XRISM Observations of Accretion Disk Corona in Cyg X-2

TL;DR

This study leverages XRISM Resolve to obtain high-resolution X-ray spectra of Cygnus X-2, focusing on Fe-K emission from the accretion disk corona (ADC) and the search for ultra-fast outflows (UFOs). The continuum is modeled with two thermal components plus a soft residual, revealing two broad Fe-K emission features with a velocity dispersion of $σ_v = 2300^{+900}_{-700}$ km s$^{-1}$, larger than in GX 340+0 and previous Chandra results. A marginal blue absorption near 9 keV suggests a UFO with $v \approx -0.26$ to $-0.29\,c$, and a mass-loss rate of $\dot{M} \sim 10^{-8}$ M$_\odot$ yr$^{-1}$; XSTAR and ionabs modeling require two emission zones with high ionization and substantial $N_H$. The results imply a highly turbulent ADC and a near-Eddington accretion regime in Cyg X-2, with broader implications for disk–wind physics in Z-sources. NICER data further support the UFO interpretation, underscoring the presence of energetic outflows in this system.

Abstract

We present the high-resolution X-ray spectrum of the Z-source Cygnus X-2, obtained with X-Ray Imaging and Spectroscopy Mission (XRISM). The observations have enabled a precise characterization of the Fe-K emission lines from the accretion disk corona (ADC) and a possible detection of an ultra-fast outflow (UFO). The ADC component has at least two distinct regions. The Fe-K emission lines are remarkably broad, with a velocity dispersion of $σ_v=2300^{+900}_{-700}$~km~s$^{-1}$. This can be larger than what was observed in previous Chandra observations ($1120\pm870$~km~s$^{-1}$) and recent XRISM observations of the similar Z-source GX 340+0 (360--800~km~s$^{-1}$). Furthermore, we marginally detect a blueshifted absorption feature, which we identify as either \ion{Fe}{26} or \ion{Fe}{25}, with outflow velocities of $-0.29c$ or $-0.26c$, respectively. The mass loss rate is $\dot{M}\sim10^{-8}M_\odot\,\mathrm{yr}^{-1}$. Our findings suggest that the ADC in Cyg X-2 has a kinematically active environment with a high degree of turbulence and kinetic energy.

Figures (8)

• Figure 1: The mosaic image in Resolve. The five fields of view used in the spectral analysis are shown by color.
• Figure 2: (Upper) Xtend light curve with a time bin of 512 s. The data we used for the spectral analysis is shown in red. (Lower) Color-color plot based on the Xtend light curve.
• Figure 3: The event branching ratio against the count rate in each pixel. The model line is overplotted, calculated under the assumption that the events follow the Poisson distribution. We use the pixels with count rates larger than 0.2 cts s$^{-1}$ pix$^{-1}$ (vertical line). Hp: High-resolution primary, Mp: Mid-resolution primary, Ms: Mid-resolution secondary, Lp: Low-resolution Primary, and Ls: Low-resolution secondary.
• Figure 4: Continuum fitting. (Upper) The upper panel shows the $\nu F_\nu$ plot, and the lower panel shows (data$-$model)/error. The Xtend and Resolve data are shown in black and red, respectively. (Lower) Same as the upper one, but only Resolve data with more sparse binning (80 eV).
• Figure 5: The Resolve model fitting with two positive Gaussian at the Fe-K band and a negative Gaussian in absorption for the UFO. The dotted line shows the continuum.