Stellar Wind Accretion in GX301-2: Evidence for a High-density Stream

TL;DR

This study analyzes decade-long X-ray observations of GX301-2 to understand how a high-density gas stream influences accretion, revealing that variations in luminosity are mainly driven by changes in wind and stream densities.

Contribution

The paper presents a detailed model of wind and stream dynamics in GX301-2, constraining system parameters and demonstrating the stream's significant role in accretion variability.

Findings

Stream mass-loss rate is 2.5 times that of the stellar wind.

Lower inclination models better fit observed data.

Changes in luminosity are linked to wind and stream density variations.

Abstract

The X-ray binary system GX301-2 consists of a neutron star in an eccentric orbit accreting from the massive early-type star WRAY 977. It has previously been shown that the X-ray orbital light curve is consistent with existence of a gas stream flowing out from Wray 977 in addition to its strong stellar wind. Here, X-ray monitoring observations by the Rossi X-ray Timing Explorer (RXTE)/ All-Sky-Monitor (ASM) and pointed observations by the RXTE/ Proportional Counter Array (PCA) over the past decade are analyzed. We analyze both the flux and column density dependence on orbital phase. The wind and stream dynamics are calculated for various system inclinations, companion rotation rates and wind velocities, as well as parametrized by the stream width and density. These calculations are used as inputs to determine both the expected accretion luminosity and the column density along the line-of-sight to the neutron star. The model luminosity and column density are compared to observed flux and column density vs. orbital phase, to constrain the properties of the stellar wind and the gas stream. We find that the change between bright and medium intensity levels is primarily due to decreased mass loss in the stellar wind, but the change between medium and dim intensity levels is primarily due to decreased stream density. The mass-loss rate in the stream exceeds that in the stellar wind by a factor of 2.5. The quality of the model fits is significantly better for lower inclinations, favoring a mass for WRAY 977 of 53 to 62 Msun.