X-ray study of complex absorption and reflection in the intermediate polar FO Aqr

The intermediate polar FO Aqr has exhibited various accretion states from 2016 to 2021, with noticeable spectral variability linked to changes in accretion dynamics. This study provides a comprehensive X-ray analysis of multiple observations across these states. NuSTAR data reveal significant reflection in the 10–30 keV range, suggesting that emission originates close to the white dwarf’s surface. XMM-Newton observations identify a complex, ionized partial covering absorber intrinsic to the system, with absorption levels modulated by both spin and orbital cycles, likely located in the accretion curtain/stream and stream-disk interaction region. A comparison of datasets across accretion states shows that in low states, the system emits and absorbs less photons than in high states. This is due to the presence of less material throughout the accretion process, supporting prior theories that reduced mass transfer from the secondary star to the white dwarf drives these low states.