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

Abstract

We present the results of Spitzer spectroscopic observations of two highly obscured massive X-ray binaries: IGR J16318-4848 and GX 301-2. 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 H I recombination lines, many forbidden lines of low ionization potential, 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 photodissociated regions. It appears difficult to attribute the total optical extinction of both sources to the hot and warm dust, which suggests that there could be an otherwise observable colder dust component responsible for 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 for investigating highly obscured X-ray binaries.