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CDF-S XT1 is a fast-rising non-thermal X-ray transient detected by\textit{Chandra} in the Deep-Field South Survey. Although various hypotheseshave been suggested, the origin of this transient remains unclear. Here, weshow that the observations of CDF-S XT1 are well explained as the X-rayafterglow produced by a relativistic structured jet viewed off-axis. We measureproperties of the jet, showing that they are similar to those of GRB170817A,albeit at cosmological distances. We measure the observers viewing angle to be$\theta_{\textrm{obs}} = 10^{\circ}\pm3^{\circ}$ and the core of theultra-relativistic jet to be $\theta_{\textrm{core}} =4.4^{\circ}\pm0.9^{\circ}$, where the uncertainties are the $68\%$ credibleinterval. The inferred properties and host galaxy combined with Hubble, radio,and optical non detections favour the hypothesis that CDF-S XT1 is the off-axisafterglow of a binary neutron star merger. We find that other previouslysuggested hypotheses are unable to explain all properties of CDF-S XT1. At aredshift of $z=2.23$, this is potentially the most distant observed neutronstar merger to date and the first orphan afterglow of a short gamma-ray burst.We discuss the implications of a binary neutron star merger at such a highredshift for the star-formation rate in the early Universe, the nucleosynthesisof heavy elements, and the prospect of identifying other off-axis afterglows.

CDF-S XT1: The off-axis afterglow of a neutron star merger at $z=2.23$