XRISM/Resolve observations of Hercules X-1: vertical structure and kinematics of the disk wind

TL;DR

The study tackles the challenge of quantifying 3D accretion-disk winds in X-ray binaries by exploiting Her X-1's warped, precessing disk to sample wind vertical structure and applying XRISM/Resolve's high-resolution spectroscopy. A large multi-instrument campaign (XRISM ≈210 ks, XMM-Newton, NuSTAR, Chandra) enables time-resolved analyses using both phenomenological slab and physical pion wind models, bolstered by broad-band SED constraints from simultaneous data. The analysis detects orbital motion in the wind with amplitude $170 ext{ km s}^{-1}$; after correction, the wind velocity rises from $250 ext{ km s}^{-1}$ to $600 ext{ km s}^{-1}$ as height above the disk increases, while the column density declines and the ionization parameter $ ext{log}(\xi/ ext{erg cm s}^{-1})$ grows from about $3.65$ to $3.9$. These results illuminate the vertical wind structure, constrain launching and acceleration scenarios, and reveal hints of a second wind component linked to orbital phase, illustrating XRISM's power to probe wind physics in accreting systems.

Abstract

X-ray binary accretion disk winds can carry away a significant fraction of the matter transferred from the companion and hence strongly affect the accretion flow and the long-term evolution of the binary. However, accurate mass outflow rate measurements are challenging due to uncertainties in our understanding of the 3D wind structure. Most studies employ absorption line spectroscopy that only gives us a single sightline through the wind streamlines. Hercules X-1 is a peculiar X-ray binary which allows us to avoid this issue, as its warped, precessing accretion disk naturally presents a range of sightlines through the vertical structure of its disk wind. Here we present the first results from a large, coordinated campaign on Her X-1 led by the new XRISM observatory (with an exposure of 210 ks) and supported by XMM-Newton, NuSTAR and Chandra. We perform a time-resolved analysis and constrain the wind properties. With XRISM/Resolve, we directly detect the Her X-1 orbital motion with an amplitude of 170 km/s in the evolution of the wind velocity. After correcting for this effect, we observe an increase in wind velocity from 250 km/s to 600 km/s as the wind rises to greater heights above the disk. The wind column density decreases with increasing height, as expected, but its ionization parameter log($ξ$/erg cm s$^{-1}$) evolves only weakly from 3.65 to 3.9 as the wind expands away. Additionally, we detect a new orbital dependence of the wind properties, revealing a likely second component that appears only briefly after the eclipse by the secondary star.

Figures (9)

• Figure 1: Raw XRISM lightcurve (events of all qualities included) from the September 2024 campaign on Her X-1. Time T=0 corresponds to the beginning of the XRISM observation at MJD=60563.09653. Horizontal arrows show the overlaps of simultaneous observations with XMM-Newton, Chandra and NuSTAR as well as their durations. The approximate clean exposures are given in the legend for each observatory. Some of the notable events are described, including our numbering scheme of individual Her X-1 orbits.
• Figure 2: Comparison between the time-averaged high-flux XRISM/Resolve spectrum, focusing on the Fe K energy band, with simultaneous data from XMM-Newton EPIC-pn. The most notable spectral features are described and their rest-frame energies are shown with blue vertical dashed lines. The EPIC-pn data were shifted in energy to account for the known gain shift issue at high count rates Kosec+22, and also in flux by a constant for visual purposes.
• Figure 3: Comparison of the Fe XXVI region between the 3 Her X-1 high-flux orbits. Fe XXVI absorption originates in the disk wind, and clear variability is observed from orbit to orbit. The line optical depth decreases, and it becomes broader and shifts to higher energies, indicating that the wind increases in velocity as well as velocity width.
• Figure 4: Time-averaged high-flux XRISM spectrum of Her X-1 (all 3 orbits combined), fitted with the phenomenological emission and absorption spectral model. The primary continuum is shown in blue. Two highly broadened emission lines are required in addition to the primary continuum for a satisfactory fit of the Fe K complex (blue and magenta). Narrow Fe I K$\alpha$ and K$\beta$ emission lines are shown in red. On top of this continuum model, we apply disk wind absorption (using the slab model in this example) which produces the narrow absorption lines across the Fe K band. The lower panel contains the residuals to the fit containing all spectral components.
• Figure 5: Disk wind velocity versus Her X-1 precession phase and orbital phase. The best-fitting velocity with no corrections applied is in red. Clear orbital motion is observed in the evolution of this velocity. In black, we show the velocity evolution after correcting for the orbital motion of the neutron star. Her X-1 eclipses by the secondary star are shown with green horizontal lines.