Revisiting orbit fits for the Eris-Dysnomia system

Eris and Dysnomia system is the quintessential binary of the Kuiper Belt. The most recent results suggest that the system is doubly synchronous (Szakats et al., 2023) with the principal period of P=15.8 days. The orbit of Dysnomia shows a non-Keplerian behavior (Holler et al., 2021), possibly introduced by the data systematics, shape effects of Eris and/or Dysnomia, or yet to be discovered inner satellite. We will present our orbit fit to the relative astrometry published in Holler et al., (2021) and discuss modeling of various hypotheses. For example, we investigated center-of-body vs center-of-light effects for various albedo models for Eris, but these do not introduce significant change in the residuals. Our preferred scenario is that there is an unseen inner moon in the mean-motion resonance with Dysnomia. We modeled presence of the inner satellite as c22, and we investigated the most likely location for this object based on root-mean-square of the residuals. The data arc extends from 2005-2018 during which the in-orbit differences between the purely Keplerian and non-Keplerian orbit fits grows to ~1500 km, or equivalent to 20 mas. The orbit fit is particularly sensitive to the high-precision (few mas) astrometry obtained in 2015-2018. We will discuss current orbital uncertainties and their impact on potential non-Keplerian nature of the orbit of Dysnomia.