The environment of the wind-wind collision region of $\eta$ Carinae

TL;DR

This study uses NuSTAR X-ray observations of $ta$ Carinae around periastron to analyze the wind collision region, revealing its geometry, variability, and physical components in detail.

Contribution

It provides new insights into the geometry and variability of the wind collision zone using high-resolution X-ray data from NuSTAR.

Findings

X-ray flux varies with periastron, indicating changes in the collision zone.

Partial disruption of the wind collision region occurs near periastron.

Fe K$b1$ line broadening suggests complex shock front electron heating.

Abstract

$\eta$ Carinae is a colliding wind binary hosting two of the most massive stars and featuring the strongest wind collision mechanical luminosity. The wind collision region of this system is detected in X-rays and $\gamma$-rays and offers a unique laboratory for the study of particle acceleration and wind magneto-hydrodynamics. Our main goal is to use X-ray observations of $\eta$ Carinae around periastron to constrain the wind collision zone geometry and understand the reasons for its variability. We analysed 10 Nuclear Spectroscopic Telescope Array (NuSTAR) observations, which were obtained around the 2014 periastron. The NuSTAR array monitored the source from 3 to 30 keV, which allowed us to grasp the continuum and absorption parameters with very good accuracy. We were able to identify several physical components and probe their variability. The X-ray flux varied in a similar way as observed during previous periastrons and largely as expected if generated in the wind collision region. The flux detected within ~10 days of periastron is lower than expected, suggesting a partial disruption of the central region of the wind collision zone. The Fe K$\alpha$ line is likely broadened by the electrons heated along the complex shock fronts. The variability of its equivalent width indicates that the fluorescence region has a complex geometry and that the source obscuration varies quickly with the line of sight.