Time-resolved HST and IUE UV spectroscopy of the Intermediate Polar FO Aqr

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Time resolved spectroscopy of the Intermediate Polar FO Aqr reveals the presence of multiple periodicities in the UV range. A strong orbital modulation dominates both continuum and emission line flux variabilities, while line velocity motions are only detected at the rotational frequency. A prominent orbital periodicity is also observed in coordinated optical photometry, where FO Aqr was previously found to be spin dominated. The spectral dependence of the main periodicities shows the presence of multi-temperature components in FO Aqr and for the first time a hot and a cool component in the rotational modulation. From a comparison with previous UV and optical data obtained in 1990, no spectral variations in the orbital and rotational variabilities are detected, indicating no significant changes in the effects of X-ray illumination but rather a shrinking of the accretion curtain accompained by an increase in size of the thickened part of the accretion disc. These observations, consistent with the recently discovered long term trend in the X-ray pulsation amplitudes, independently confirm a change in the accretion mode in FO Aqr, which switched from a disc-fed into a disc-overflow state, likely triggered by mass accretion variations.