The soft X-ray eclipses of HU Aqr

We present the results of extended monitoring observations in soft X-rays o f the bright eclipsing polar (AM Herculis star) HU Aqr. It was observed bet ween 1990 and 1998 by ROSAT for a total of 230 ksec using the PSPC and the HRI detectors and by EUVE with the Deep Survey Imager and the Spectrometer for a total of 580 ksec. The overall X-ray brightness of HU Aqr varied due to changes of the mass accretion rate by a factor of 40 over that period of time. At all occasions the X-ray light curve was characterized by a marked on-off behavior due to the self-eclipse of the accreting pole. The X-ray l ight curve showed eclipses by the companion star, the accretion stream and by an accretion curtain raised between the two stars in the binary. Narrow dips prior to the stellar eclipse are caused by the transit of the outer ac cretion stream. These dips display marked phase shifts, thus indicating a l arge movement of the threading region, where the stream couples to the magn etic field. These shifts are shown to be related to changes of the mass acc retion rate. Correspondingly, the spot longitude varied between 34 degrees and 50 degrees. The X-ray light curves display clear evidence for the prese nce of an accretion curtain, which is raised all along the ballistic accret ion stream down to the region where the bulk of matter couples onto magneti c field lines. A lower limit to the mass accretion rate in the curtain is 6 x 10(-12) M-./yr, which is of order 10% of the total mass accretion rate. A linear fit to all available eclipse egress times yields an updated orbita l ephemeris of the system: BJED(T-o) = 2449217.345872(35) +E x 0.0868204161 95(47) with T-o the time of eclipse of the white dwarf centre of mass (BJED : barycentric Julian ephemeris time). The inclusion of a quadratic term giv es a better fit to the data but is not regarded as indication of a period c hange or asynchronous rotation but by a migration of the accretion spot ove r the surface of the white dwarf. For one particular data set obtained in a high accretion state, detailed light curve modeling was possible. The egre ss from eclipse lasted 1.3 s, which constrained the azimuthal extent of the accretion spot to less than 4 degrees or 450 km. The spot extended vertica lly by less than or equal to 0.015 R-wd. A comparison of the width of the s tream dip and the extent of the accretion spot shows, that only the inner 6 0-80% of the stream are dense enough to fire the soft X-ray engine. During the eclipse, HU Aqr was detected at a flux level of 6 x 10(-14) erg cm(-1) s(-1). The implied X-ray luminosity is L-x = 2.2 x 10(29) erg s(-1), compar able with X-ray emission from single, late-type, active stars.