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

Abstract

The supermassive binary system, $η$ 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$α$ band between 6.2 and 7.1 keV, obtained with the Resolve X-ray microcalorimeter. The hydrogen-like (Ly$α$) and helium-like (He$α$) 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$α$ 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.

Figures (9)

• Figure 1: Left: XRISM$\eta$ Carinae observations in the 2$-$10 keV flux modulation plot folded with the binary orbital period, obtained with , , and since 1996 Corcoran2017aEspinoza2022a The crosses show the 2$-$10 keV fluxes during the XRISM observations ( black: XRI231123, red: XRI240609), which are similar to those observed during earlier epochs near their orbital phases. Right: Companion star's locations in a binary orbit during the XRISM observations.
• Figure 2: Resolve spectra of $\eta$ Carinae during XRI231123 ( black) and XRI240609 ( red).
• Figure 3: Resolve Fe K$\alpha$ band spectrum of $\eta$ Carinae in XRI240609. The solid red line shows the best-fit optically thin, collisional equilibrium one-temperature thermal plasma ( bapec) model derived from a fit to the data in the top red bar ranges. The blue line barely above the x-axis shows the estimated non-X-ray background spectrum.
• Figure 4: Fluorescent Fe K$\alpha$ ( left) and K$\beta$ ( right) line spectra ( black). The line components are fitted by empirical models in Holzer1997a, smoothed with the gsmooth Gaussian convolution function. For the K$\alpha$ lines, the original model exhibits a non-negligible residual between the K$\alpha_1$ and K$\alpha_2$ lines ( orange). We thus allow their line normalizations to be independently varied ( green). For the K$\beta$ lines, we employ a model that ties the redshift and $v_{\sigma6}$ smoothing parameters to those of the K$\alpha$ model ( green). However, the best-fit model shows a marginal excess on the blue side, and a model with a free redshift parameter reproduces this excess better ( orange). We employ the models in green for the entire model fit. The green dotted lines shows the line components, the grey lines represent a thermal model for the continuum, and the blue lines barely above the x-axis represent the non-X-ray background.
• Figure 5: Best-fit model of the Fe K$\alpha$ band spectrum between 6.2$-$7.1 keV ( top: XRI240609, bottom: XRI231123). The black and grey lines represent the total and the thermal plasma emissions. The blue, purple, and red lines show the blue wing, narrow, and red wing components, and the orange line represents the Compton scattering of these components. The green line shows the Fe fluorescent K$\alpha$ and K$\beta$ lines, and the cyan line shows the Compton scattering of the K$\alpha$ line. The non-X-ray background background, included in the model, is below this plotting scale.