Constraining the size, shape, and albedo of the large trans-Neptunian object (28978) Ixion with multi-chord stellar occultations

Context. Trans-Neptunian objects (TNOs) are among the most primitive remnants of the early Solar System. Determining their sizes, shapes, albedos, and surface properties is essential for understanding their origin and evolution. Stellar occultations provide highly accurate size and shape information for TNOs, while photometry constrains their albedo and surface colours. (28978) Ixion is one of the largest TNOs and a prominent Plutino, making it a key target for comparative studies. Aims. The aim of this work is to constrain Ixion's projected size, shape, absolute magnitude, geometric albedo, and surface colours, and to search for evidence of an atmosphere or circum-object material. Methods. We conducted a series of campaigns targeting stellar occultations by Ixion between 2020 and 2023 as part of the Lucky Star collaboration, gathering 51 observations from eight events, including 30 positive detections. Five multi-chord events were used for a global limb fit, enabling an accurate reconstruction of Ixion's projected shape. Calibrated photometric data, including new and archival measurements, were analysed to derive its absolute magnitude, phase-curve parameters, and broadband colours. Results. The multi-chord occultations reveal a slightly elongated limb that is well represented by a single projected ellipse with semi-axes a = 363.42 −3.85/+3.53 km and b = 333.98 −4.96/+7.07 km, yielding R_equiv = 348.39 −4.43/+5.37 km (D_equiv = 696.78 −8.87/+10.75 km), and an apparent oblateness ∊' = 0.081 −0.010/+0.004. The geometry is consistent with a moderately flattened, nearly spheroidal body that may show slight departures from axial symmetry. The typical radial residuals (~10 km) support a largely stable shape across the observed epochs, with modest epoch-dependent variations. The phase-curve fit gives H_V = 3.845 ± 0.006, β = 0.1301 ± 0.0078 mag deg^−1, and p_V = 0.106 −0.003/+0.003. Multi-band photometry yields B − V = 1.06 ± 0.03, V − R = 0.61 ± 0.02, and R − I = 0.54 ± 0.03, which is consistent with moderately red TNO surfaces. No atmosphere or circum-object material was detected down to our sensitivity limits. The best-sampled event (13 October 2020) also allowed us to measure the angular diameter of the occulted star Gaia DR3 4056440205544338944, θ_⋆ = 0.670 ± 0.010 mas, which corresponds to R_⋆ = 128 ± 10 R_⊙ at the Gaia distance.