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

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 ($e\simeq 0.9$), 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 ($N_{\mathrm{H}}(t)$) 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, $N_{\mathrm{H}}(t)$ - serves as a potentially stringent constraint on the binary orientation. We compare the observed $N_{\mathrm{H}}(t)$ 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.

arXiv commentAccepted to ApJ

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