Herschel observations of dust around the high-mass X-ray binary GX 301-2

TL;DR

This study uses Herschel infrared observations to characterize the dust and gas environment around the obscured high-mass X-ray binary GX 301-2, revealing a likely circumstellar disk structure that informs its evolutionary stage.

Contribution

First detection of GX 301-2 at 70 and 100 micrometers, modeling of its circumstellar environment, and evidence supporting a disk-like structure around the system.

Findings

Detection at 70 and 100 micrometers for the first time

A circumstellar disk of ~8 AU is favored

The environment includes free-free emission from stellar wind

Abstract

We aim at characterising the structure of the gas and dust around the high mass X-ray binary GX 301-2, a highly obscured X-ray binary hosting a hypergiant star and a neutron star, in order to better constrain its evolution. We used Herschel PACS to observe GX 301-2 in the far infrared and completed the spectral energy distribution of the source using published data or catalogs, from the optical to the radio range (0.4 to 4x10^4 micrometer). GX 301-2 is detected for the first time at 70 and 100 micrometer. We fitted different models of circumstellar environments to the data. All tested models are statistically acceptable, and consistent with a hypergiant star at ~3 kpc. We found that the addition of a free-free emission component from the strong stellar wind is required and could dominate the far infrared flux. Through comparisons with similar systems and discussion on the estimated model parameters, we favour a disk-like circumstellar environment of ~8 AU that would enshroud the binary system. The temperature goes down to ~200 K at the edge of the disk, allowing for dust formation. This disk is probably a rimmed viscous disk with an inner rim at the temperature of the dust sublimation temperature (~1500 K). The similarities between the hypergiant GX 301-2, B[e] supergiants and the highly obscured X-ray binaries (in particular IGR J16318-4848) are strengthened. GX 301-2 might represent a transition stage in the evolution of massive stars in binary systems, connecting supergiant B[e] systems to luminous blue variables.