The Rich Mid-Infrared Environments of Two Highly-Obscured X-ray Binaries: Spitzer Observations of IGR J16318-4848 and GX 301-2

TL;DR

This study uses Spitzer mid-infrared spectroscopy to explore the complex environments of two highly-obscured X-ray binaries, revealing rich dust and gas features that suggest a new evolutionary phase for such systems.

Contribution

First detailed mid-infrared spectral analysis of these obscured X-ray binaries, uncovering their complex dust and gas environments and potential evolutionary implications.

Findings

Detection of hot and warm dust components with silicate absorption

Presence of multiple emission lines indicating ionized and molecular regions

Similarity to Luminous Blue Variables suggesting a new evolutionary phase

Abstract

We present the results of Spitzer mid-infrared spectroscopic observations of two highly-obscured massive X-ray binaries: IGR J16318-4848 and GX301-2. Our observations reveal for the first time the extremely rich mid-infrared environments of this type of source, including multiple continuum emission components (a hot component with T > 700 K and a warm component with T ~ 180 K) with apparent silicate absorption features, numerous HI recombination lines, many forbidden ionic lines of low ionization potentials, and pure rotational H2 lines. This indicates that both sources have hot and warm circumstellar dust, ionized stellar winds, extended low-density ionized regions, and photo-dissociated regions. It appears difficult to attribute the total optical extinction of both sources to the hot and warm dust components, which suggests that there could be an otherwise observable colder dust component responsible for the most of the optical extinction and silicate absorption features. The observed mid-infrared spectra are similar to those from Luminous Blue Variables, indicating that the highly-obscured massive X-ray binaries may represent a previously unknown evolutionary phase of X-ray binaries with early-type optical companions. Our results highlight the importance and utility of mid-infrared spectroscopy to investigate highly-obscured X-ray binaries.