Unveiling the white dwarf in the eclipsing polar HU Aquarii

We present an analysis of high-speed u- and r-band photometry of the eclipsing polar HU Aqr that was obtained with ULTRACAM mounted on the VLT. The observations were performed during a low state, permitting us for the first time to determine the contact points of the white dwarf. Using LCURVE we could determine its size, and hence mass, with a direct method and with unprecedented accuracy. We determined the mass of the white dwarf as 0.78 ± 0.02 M⊙, the mass ratio Q = MWD/Msec = 4.59, and the orbital inclination i = 87.​​° 4 ± 0.​​° 9. An extended warm region with a central temperature of ∼33 000 K was observed in the u-band at the location of the previous high-state accretion spot. Weak accretion was ongoing in the low state that led to cyclotron emission that could best be studied with the r-band data. It has a diameter of only 3° to 4° and is located much closer to the binary meridian than the accretion-heated region studied in the u-band. The longitudinal shift of the two accretion regions is of order 30°, due to early and late coupling of accreted matter onto the magnetic field lines in low and high accretion states, respectively. The low-state cyclotron-emitting region has a vertical extent of 0.005 − 0.016 RWD, a value that seems to be correlated to the instantaneous accretion rate.