A changing wind collision

TL;DR

This study reports the first detection of a global change in the X-ray emission from the wind-wind collision in the HD5980 system, revealing increased flux and spectral hardening linked to evolving stellar wind properties.

Contribution

It provides the first observational evidence of a changing wind-wind collision in a massive star system and compares observations with analytical models to understand the phenomena.

Findings

X-ray flux increased by a factor of ~2.5 and slightly hardened.

The lightcurve shape remained consistent despite brightness changes.

Models partially reproduce observed changes but predict different behavior at apastron.

Abstract

We report on the first detection of a global change in the X-ray emitting properties of a wind-wind collision, thanks to XMM-Newton observations of the massive SMC system HD5980. While its lightcurve had remained unchanged between 2000 and 2005, the X-ray flux has now increased by a factor of ~2.5, and slightly hardened. The new observations also extend the observational coverage over the entire orbit, pinpointing the lightcurve shape. It has not varied much despite the large overall brightening, and a tight correlation of fluxes with orbital separation is found, without any hysteresis effect. Moreover, the absence of eclipses and of absorption effects related to orientation suggests a large size for the X-ray emitting region. Simple analytical models of the wind-wind collision, considering the varying wind properties of the eruptive component in HD5980, are able to reproduce the recent hardening and the flux-separation relationship, at least qualitatively, but they predict a hardening at apastron and little change in mean flux, contrary to observations. The brightness change could then possibly be related to a recently theorized phenomenon linked to the varying strength of thin-shell instabilities in shocked wind regions.