First look at Vela X-1 with XRISM: A simultaneous campaign with XMM-Newton and NuSTAR

TL;DR

This work presents the inaugural XRISM/Resolve observation of the wind-fed HMXB Vela X-1 taken during inferior conjunction, with simultaneous XMM-Newton and NuSTAR coverage to capture absorption variability and Fe K-region evolution. Through absorption-resolved spectroscopy across soft and hard hardness-ratio intervals, the authors detect a rapid HR transition coinciding with an increase in the wind column density, indicative of accretion-wake crossing the line of sight. High-resolution XRISM spectra reveal a Fe Kα doublet (and Fe Kβ) from near-neutral/lowly ionised material and constrain line widths to wind-velocity scales, consistent with clumps in the dense wind near the neutron star. The results highlight XRISM’s capability to dissect wind–accretion dynamics in X-ray binaries and establish a framework for future time-resolved, high-resolution spectral studies using variational inference methods like jaxspec.

Abstract

High-Mass X-ray Binaries (HMXBs) serve as useful laboratories for exploring the behaviour of accreted matter onto compact objects and for probing the complex wind environments of massive stars. These investigations are essential for understanding stellar life cycles and the dynamics of the Milky Way, and they are prominent topics in the science cases for XRISM and NewAthena. We report, for the first time, a XRISM observation of the HMXB Vela X-1, conducted during the first cycle of the XRISM general observer programme and complemented by simultaneous XMM-Newton and NuSTAR coverage. This campaign targeted a critical orbital phase -- when the neutron star is in inferior conjunction -- during which significant changes in absorption are expected. We performed absorption-resolved spectral analyses during two time intervals of interest: the soft and hard hardness ratio (HR) intervals, as it is strongly correlated with absorption variability. We observed a sudden transition in the HR from a soft to a hard state, coinciding with an increase in the absorption column density. This is likely attributed to the onset of the accretion structure crossing our line of sight. With XRISM/Resolve, we also investigated the Fe K region, and we report for the first time the presence of a Fe K$α$ doublet in the spectrum of Vela X-1, together with the presence of already known Fe K$β$ and Ni K$α$ lines that are produced in cold clumps embedded in the hot ionised wind. The measured line velocities of the order of $10^2 \ \mathrm{km\,s^{-1}}$ are consistent with production sites in the vicinity of the neutron star. This precursor study with Vela X-1 shows the potential of XRISM in studying in unprecedented details the spectral evolution of wind-accreting X-ray binaries.

Figures (6)

• Figure 1: Sketch of the Vela X-1 system showing the orbital phases covered by the neutron star during our XRISM observation (light blue) and during the ER observation (dark brown). In this image, the observer is facing the system at the bottom of the picture, and $\phi_{\rm{orb}} = 0$ is defined at the eclipse.
• Figure 2:
• Figure 3: Folded XRISM/Resolve spectrum (light blue) averaged over the hard HR interval. The red solid line corresponds to the base model we used to test for the presence of additional line components, indicated by the grey solid line (see Sect. \ref{['subsubsection:lines']}). Lines for which a detection could be resolved are labelled by their names; the others are labelled by numbers. The corresponding ratio residuals in the lower panels are computed as data/model. For plotting purposes, the data were binned to a minimum of 150 counts/bin.
• Figure 4: Comparison of the expected log predictive densities between the base model and the same model adding one line component at a time. The base model includes the Fe K$\alpha_1$, Fe K$\alpha_2$, and Fe K$\beta$ lines.
• Figure 5: Folded spectra for GTI 1 obtained with XMM-Newton/EPIC-pn (orange), NuSTAR/FPMA (red) and FPMB (dark blue), and XRISM/Resolve (light blue). Solid lines show the best-fit model and corresponding ratio in the bottom panel computed as data/model. A zoom in the Fe region is shown to highlight the resolution capabilities of Resolve. For plotting and readability purposes, the NuSTAR and XMM-Newton data are not displayed in the zoomed window and the XRISM/Resolve data are binned to a minimum of 150 counts/bin.