XRISM Observations of Orbital-phase-dependent Fe K <i>α</i> Emission Using High-resolution X-Ray Spectroscopy in the Wind-fed High-mass X-Ray Binary 4U 1700–37

Abstract We report the first XRISM observations of the wind-fed high-mass X-ray binary 4U 1700–37, focusing on the orbital-phase dependence of the Fe K α fluorescence line. The observation, conducted over a ∼1.4 day interval in 2025 February, spans approximately 40% of the 3.4 day orbital period and provides sufficient photon statistics for phase-resolved spectroscopy with eV-scale energy resolution. XRISM/Xtend constrains the orbital-phase-dependent absorption through its broad bandpass, while XRISM/Resolve enables precise diagnostics in the Fe K band, including Fe K α doublet, Fe K β , the Fe K absorption edge, and Ni K α . By dividing the data into orbital-phase bins of width Δ ϕ orb = 0.1, we detect statistically significant variations in the centroid energy of the Fe K α line after accounting for both statistical and systematic uncertainties. The Fe K β line exhibits a qualitatively similar orbital-phase dependence. The measured redshift varies smoothly with orbital phase, reaching (−2.2 ± 0.3) × 10 −4 at ϕ orb = 0.5–0.6 and (7.7 ± 0.6) × 10 −4 at ϕ orb = 0.8–0.88; intermediate values are observed at ϕ orb = 0.7–0.8. When described phenomenologically, the measured centroid shifts are well approximated by a sinusoidal modulation with a characteristic velocity scale of ∼200 km s −1 . The observed phase dependence indicates that the Fe K α emission does not simply trace the orbital motion of the compact object but is influenced by large-scale velocity structures within an asymmetric stellar wind. Together with recent IXPE detections of high and variable X-ray polarization, our results provide an observational context for future studies of the circumstellar environment of 4U 1700–37, a system hosting an unusually massive compact object candidate.