2019MNRAS.489.1919T

We present the results of our year-long afterglow monitoring of GW 170817, the first binary neutron star merger detected by Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) and Advanced Virgo. New observations with the Australian Telescope Compact Array and the ChandraX-ray Telescope were used to constrain its late-time behaviour. The broad-band emission, from radio to X-rays, is well-described by a simple power-law spectrum with index β ∼ 0.585 at all epochs. After an initial shallow rise ∝t0.9, the afterglow displayed a smooth turnover, reaching a peak X-ray luminosity of LX ≃ 5 x 1039 erg s-1 at 160 d, and has now entered a phase of rapid decline, approximately ∝t-2. The latest temporal trend challenges most models of choked jet/cocoon systems, and is instead consistent with the emergence of a relativistic structured jet seen at an angle of ≃22* from its axis. Within such model, the properties of the explosion (such as its blast wave energy EK ≃ 2 x 1050 erg, jet width θc ≃ 4*, and ambient density n ≃ 3 x 10-3 cm-3) fit well within the range of properties of cosmological short gamma-ray bursts.