TITLE: GCN CIRCULAR NUMBER: 23139 SUBJECT: LIGO/Virgo GW170817: A steep decline in the radio light curve and prediction for the X-rays DATE: 18/08/13 18:38:03 GMT FROM: Tara Murphy at U of Sydney D. Dobie (University of Sydney), K. Mooley (Caltech), T. Murphy (University of Sydney), D. Kaplan (UWM), E. Lenc (CSIRO), A. Corsi (TTU), D. Frail (NRAO), report on behalf of a larger collaboration Our continued observations of GW170817 with the ATCA and the VLA up to 300 days post-merger (Mooley et al. in prep) confirm the t^(-2) decline in the radio light curve initially reported in Mooley et al. 2018 (arXiv:1806.09693). Such a slope rules out a cocoon-dominated outflow at late times, and is instead the classic signature of a relativistic jet, consistent with the VLBI result from Mooley et al. 2018. The t^(-2) decline is also expected in the X-ray light curve, and may be confirmed by the Chandra observation carried out on 2018 Aug 10.