Late-time Evolution of Afterglows from Off-Axis Neutron-Star Mergers

Gravitational-wave-detected neutron star mergers provide an opportunity to investigate short gamma-ray burst (GRB) jet afterglows without the GRB trigger. Here we show that the postpeak afterglow decline can distinguish between an initially ultrarelativistic jet viewed off-axis and a mildly relativistic wide-angle outflow. Post-peak the afterglow flux will decline as Fν ∝ t −α. The steepest decline for a jet afterglow is α > 3p/4 or > (3p + 1)/4, for an observation frequency below and above the cooling frequency, respectively, where p is the power-law index of the electron energy distribution. The steepest decline for a mildly relativistic outflow, with initial Lorentz factor 0 2, is α (15p − 19)/10 or α (15p − 18)/10, in the respective spectral regimes. If the afterglow from GW170817 fades with a maximum index α > 1.5, then we are observing the core of an initially ultrarelativistic jet viewed off the central axis, while a decline with α 1.4 after ∼5–10 peak times indicates that a wide-angled and initially 0 2 outflow is responsible. At twice the peak time, the two outflow models fall on opposite sides of α ≈ 1. So far, two post-peak X-ray data points at 160 and 260 d suggest a decline consistent with an off-axis jet afterglow. Follow-up observations over the next 1–2 yr will test this model.