  Compact binary mergers, with neutron stars or neutron star and black-hole
components, are thought to produce various electromagnetic counterparts: short
gamma-ray bursts (GRBs) from ultra-relativistic jets followed by broadband
afterglow; semi-isotropic kilonova from radioactive decay of r-process
elements; and late time radio flares; etc. If the jets from such mergers follow
a similar power-law distribution of Lorentz factors as other astrophysical jets
then the population of merger jets will be dominated by low-{\Gamma} values.
The prompt gamma-rays associated with short GRBs would be suppressed for a
low-{\Gamma} jet and the jet energy will be released as X-ray/optical/radio
transients when a shock forms in the ambient medium. Using Monte Carlo
simulations, we study the properties of such transients as candidate
electromagnetic counterparts to gravitational wave sources detectable by
LIGO/Virgo. Approximately 78% of merger-jets result in failed GRB with optical
peaks 14-22 magnitude and an all-sky rate of 2-3 per year.
