We show the peak magnitude for orphan afterglows from the jets of gravitational wave ( GW ) detected black-hole/neutron star - neutron star ( BH/NS-NS ) mergers highly depends on the jet half-opening angle \theta _ { j } .
Short \gamma -ray bursts ( GRB ) with a homogeneous jet structure and \theta _ { j } > 10 ^ { \circ } , the orphan afterglow viewed at the typical inclination for a GW detected event , 38 ^ { \circ } , is brighter at optical frequencies than the comparable macronova emission .
Structured jets , where the energetics and Lorentz factor \Gamma vary with angle from the central axis , may have low- \Gamma components where the prompt emission is suppressed ; GW electromagnetic ( EM ) counterparts may reveal a population of failed-GRB orphan afterglows .
Using a Monte Carlo method assuming a NS-NS detection limit we show the fraction of GW-EM counterparts from homogeneous , two-component , power-law structured , and Gaussian jets where the variable structure models include a wide low energy and \Gamma component : for homogeneous jets , with a \theta _ { j } = 6 ^ { \circ } and typical short GRB parameters , we find r -band magnitude m _ { r } \leq 21 counterparts for \sim 13.6 \% of GW detected mergers ; where jet structure extends to a half-opening angle of 25 ^ { \circ } , two-component jets produce m _ { r } \leq 21 counterparts in \sim 30 \% of GW detected mergers ; power-law structured jets result in \sim 37 \% ; and Gaussian jets with our parameters \sim 13 \% .
We show the features in the lightcurves from orphan afterglows can be used to indicate the presence of extended structure .