• Corpus ID: 235125645

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

@inproceedings{Sarin2021CDFSXT,
  title={CDF-S XT1: The off-axis afterglow of a neutron star merger at \$z=2.23\$},
  author={Nikhil Sarin and Gregory Ashton and Paul D. Lasky and Kendall Ackley and Y.-L. Mong and Duncan K. Galloway},
  year={2021}
}

• N. Sarin, G. Ashton, +3 authors D. Galloway
• Published 21 May 2021
• Physics

CDF-S XT1 is a fast-rising non-thermal X-ray transient detected by Chandra in the Deep-Field South Survey. Although various hypotheses have been suggested, the origin of this transient remains unclear. Here, we show that the observations of CDF-S XT1 are well explained as the X-ray afterglow produced by a relativistic structured jet viewed off-axis. We measure properties of the jet, showing that they are similar to those of GRB170817A, albeit at cosmological distances. We measure the observers viewing angle to be θobs = 10 ◦ ± 3◦ and the core of the ultra-relativistic jet to be θcore = 4.4◦ ± 0.9◦, where the uncertainties are the 68% credible interval. The inferred properties and host galaxy combined with Hubble, radio, and optical non detections favour the hypothesis that CDF-S XT1 is the off-axis afterglow of a binary neutron star merger. We find that other previously suggested hypotheses are unable to explain all properties of CDF-S XT1. At a redshift of z = 2.23, this is potentially the most distant observed neutron star 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 high redshift for the star-formation rate in the early Universe, the nucleosynthesis of heavy elements, and the prospect of identifying other off-axis afterglows.