Researchers identified red and blue shifts in X-ray photons caused by the orbital motion of the XRISM telescope rather than the observed source. The instrument completes an orbit around Earth every 90 minutes, leading to periodic energy variations in incoming photons.
The team analyzed a narrow iron line at approximately 6.4 keV. While the energy shift for a single photon is about 0.14 eV, which is below the instrument's resolution, statistical analysis of multiple observations allowed researchers to detect the effect by grouping data by orbital phase.
Using a custom algorithm to track orbital periods via signal gaps caused by Earth occultation, scientists measured an average velocity difference of 28 km/s across four observations of molecular clouds near the galactic center. The expected velocity was 14.7 km/s.
The study yielded a p-value of 0.0084, rejecting the null hypothesis of no orbital modulation at a 5% significance level. Researchers investigated potential instrumental errors, such as temperature fluctuations or calibration issues, but found no evidence to explain the results.
Although current resolution requires statistical correction for this effect, future high-resolution spectrometers will necessitate individual photon energy adjustments. Authors recommend including orbital telemetry in standard observation files to prevent misinterpreting telescope motion as cosmic phenomena.
