An X-ray view of HD166734, a massive supergiant system

TL;DR

This study uses X-ray observations to analyze the wind-wind collision in the massive binary HD166734, revealing phase-locked variations, a possible shock collapse at periastron, and complex emission behaviors.

Contribution

It provides the first detailed phase-resolved X-ray analysis of HD166734, highlighting wind collision dynamics and variability patterns not previously documented.

Findings

X-ray flux varies by an order of magnitude with orbital phase.

A long minimum state suggests possible shock collapse at periastron.

X-ray emission correlates with orbital separation, indicating radiative collision processes.

Abstract

The X-ray emission of the O+O binary HD166734 was monitored using Swift and XMM-Newton observatories, leading to the discovery of phase-locked variations. The presence of an f line in the He-like triplets further supports a wind-wind collision as the main source of the X-rays in HD166734. While temperature and absorption do not vary significantly along the orbit, the X-ray emission strength varies by one order of magnitude, with a long minimum state (Delta(phi)~0.1) occurring after a steep decrease. The flux at minimum is compatible with the intrinsic emission of the O-stars in the system, suggesting a possible disappearance of colliding wind emission. While this minimum cannot be explained by eclipse or occultation effects, a shock collapse may occur at periastron in view of the wind properties. Afterwards, the recovery is long, with an X-ray flux proportional to the separation d (in hard band) or to d^2 (in soft band). This is incompatible with an adiabatic nature for the collision (which would instead lead to "F_X propto 1/d"), but could be reconciled with a radiative character of the collision, though predicted temperatures are lower and more variable than in observations. An increase in flux around phi~0.65 and the global asymmetry of the light curve remain unexplained, however.