Interstellar dust along the line of sight of GX 3+1 (1909.00652v1)

Abstract: Studying absorption and scattering of X-ray radiation by interstellar dust grains allows us to access the physical and chemical properties of cosmic grains even in the densest regions of the Galaxy. We aim at characterising the dust silicate population which presents clear absorption features in the energy band covered by the Chandra X-ray Observatory. Through these absorption features, in principle, it is possible to infer the size distribution, composition, and structure of silicate in the interstellar medium. In particular, in this work, we investigate the magnesium and silicon K-edges. By using newly acquired synchrotron measurements, we build X-ray extinction models for fifteen dust candidates. These models, adapted for astrophysical analysis, and implemented in the Spex spectral fitting program, are used to reproduce the dust absorption features observed in the spectrum of the bright low mass X-ray binary GX 3+1 which is used as a background source. With the simultaneous analysis of the two edges we test two different size distributions of dust: one corresponding to the standard Mathis-Rumpl-Nordsieck model and one considering larger grains ($n(a) \propto a_i^{-3.5}$ with $0.005<a_1<0.25$ and $0.05<a_2<0.5$, respectively, with $a$ the grain size). These distributions may be representative of the complex Galactic region towards this source. We find that up to $70\%$ of dust is constituted by amorphous olivine. We discuss the crystallinity of the cosmic dust found along this line of sight. Both magnesium and silicon are highly depleted into dust ($\delta_{Z} = 0.89\ \rm{and}\ 0.94$, respectively) while their total abundance does not depart from solar values.

• The paper utilizes X-ray absorption spectroscopy to characterize interstellar dust toward GX 3+1, identifying amorphous olivine as a major component.
• The study reports high depletion values for Mg (0.89) and Si (0.94) and finds that including large grains significantly improves dust model accuracy.
• This research demonstrates the utility of X-ray spectroscopy for studying dust in dense environments, impacting models of dust composition and evolution.

Overview of "Interstellar dust along the line of sight of GX 3+1"

This paper tackles the complex issue of characterizing interstellar dust in the line of sight of the low-mass X-ray binary GX 3+1. By leveraging X-ray absorption spectroscopy, the paper focuses on the Mg and Si K-edges, which are particularly insightful for understanding silicate dust—notably underrepresented in conventional infrared studies due to X-ray's enhanced sensitivity to dense regions of the ISM.

Objectives and Methodology

The authors aim to better understand the composition, size distribution, and crystallinity of silicate dust. This is achieved through synchrotron-based extinction models for fifteen dust candidates that are integrated within the Spex spectral fitting program. These models are used to perform simultaneous analysis on Chandra X-ray observations, specifically focusing on the magnesium and silicon edges. The researchers explore two dust size distributions: the standard Mathis-Rumpl-Nordsieck (MRN) model and a large-grain size distribution (LMRN). This dual approach helps them test the fit against two potential grain size assumptions.

Key Findings

The analysis reveals that a substantial portion of the dust—up to 70%—is composed of amorphous olivine. Additionally, the paper finds high depletion values for magnesium and silicon, with depletion indices of 0.89 and 0.94, respectively. Despite this, the total elemental abundances do not significantly diverge from solar values. The inclusion of a mixed grain size model, comprising both MRN and LMRN distributions, yields the most accurate fit, highlighting the necessity of considering larger grains to accurately model interstellar dust.

Implications

This research demonstrates the utility of X-ray spectroscopy in probing interstellar dust, particularly in dense environments where traditional methods fall short. The findings may have implications for understanding dust composition across various galactic environments, potentially influencing models of dust grain evolution and formation processes in the galaxy. Moreover, the simultaneous consideration of both Mg and Si edges provides a more comprehensive picture that could be pivotal for future X-ray astronomy missions.

Future Prospects

The paper opens the door for further research into the use of X-ray absorption features to paper cosmic dust, particularly in challenging environments. The methodologies employed, including enhanced synchrotron-based modeling, could be further refined and expanded in future work. Additionally, resolving the residuals in the Si K-edge region and exploring the potential impact of instrument calibration or physical features on these measurements will be critical in advancing the field.

Overall, this paper provides a significant step forward in understanding the character and distribution of interstellar dust, particularly in the complex region toward the GX 3+1 binary system. Through careful modeling and analysis, it yields valuable insights that may inform both theoretical and observational astrophysics.