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

Abstract

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~{\sc xxv} K$\hat{I}\pm$ components show coherent orbital modulation, yielding $K_1=58.1\pm 8.5\mathrm{km}{\mathrm{s}}^{-1}$. 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 $M_1=0.79\pm 0.04M_{\odot }$. 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.

Accepted for publication in ApJL, 13 pages, 6 figures