas content.