Tracing the Orbital Motion of the Accreting White Dwarf in EX Hydrae with XRISM/Resolve

Measuring the masses of accreting white dwarfs (WDs) is crucial for understanding their evolution and the physics of accretion. High-resolution X-ray spectroscopy can trace the WD motion through Doppler shifts of emission lines formed close to the WD. We report an 83 ks XRISM/Resolve observation of the intermediate polar EX Hydrae and measure the orbital modulation of individual Fe K-shell line centroids. The Fe xxv K alpha components show coherent orbital modulation, yielding K 1 = 58.1 plus or minus 8.5 km s-1. This is the first detection of orbital modulation in individual Fe K-shell lines from an accreting WD, made possible by the high spectral resolution of Resolve and its frequent in-orbit gain calibration. The measured K 1 is consistent with optical/UV K 1 measurements, providing a cross-check that these distinct tracers follow the WD orbital motion. Combining this X-ray measurement with literature orbital parameters, we derive a WD mass of M 1 = 0.79 plus or minus 0.04 M. These results demonstrate that high-resolution X-ray spectroscopy can use individual Fe K-shell line centroids to trace WD orbital motion in accreting WDs.