2004 TT357: A Potential Contact Binary in the Trans-Neptunian Belt

We report photometric observations of the trans-Neptunian object 2004 TT₃₅₇ obtained in 2015 and 2017 using the 4.3 m Lowell’s Discovery Channel Telescope. We derive a rotational period of 7.79 ± 0.01 hr and a peak-to-peak lightcurve amplitude of 0.76 ± 0.03 mag. 2004 TT₃₅₇ displays a large variability that can be explained by a very elongated single object or can be due to a contact/close binary. The most likely scenario is that 2004 TT₃₅₇ is a contact binary. If it is in hydrostatic equilibrium, we find that the lightcurve can be explained by a system with a mass ratio q _min = 0.45 ± 0.05, and a density of ρ _min = 2 g cm^-3, or less likely a system with q _max = 0.8 ± 0.05, and ρ _max = 5 g cm^-3. Considering a single triaxial ellipsoid in hydrostatic equilibrium, we derive a lower limit to the density of 0.78 g cm^-3, and an elongation (a/b) of 2.01 assuming an equatorial view. From Hubble Space Telescope data, we report no resolved companion orbiting 2004 TT₃₅₇. Despite an expected high fraction of contact binaries in the trans-Neptunian belt, 2001 QG₂₉₈ is the unique confirmed contact binary in the trans-Neptunian belt, and 2004 TT₃₅₇ is only the second candidate to this class of systems, with 2003 SQ₃₁₇.