NICER X-ray Observations of Eta Carinae During its Most Recent Periastron Passage

TL;DR

This paper presents high-precision X-ray observations of Eta Carinae during its 2020 periastron, revealing the shortest X-ray minimum, detailed shock and column density variations, and insights into the ejecta's kinetic luminosity.

Contribution

It provides new constraints on the timing and characteristics of the X-ray minimum and shock behavior during Eta Carinae's periastron passage, with detailed analysis of column density and ejecta luminosity.

Findings

Shortest X-ray minimum observed so far (~16 days).

Column density increases before minimum, indicating dense clumps near shock apex.

Derived kinetic X-ray luminosity of ejecta near the Great Eruption (~10^41 ergs s^-1).

Abstract

We report high-precision X-ray monitoring observations in the 0.4-10 keV band of the luminous, long-period colliding-wind binary Eta Carinae up to and through its most recent X-ray minimum/periastron passage in February 2020. Eta Carinae reached its observed maximum X-ray flux on 7 January 2020, at a flux level of $3.30 \times 10^{-10}$ ergs s$^{-1}$ cm$^{-2}$, followed by a rapid plunge to its observed minimum flux, $0.03 \times 10^{-10}$ ergs s$^{-1}$ cm$^{-2}$ near 17 February 2020. The NICER observations show an X-ray recovery from minimum of only $\sim$16 days, the shortest X-ray minimum observed so far. We provide new constraints of the "deep" and "shallow" minimum intervals. Variations in the characteristic X-ray temperature of the hottest observed X-ray emission indicate that the apex of the wind-wind "bow shock" enters the companion's wind acceleration zone about 81 days before the start of the X-ray minimum. There is a step-like increase in column density just before the X-ray minimum, probably associated with the presence of dense clumps near the shock apex. During recovery and after, the column density shows a smooth decline, which agrees with previous $N_{H}$ measurements made by SWIFT at the same orbital phase, indicating that changes in mass-loss rate are only a few percent over the two cycles. Finally, we use the variations in the X-ray flux of the outer ejecta seen by NICER to derive a kinetic X-ray luminosity of the ejecta of $\sim 10^{41}$ ergs s$^{-1}$ near the time of the "Great Eruption'.