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

Kavli Affiliate: David A. Principe

| First 5 Authors: Amit Kashi, David A. Principe, Noam Soker, Joel H. Kastner,

| Summary:

The massive binary system Eta Carinae is characterized by intense colliding
winds that form shocks and emit X-rays. The system is highly eccentric
($esimeq0.9$), 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.

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