A year in the life of GW 170817: the rise and fall of a structured jet from a binary neutron star merger

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 Chandra X-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 ∝t^0.9, the afterglow displayed a smooth turnover, reaching a peak X-ray luminosity of L_X ≈ 5 × 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 E_K ≈ 2 × 10⁵⁰ erg, jet width θ_c ≈ 4°, and ambient density n ≈ 3 × 10^-3 cm^-3) fit well within the range of properties of cosmological short gamma-ray bursts.