Direct spectroscopic observation of matter falling onto a compact stellar object

High Mass X-ray Binaries (HMXB) are systems with a massive star (> 8 Msol) and a neutron star or black hole companion, which accretes matter from the stellar wind. The companion emits X-rays with luminosity proportional to the accretion rate. Despite decades of observation, wind-fed accretion was never directly observed and remains poorly understood. Using the new XRISM/Resolve X-ray spectrometer, we report on the first direct detection of matter falling onto the neutron star in the HMXB GX 301-2, which can fully account for the observed X-ray luminosity. We also demonstrate how accretion during GX 301-2 flares varies between near-direct radial inflow and transverse, potentially disk-like, flow. Observations of inflowing matter thus constitute a direct test of accretion hypotheses, allowing a deeper understanding of HMXB systems.