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 post-peak afterglow decline can distinguish between an initially ultra-relativistic jet viewed off-axis and a mildly relativistic wide-angle outflow .
Post-peak the afterglow flux will decline as F _ { \nu } \propto t ^ { - \alpha } .
The steepest decline for a jet afterglow is \alpha > 3 p / 4 or > ( 3 p + 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 } \lesssim 2 , is \alpha \lesssim ( 15 p - 19 ) / 10 or \alpha \lesssim ( 15 p - 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 ultra-relativistic jet viewed off the central axis , while a decline with \alpha \lesssim 1.4 after \sim 5 –10 peak times indicates that a wide angled and initially \Gamma _ { 0 } \lesssim 2 outflow is responsible .
At twice the peak time , the two outflow models fall on opposite sides of \alpha \approx 1 .
So far , two post-peak X-ray data points at 160 and 260 days suggest a decline consistent with an off-axis jet afterglow .
Follow-up observations over the next 1–2 years will test this model .