On the detectability of CO molecules in the Interstellar Medium via X-ray spectroscopy

We present a study of the detectability of CO molecules in the Galactic interstellar medium using high-resolution X-ray spectra obtained with the XMM-Newton Reflection Grating Spectrometer. We analyzed 10 bright low mass X-ray binaries (LMXBs) to study the CO contribution in their line-of-sights. A total of 25 observations were fitted with the ISMabs X-ray absorption model which includes photoabsorption cross-sections for O I, O II, O III and CO. We performed a Monte-Carlo (MC) simulation analysis of the goodness of fit in order to estimate the significance of the CO detection. We determine that the statistical analysis prevents a significant detection of CO molecular X-ray absorption features, except for the lines-of-sight toward XTE~J1718-330 and 4U~1636-53. In the case of XTE~J1817-330, this is the first report of the presence of CO along its line-of-sight. Our results reinforce the conclusion that molecules have a minor contribution to the absorption features in the O K-edge spectral region. We estimate a CO column density lower limit to perform a significant detection with XMM-Newton of N(CO) > 6x10^{16} cm-2 for typical exposure timesComment: 7 pages, 5 figure

On the discrepancy between the X-ray and UV absorption measurements of O VI in the local ISM

The total amount of O VI present in the interstellar medium (ISM) obtained via absorption measurements in UV and X-ray spectra is currently in disagreement, with the latter being significantly larger (by a factor of 10 or more) than the former. Previous works have proposed that the blend of the O VIKα line (22.032 Å) with the O II Kβ-L12 line (22.04 Å) could account for the stronger absorption observed in the X-ray spectra. Here, we present a detailed study of the oxygen absorption in the local ISM, implementing our new model IGMabs which includes photoabsorption cross-sections of highly ionized species of abundant elements as well as turbulence broadening. By analysing high-resolution Chandra spectra of 13 low-mass X-ray binaries (LMXBs) and 29 extragalactic sources, we have estimated the column densities of O I – O III and from O VI – O VIII along multiple line-of-sights. We find that in most cases the O II Kβ-L12 line accounts for <30 per cent of the total O VI K α + O II K β. We conclude that the amount of O II predicted by our model is still insufficient to explain the discrepancy between X-ray and UV measurements of O VI column densities

On the discrepancy between the X-ray and UV absorption measurements of OVI in the local ISM

The total amount of OVI present in the interstellar medium (ISM) obtained via absorption measurements in UV and X-ray spectra is currently in disagreement, with the latter being significantly larger (by a factor of 10 or more) than the former. Previous works have proposed that the blend of the OVI Ka line (22.032 A) with the OII Kb-L12 line (22.04 A) could account for the stronger absorption observed in the X-ray spectra. Here we present a detailed study of the oxygen absorption in the local ISM, implementing our new model IGMabs, which includes photoabsorption cross-sections of highly ionized species of abundant elements as well as turbulence broadening. By analyzing high-resolution Chandra spectra of 13 low mass X-ray binaries (LMXBs) and 29 extragalactic sources, we have estimated the column densities of OI-OIII and from OVI-OVIII along multiple line-of-sights. We find that in most cases the OII Kb-L12 line accounts for < 30% of the total OVI Ka + OII Kb. We conclude that the amount of OII predicted by our model is still insufficient to explain the discrepancy between X-ray and UV measurements of OVI column densities.Comment: 5 pages, 3 figure

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

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

Carbon X-ray absorption in the local ISM: Fingerprintsin X-ray Novae spectra

We present a study of the C K-edge using high-resolution LETGS Chandraspectra of four novae during their super-soft-source (SSS) phase. We identified absorption lines due to C II Kα, C III Kα, and C III Kβ resonances. We used these astronomical observations to perform a benchmarking of the atomic data, which involves wavelength shifts of the resonances and photoionization cross-sections. We used improved atomic data to estimate the C II and C III column densities. The absence of physical shifts for the absorption lines, the consistence of the column densities between multiple observations, and the high temperature required for the SSS nova atmosphere modelling support our conclusion about an interstellar medium origin of the respective absorption lines. Assuming a collisional ionization equilibrium plasma the maximum temperature derived from the ratio of C II/C III column densities of the absorbers correspond to T_(max) < 3.05 × 10^4 K

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

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 (EXTRAS) which provides equivalent N_H values from X-ray spectral fits of observations within the XMM-Newton Data Release. EXTRAS include multiple fits for each source, allowing an accurate determination of the equivalent column densities, which depends on the continuum modelling of the spectra. A cross-correlation between the EXTRAS catalogue and the first Gaia Data Release was performed in order to obtain accurate parallax and distance measurements. We use a Bayesian method explained in Rezaei Kh. et al. (2017) 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 could not have been modelled 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 catalogues. However, given the systematic uncertainties connected to the source identification and to the dependence of N_H on the spectral model, the present maps should be considered qualitatively at this point

Oxygen, neon, and iron X-ray absorption in the local interstellar medium

Aims. We present a detailed study of X-ray absorption in the local interstellar medium by analyzing the X-ray spectra of 24 galactic sources obtained with the Chandra High Energy Transmission Grating Spectrometer and the XMM-Newton Reflection Grating Spectrometer. Methods. By modeling the continuum with a simple broken power-law and by implementing the new ISMab