Photometric masses for long period CVs: the case study of CSS131106

Abstract

We present high-speed photometry of the eclipsing cataclysmic variable CSS131106 J052412+004148. We determine the system parameters by modelling the eclipse lightcurve using the photometric eclipse method, in which the mass ratio is determined from the relative timings of the white dwarf and bright spot eclipses. Despite the blended white dwarf and bright spot ingress, typical of longer period cataclysmic variables, we perform simulations that show we are able to reliably constrain the component masses. We find a mass ratio of $q = 0.81 \pm 0.06$ and inclination $i = 78.5 \pm 0.7$ degrees. The white dwarf and donor masses were found to be $M_{w} = 0.72 \pm 0.04 \, M_{\odot}$ and $M_{d} = 0.58 \pm 0.06 \, M_{\odot}$ respectively. The white dwarf temperature was estimated to be $T_{\rm eff} = 18~500 \pm 2~000$ K, implying a moderate accretion rate of $\dot{M} = 3 \pm 1 \times 10^{-10} M_{\odot}$ yr$^{-1}$. The donor in CSS131106 J052412+004148 joins two other long-period cataclysmic variables (IP Peg and HS 0220+0031) in being unusually small for its mass, even when compared to detached M-dwarfs. The donors in all three systems are also unusually cool for their mass. We discuss possible explanations for the small radii and cool temperatures of the donors in these systems, but find no viable explanation for their properties.

Figures (8)

• Figure 1: Flux calibrated ULTRACAM lightcurves of CSS131106 in (top-to bottom) $u'$, $g'$ and $r'$ bands.
• Figure 2: Model fit to simulated eclipse lightcurve with a blended white dwarf and bright spot ingress.
• Figure 3: Posterior distributions of mass ratio and inclination compared to true values for the simulated lightcurve. Histograms show posterior probability samples from the MCMC chains. The black vertical lines denote the true values of the parameters.
• Figure 4: Phase folded and binned $r'$-band lightcurves together with model fits. From top to bottom, the panels show the 2016 data, the 2018-01-17 data, and the binned lightcurves of the remaining data. Data are shown as a black line with error bars. Model fits are shown in red. Also shown are the different components of the model: the white dwarf (cyan), bright spot (dark blue), accretion disc (dark red) and donor star (purple). The lower panels show the residuals of the fit (black line with errors) and the 1$\sigma$ posterior of the Gaussian process model to the residuals (red band).
• Figure 5: Phase folded and binned $g'$-band lightcurves together with model fits. From top to bottom, the panels show the 2016 data, the 2018-01-17 data, and the binned lightcurves of remaining data. Markers and colours are as described in figure \ref{['fig:rmodelfits']}.