The X-ray properties of Eta Carinae during its 2020 X-ray minimum

TL;DR

This study analyzes NICER X-ray spectra of Eta Carinae during its 2020 minimum, revealing insights into the binary's orientation, wind interactions, and recent wind weakening through spectral and geometric modeling.

Contribution

It provides the first detailed spectral analysis of Eta Carinae's 2020 X-ray minimum and constrains the binary orientation and wind dynamics using $N_{H}(t)$ data.

Findings

Column density variations support the secondary star being closer at periastron.

2020 minimum showed the fastest recovery, indicating wind weakening.

Spectral analysis constrains the binary orientation to $ ext{omega} ightarrow 90^ ext{o}$.

Abstract

The massive binary system Eta Carinae is characterized by intense colliding winds that form shocks and emit X-rays. The system is highly eccentric ($e\simeq0.9$), resulting in modulated X-ray emission during its 5.54 year orbit. The X-ray flux increases in the months prior to periastron passage, exhibiting strong flares, then rapidly declines to a flat minimum lasting a few weeks, followed by a gradual recovery. We present Neutron Star Interior Composition Explorer (NICER) telescope spectra obtained before, during, and after the 2020 X-ray minimum, and perform spectral analysis to establish the temporal behavior of X-ray flux and X-ray-absorbing column density ($N_{\rm H}(t)$) for the 2-10 keV and 5-10 keV energy ranges. The latter range is dominated by the stellar wind collision region and, therefore, these spectral parameters - in particular, $N_{\rm H}(t)$ - serves as a potentially stringent constraint on the binary orientation. We compare the observed $N_{\rm H}(t)$ results to the behavior predicted by a simple geometrical model in an attempt to ascertain which star is closer to us at periastron: the more massive primary ($\omega \simeq 240$-$270^\circ$), or the secondary ($\omega \simeq 90^\circ$). We find that the variations in column density, both far from periastron and around periastron passage, support the latter configuration ($\omega \simeq 90^\circ$). The 2020 X-ray minimum showed the fastest recovery among the last five minima, providing additional evidence for a recent weakening of the primary star's wind.