First Detection of the Doppler Shift of X-Ray Photons Due to a Telescope’s Orbital Motion

We have detected for the first time the redshifting and blueshifting of X-ray photons due to the orbital motion of an X-ray telescope around the Earth. Current and planned X-ray telescopes like XRISM/Resolve and NewAthena can achieve energy resolutions R = E /Δ E ≈ 1000, requiring spectroscopic calibration of physical effects such as the local standard of rest and the orbit of the Earth around the Sun. We construct an a priori method for detecting a heretofore unexamined effect: the modulation of redshift due to the motion of an X-ray telescope around the Earth. We apply this pipeline to several XRISM/Resolve observations of narrow Fe K α emission lines from Galactic center molecular clouds, and we detect evidence of orbital redshift modulation in the Fe K α emission at the 95% confidence level. Our findings suggest that orbital redshift modulation is already present and detectable in X-ray data and motivate reexamination of how X-ray photon data are packaged in the emerging era of high-resolution X-ray spectroscopy.