Advancing Precision Particle Background Estimation for Future X-Ray Missions: Correlated Variability between the Alpha Magnetic Spectrometer and Chandra/XMM-Newton

Galactic cosmic-ray (GCR) particles have a significant impact on the particle-induced background of X-ray observatories, and their flux exhibits substantial temporal variability, potentially influencing background levels. In this study, we present 1 day binned high-energy reject rates derived from the Chandra-ACIS and XMM-Newton EPIC-pn instruments, serving as proxies for the GCR particle flux. We systematically analyze the ACIS and EPIC-pn reject rates and compare them with the AMS proton flux. Our analysis initially reveals robust correlations between the AMS proton flux and the ACIS/EPIC-pn reject rates when binned over 27 day intervals. However, a closer examination reveals substantial fluctuations within each 27 day bin, indicating shorter-term variability. Upon daily binning, we observe finer temporal structures in the data sets, demonstrating the presence of recurrent variations with periods of ∼25 days and 23 days in the ACIS and EPIC-pn reject rates, respectively, spanning the years 2014–2018. Notably, during the 2016–2017 period, we additionally detect periodicities of ∼13.5 days and 9 days in the ACIS and EPIC-pn reject rates, respectively. Intriguingly, we observe a time lag of ∼6 days between the AMS proton flux and the ACIS/EPIC-pn reject rates during the second half of 2016. This time lag is not visible before 2016 and after 2017. The underlying physical mechanisms responsible for this time lag remain a subject of ongoing investigation.