The substellar companion in the eclipsing white dwarf binary SDSS J141126.20+200911.1

Abstract

We present high time resolution SDSS-$g^{\prime }$ and SDSS-$z^{\prime }$ light curves of the primary eclipse in SDSS J141126.20+200911.1, together with time-resolved X-Shooter spectroscopy and near-infrared $JH{K}_s$ photometry. Our observations confirm the substellar nature of the companion, making SDSS J141126.20+200911.1 the first eclipsing white dwarf/brown dwarf binary known. We measure a (white dwarf model dependent) mass and radius for the brown dwarf companion of $M_2=0.050\pm 0.002$ $M_{\odot }$ and $R_2=0.072\pm 0.004$ $M_{\odot }$, respectively. The lack of a robust detection of the companion light in the $z^{\prime }$-band eclipse constrains the spectral type of the companion to be later than L5. Comparing the NIR photometry to the expected white dwarf flux reveals a clear $K_s$-band excess, suggesting a spectral type in the range L7-T1. The radius measurement is consistent with the predictions of evolutionary models, and suggests a system age in excess of three Gyr. The low companion mass is inconsistent with the inferred spectral type of L7-T1, instead predicting a spectral type nearer T5. This indicates that irradiation of the companion in SDSS J1411 could be causing a significant temperature increase, at least on one hemisphere.

11 pages, 10 figures, accepted for publication in MNRAS