Figure 2 shows a cartoon picture of the four primary EM signals that have been previously suggested as potential counterparts to binary neutron star mergers \cite{Metzger2012}.
  1. Short-Duration Gamma-Ray Burst: Given the plethora of circumstantial evidence connecting short-duration gamma-ray bursts (GRBs) to binary neutron star mergers, this was one of the first proposed counterparts for gravitational wave detections \cite{Eichler_1989}.
  2. Gamma-Ray Burst Afterglow: Even if the GRB itself is not detected (for instance, due to lack of sky coverage by high-energy satellites at the time of merger), the longer-lived X-ray, optical, and/or radio emission could be detected.  This includes both afterglows viewed "on-axis" (within the opening angle of the collimated jet), or even off-axis, when the jet spreads laterally and illuminates an increasing fraction of the sky (so-called "orphan" afterglows; \cite{Rhoads_1999}).
  3. Kilonova (also known as a "macronova"): Resulting from nucleosynthesis in the neutron-rich ejecta, this short-lived transient was predicted to be largely isotropic, and so was posed as a particularly promising counterpart \cite{Li_1998}.
  4. Merger Radio Remnant: Analogous to radio emission viewed at late times from supernovae (aka, the "remnant"), when the outgoing blastwave shock heats sufficient material in the circumstellar medium, the neutron-rich ejecta will eventually give rise to radio emission, though on relatively late time scales (~ years; \cite{Nakar_2011}).
I will discuss these potential counterparts in greater detail in the sections that follow.