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

Current and planned X-ray telescopes such as XRISM/Resolve and NewAthena can achieve energy resolutions of R = E / ΔE ≈ 1000, making spectroscopic calibration necessary for physical effects including the local standard of rest and Earth's orbit around the Sun. The authors identify a previously unexamined effect: modulation of redshift due to an X-ray telescope's motion around Earth.

They construct an a priori method for detecting this orbital redshift modulation and apply the pipeline to several XRISM/Resolve observations of narrow iron K-alpha emission lines from galactic-center molecular clouds.

The analysis yields evidence of orbital redshift modulation in the K-alpha emission at the 95% confidence level, representing the first detection of redshifting and blueshifting of X-ray photons from a telescope's orbital motion. The findings suggest this modulation is already present and detectable in X-ray data, and motivate reexamination of how X-ray photon data is packaged in the emerging era of high-resolution X-ray spectroscopy.