Unveiling the white dwarf in the eclipsing polar HU Aquarii

TL;DR

This study precisely measures the white dwarf's mass and accretion regions in HU Aquarii during a low state, revealing detailed accretion geometry and magnetic field interactions with unprecedented accuracy.

Contribution

First direct and highly accurate measurement of the white dwarf's size and mass in HU Aquarii using low-state photometry and light curve modeling.

Findings

White dwarf mass is 0.78 ± 0.02 Msun.

Accretion regions show a 30° longitudinal shift.

Detected a warm region at ~33,000 K in the low state.

Abstract

We present an analysis of high-speed u- and r-band photometry of the eclipsing polar HU Aquarii 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 Msun, 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 3deg to 4deg 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 30deg, 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.