The unusual Kuiper belt object 2003 SQ317

We report photometric observations of Kuiper belt object 2003 SQ₃₁₇ obtained between 2011 August 21 and November 1 at the 3.58 m New Technology Telescope, La Silla. We obtained a rotational light curve for 2003 SQ₃₁₇ with a large peak-to-peak photometric range, Δm = 0.85 ± 0.05 mag, and a periodicity, P = 7.210 ± 0.001 h. We also measure a nearly neutral broad-band colour B - R = 1.05 ± 0.18 mag and a phase function with slope β = 0.95 ± 0.41 mag deg^-1. The large light-curve range implies an extremely elongated shape for 2003 SQ₃₁₇, possibly as a single elongated object but most simply explained as a compact binary. If modelled as a compact binary near hydrostatic equilibrium, the bulk density of 2003 SQ₃₁₇ is near 2670 kg m^-3. If 2003 SQ₃₁₇ is instead a single, elongated object, then its equilibrium density is about 860 kg m^-3. These density estimates become uncertain at the 30 per cent level if we relax the hydrostatic assumption and account for solid, `rubble pile'-type configurations. 2003 SQ₃₁₇ has been associated with the Haumea family based on its orbital parameters and near-infrared colour; we discuss our findings in this context. If confirmed as a close binary, 2003 SQ₃₁₇ will be the second object of its kind identified in the Kuiper belt.