Hunting high and low: XMM monitoring of the eclipsing polar HU Aquarii

Aims. We want to study the temporal and spectral behaviour of HU Aqr in the X-ray domain during different accretion states. Methods. We obtained spectra and light curves from four different XMM-Newton pointings covering intermediate and low states. The X-ray observations were accompanied with high time resolution photometry obtained with the Optima and ULTRACAM instruments. Results. On two occasions in May 2002 and 2003 HU Aqr was found in an intermediate state with the accretion rate reduced by a factor of 50 compared to earlier high state measurements. X-ray spectra in the intermediate state can be described by a model containing a blackbody component and hot thermal plasma. Contrary to the high state the ratio between soft and hard X-ray flux is nearly balanced. In agreement with previous measurements we observed a migration of the accretion spot and stream towards the line connecting both stars. The brightness of HU Aqr was further reduced by a factor of 80 during two low states in October 2003 and May 2005, where it was detected at a luminosity of only $L_{\rm X} = 4.7\times10^{28}$ erg s-1 . This luminosity would fit well with an active coronal emitter, but the relatively high plasma temperatures of 3.5 and 2.0 keV are more compatible with residual accretion. We updated the eclipse ephemeris of HU Aqr based on the eclipse egress of the accretion spot measured in various wavelength bands. The $\rm (O{-}C)$-diagram of the observed accretion spot eclipse timings reveals complex deviations from a linear trend, which can be explained by a constant or cyclic period change or a combination thereof. The quadratic term implies a period decrease at a rate of $\dot{P}_{\rm orb} = -7..-11 \times 10^{-12}$ s s-1. In case the observed period change reflects a true angular momentum loss, this would be a factor of 30 larger than given by gravitational radiation