High Spectral Resolution X-ray Observations of the Evolved Supermassive Stellar Binary System $\eta$ Carinae - Iron K$\alpha$ Band Profile Revealed with XRISM

TL;DR

High-resolution X-ray spectra of $ta$ Carinae reveal complex wind interactions, velocity components, and hydrogen depletion, providing new insights into the system's wind-wind collision dynamics and composition.

Contribution

This study presents the first high-resolution iron K$lpha$ band spectra of $ta$ Carinae, revealing detailed velocity components and wind properties during periastron approach.

Findings

Three velocity components identified in iron lines.

Detection of a Compton shoulder for the first time.

Evidence of hydrogen depletion in the primary wind.

Abstract

The supermassive binary system, $\eta$ Carinae, is experiencing enormous wind-driven mass loss at a rate unparalleled in the rest of the Galaxy. Their wind-wind collision (WWC) continuously produces shock heated, X-ray emitting plasmas. The XRISM X-ray observatory observed the system in 2023 and 2024 when the X-ray emission began to increase toward periastron passage in 2025. This manuscript reports unprecedentedly high-resolution X-ray spectra in the iron K$\alpha$ band between 6.2 and 7.1 keV, obtained with the Resolve X-ray microcalorimeter. The hydrogen-like (Ly$\alpha$) and helium-like (He$\alpha$) lines reveal three velocity components. Two of them are broadened with maximum velocities of 2000-3000 km/s, likely originating from the post-shock companion wind. The other is relatively narrow, with a Gaussian broadening of only ~290 km/s in 1 sigma, which may originate from the post-shock companion wind at the WWC stagnation point or penetrating the primary wind. The iron fluorescent lines exhibit a moderate blueshift and broadening with velocities at 100-200 km/s, consistent with the primary wind's velocity field. The spectra also confirm a Compton shoulder of the He$\alpha$ line complex for the first time. Both fluorescing and scattering spectral profiles indicate that the binary system is seen from the companion side during these observations. The flux ratio of the Compton scattering emission to the fluorescent line suggests substantial hydrogen depletion of the primary wind, expected from CNO-cycled hydrogen nuclear fusion gas.