The inspiral and coalescence of double neutron star ( NS-NS ) and neutron star-black hole ( NS-BH ) binaries are likely to be detected by advanced networks of ground-based gravitational wave ( GW ) interferometers .
Maximizing the science returns from such a discovery will require the identification and localization of an electromagnetic ( EM ) counterpart .
Here we critically evaluate and compare several possible counterparts , including short-duration gamma-ray bursts ( SGRBs ) , “ orphan ” optical and radio afterglows , and \sim day-long optical transients powered by the radioactive decay of heavy nuclei synthesized in the merger ejecta ( “ kilonovae ” ) .
We assess the promise of each potential counterpart in terms of four “ Cardinal Virtues ” : detectability , high fraction , identifiability , and positional accuracy .
For viewing angles within the half-opening angle of the jet ( \theta _ { obs } \lesssim \theta _ { j } ) the SGRB and associated afterglow are easily detectable within the range of Advanced LIGO/Virgo if the jet energy ( E _ { j } ) and the circumburst density ( n ) are similar to those inferred from existing SGRB observations .
For modest off-axis angles ( \theta _ { obs } \lesssim 2 \theta _ { j } ) , the orphan optical afterglow is detectable with LSST if E _ { j, 50 } n _ { 0 } ^ { 7 / 8 } \gtrsim 0.002 ; the fraction of such events is \sim 7 \theta _ { j } ^ { 2 } \sim 0.1 .
At even larger viewing angles ( i.e. , the majority of observers ) the isotropic kilonova emission dominates , with a peak optical brightness of \sim 19 - 22 mag within the Advanced LIGO/Virgo volume , detectable with LSST using a specialized 1-day cadence .
Radio afterglow emission from an initially off-axis jet or from sub-relativistic ejecta is also isotropic , but peaks on a timescale of months-years ; this signal is detectable provided that E _ { j, 50 } n _ { 0 } ^ { 7 / 8 } ( v / c ) ^ { 11 / 4 } \gtrsim 0.2 ( for off-axis afterglows , v / c \sim 1 ) .
However , existing SGRB afterglows do not satisfy this criterion , indicating a low probability of radio detections .
Taking into account the search strategy for typical error regions of tens of square degrees , our primary conclusion is that SGRBs are the most useful EM counterparts to confirm the cosmic origin of a few GW events , and to test the association with NS-NS/NS-BH mergers .
However , for the more ambitious goal of localizing and obtaining redshifts for a large sample of GW events , kilonovae are instead preferred .
Off-axis optical afterglows will be detectable for at most \sim 10 \% of all events , while radio afterglows are promising only for the unique combination of energetic relativistic ejecta in a high density medium , and even then will require hundreds of hours of EVLA time per event spread over months-years .
Our main recommendations from this analysis are : ( i ) an all-sky \gamma -ray satellite is essential for temporal coincidence detections , and for GW searches of \gamma -ray triggered events ; ( ii ) LSST should adopt a 1-day cadence follow-up strategy , ideally with \sim 0.5 hr per pointing to cover GW error regions ( the standard 4-day cadence and depth will severely limit the probability of a unique identification ) ; and ( iii ) radio searches should only focus on the relativistic case , which requires observations for a few months .