3D mapping of the neutral X-ray absorption in the local interstellar medium: The Gaia and XMM-Newton synergy

TL;DR

This paper creates a 3D map of hydrogen density in the local interstellar medium by combining X-ray spectral data from XMM-Newton with Gaia parallax measurements, revealing small-scale structures.

Contribution

It introduces a novel Bayesian method to map interstellar hydrogen density using combined X-ray and Gaia data, capturing small-scale structures not modeled analytically.

Findings

Map reveals small-scale density structures

Demonstrates synergy of Gaia and XMM-Newton data

Provides a proof of concept for 3D interstellar mapping

Abstract

We present a three-dimensional map of the hydrogen density distribution in the Galactic interstellar medium. The hydrogen equivalent column densities were obtained from the Exploring the X-ray Transient and variable Sky project ({\sc EXTraS}) which provides equivalent $N_{\rm H}$ values from X-ray spectral fits of observations within the {\it XMM-Newton} Data Release. {\sc EXTraS} include multiple fits for each source, allowing an accurate determination of the equivalent column densities, which depends on the continuum modeling of the spectra. A cross-correlation between the {\sc EXTraS} catalogue and the first {\it Gaia} Data Release was performed in order to obtain accurate parallax and distance measurements. We use a Bayesian method explained in \citet{rez17} in order to predict the most probable distribution of the density at any arbitrary point, even for lines of sight along which there are no initial observation. The resulting map shows small-scale density structures which can not been modeled by using analytic density profiles. In this paper we present a proof of concept of the kind of science possible with the synergy of these catalogs. However, given the systematic uncertainties connected to the source identification and to the dependence of $N_{\rm H}$ on the spectral model, the present maps should be considered qualitatively at this point.