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 F-v proportional to t(-alpha). The steepest decline for a jet afterglow is alpha > 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 Gamma(0) less than or similar to 2, is alpha less than or similar to (15p - 19)/10 or alpha less than or similar to (15p - 18)/10, in the respective spectral regimes. If the afterglow from GW170817 fades with a maximum index alpha > 1.5, then we are observing the core of an initially ultrarelativistic jet viewed off the central axis, while a decline with alpha less than or similar to 1.4 after similar to 5-10 peak times indicates that a wide-angled and initially Gamma(0) less than or similar to 2 outflow is responsible. At twice the peak time, the two outflow models fall on opposite sides of alpha approximate to 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.