Towards precision particle background estimation for future X-ray missions: correlated variability between Chandra ACIS and AMS

A science goal of many future X-ray observatories is mapping the cosmic web through deep exposures of faint diffuse sources. Such observations require low background and the best possible knowledge of the remaining unrejected background. The dominant contribution to the background above 1-2 keV is from Galactic Cosmic Ray protons. Their flux and spectrum are modulated by the solar cycle but also by solar activity. For observatories in low Earth orbit, the flux and spectrum is further modulated by geomagnetic shielding. While geomagnetic background variations can be well modeled, understanding the solar variability may prove crucial to reducing background uncertainty for ESA’s Athena X-ray Observatory and other future missions in high Earth orbit and at L2. We examine the variability of the particle background as measured by ACIS on the Chandra X-ray Observatory and compare the variability to that measured by the Alpha Magnetic Spectrometer (AMS), a precision particle detector on the ISS. We show that cosmic ray proton variability measured by AMS is well matched to the ACIS background and can be used to estimate proton energies responsible for the background. In choosing an orbit, future missions must trade-off between the low, stable particle background in LEO, with the high, variable particle background in HEO and L2. Better knowledge of this variability can improve the science return from high orbit.