Compact binary mergers with neutron star and/or black-hole components are the progenitor of short gamma-ray bursts (GRB) and a candidate for gravitational waves (GW) detectable by advanced LIGO/Virgo.
By assuming a power-law distribution of Lorentz factor (Gamma) for a population of post-merger outflows, we show that low-Gamma jets produce on-axis orphan afterglow for failed GRB.
These afterglow are a suitable candidate for electromagnetic (EM) counterpart searches to GW triggers.
GRB will fail if the dissipative radius is less than the photospheric radius; adiabatic cooling, and thermalisation of the spectrum below the photosphere suppresses the prompt gamma-ray emission.
The fraction of failed GRB from a population of mergers within the aLIGO/Virgo detection volume is strongly dependent on the Gamma distribution.
Assuming a typical astrophysical jet Gamma distribution of index -1.5 for post-merger outflow of events within 300 Mpc, ~65% result in failed GRB.
Observations of GRB indicate low-Gamma outflows have wider opening-angles; for an isotropic distribution, low-Gamma jets are more likely oriented towards an observer than standard GRB.
As GW are strongest on-axis, the probability of EM counterparts oriented towards an observer is greater than the isotropic case.
By assuming 20 degrees for all failed GRB jet opening angles, and given a GW detection, ~20% will be oriented towards an observer.
The rate of on-axis orphan afterglow with an optical peak brighter than magnitude 21 for 40(400) mergers per year would be 4(40).
We show that the peak flux is typically magnitude ~18, and emission peaks ~30 hours after a merger.