Unveiling the white dwarf in the eclipsing polar HU Aquarii

TL;DR

This study leverages high-speed ULTRACAM photometry of HU Aqr taken during a low accretion state to precisely constrain the white dwarf and binary geometry in a magnetic cataclysmic variable. Using the LCURVE eclipse modeling framework with MCMC, the authors extract a robust white dwarf mass of $M_ ext{WD} = 0.78 \pm 0.02\,M_\odot$ and an orbital inclination of $i = 87.4 \pm 0.9^\circ$, along with the donor mass and the location of a heated region near the former high-state accretion spot. In the $r$-band, they identify a compact cyclotron-emitting region offset by ~$30^\circ$ in longitude, with a vertical extent $h \sim 0.005$–$0.016\,R_\mathrm{WD}$ that tracks the accretion rate, and a diameter of 3–4 degrees. The results show that a vertical extent is required to match the observed bright-phase length and demonstrate the power of eclipse-based modeling (via ULTRACAM and LCURVE) to reveal detailed accretion geometry in magnetic CVs. The public availability of the data enables reproducibility and further studies of HU Aqr's accretion behavior.

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.

Figures (10)

• Figure 1: ULTRACAM light curves of HU Aquarii in $u$(blue), $g$ (green), and $r$ (red) in original time sequence obtained on 14 - 17 May 2005. The data from 17 May are not shown here because just the eclipse was observed.
• Figure 2: ULTRACAM $u$-band light curve from 14 May 2005 with model fit overplotted. The left panels show the fit to the ingress and egress of the white dwarf eclipse, while the right panel shows the fit to the full light curve. The residuals to the fit are shown at the bottom (in standard deviations).
• Figure 3: ULTRACAM $r$-band eclipse light curve from 16 May 2005 with best-fit model shown in gray. The individual components of the model are also shown: white dwarf (blue dashed line), donor star (red dot-dashed line), cyclotron spot (green dotted line). The residuals are shown below (in standard deviations).
• Figure 4: ULTRACAM $r$-band eclipse light curves with model fits overplotted. The fit includes the eclipse of both the white dwarf photosphere and cyclotron spot. The residuals are shown below each fit (in standard deviations).
• Figure 5: UV to optical spectral energy distribution of HU Aqr. Shown are the faint-phase data only, the accretion steam-corrected HST/FOS spectrum obtained 1996, and the OM data obtained simultaneously with the ULTRACAM $u,g,r$ photometry. The model spectra for 13,500 K, 14,000 K, and 14,500 K scaled to the distance and radius of the white dwarf in HU Aqr are shown as blue lines.