XMM-Newton Survey of Local OVII Absorption Lines in the Spectra of Galactic X-ray Sources

TL;DR

This study surveys OVII absorption lines in 33 Galactic X-ray binaries using XMM-Newton, revealing hot gas distribution in the Galaxy with new detections and analyzing its properties and correlations.

Contribution

First comprehensive survey of OVII lines in Galactic XRBs, identifying new detections and analyzing hot gas distribution and properties in the Galaxy.

Findings

OVII absorption detected in 16 out of 33 XRBs

No strong correlation between OVII EWs and N_H or Galactic coordinates

Hot gas appears uniformly distributed regardless of direction

Abstract

The detection of highly ionized metal absorption lines in the X-ray spectra of the Galactic X-ray binaries (XRBs) implies the distribution of hot gas along the sightline toward the background sources. However, the origin of this hot gas is still unclear: it can arise in the hot interstellar medium (ISM), or is intrinsic to the XRBs. In this paper, we present an XMM-Newton survey of the OVII absorption lines in the spectra of Galactic XRBs. A total of 33 XRBs were selected, with 29 low mass XRBs and 4 high mass XRBs. At more than 3$\sigma$ threshold, OVII absorption line was detected in 16 targets, among which 4 were newly discovered in this work. The average line equivalent width (EW) is centered around $\sim$ 20 m\AA. Additionally we do not find strong correlations between the OVII EWs and the Galactic neutral absorption N$_{\rm H}$, the Galactic coordinates or the distance of background targets. Such non-correlation may suggest the contamination of the circumstellar material, or the lack of constrains on the line Doppler-b parameter. We also find regardless the direction of the XRBs, the OVII absorption lines were always detected when the flux of the background XRBs reach a certain level, suggesting a uniform distribution of this hot gas. We estimate a ratio of 0.004 --- 0.4 between hot and neutral phase of the interstellar medium. This is the second paper in the series following Fang et al. 2015, in which we focused on the local OVII absorption lines detected in the background AGN spectra. Detailed modeling of the hot ISM distribution will be investigated in a future paper.