occultation events by (307261) 2002 MS4

Using 13 selected chords from the 8 August 2020 event, we determined the global elliptical limb of 2002 MS4. The best-fitted ellipse, combined with the object's rotational information from the literature, constrains the object's size, shape, and albedo. Additionally, we developed a new method to characterize topography features on the object's limb.

The global limb has a semi-major axis of 412+/-10km, a semi-minor axis of 385+/-17km, and the position angle of the minor axis is 121{deg}+/-16{deg}. From this instantaneous limb, we obtained 2002 MS4's geometric albedo of p_V_=0.1+/-0.025 using an H_V_=3.63+/-0.05mag and a projected area-equivalent diameter of 796+/-24km. Significant deviations from the fitted ellipse in the northernmost limb are detected from multiple sites highlighting three distinct topographic features: one 11 km depth depression followed by a 25^+4^_-5_km height elevation next to a crater-like depression with an extension of 322+/-39km and 45.1+/-1.5km deep.

Our results present an object that is ~138km smaller in diameter than derived from thermal data, possibly indicating the presence of a so-far unknown satellite. However, within the error bars, the geometric albedo in the V-band agrees with the results published in the literature, even with the radiometric-derived albedo. This stellar occultation allowed for a multichord measurement of a large topography in a TNO for the first time.