Mergers of binaries involving neutron stars and/or black holes are the most likely source of gravitational waves for upcoming ground based interferometers such as Advanced LIGO. The sky localization of most events is expected to be coarse, hence a simultaneous electromagnetic counterpart would greatly help in decreasing uncertainties, and would provide complementary information about the system.
The most promising electromagnetic counterpart form these mergers is a supernova-like transient powered by the radioactive decay of r-process elements generated during the expansion of the non-relativistic merger ejecta (a kilonova).
The properties of the kilonova are very sensitive to the composition of the material through the optical opacity: matter containing tiny amounts of heavy r-process material leads to a transient peaking in the infrared band and on timescale of weeks, whereas matter with lighter composition peaks in the optical band on a timescale of days
The figure shows mass histograms constructed with material from a disk wind, with the central object being a black hole of variable spin. Shown are different quantities that determine the nucleosynthesis properties: electron fraction, entropy, expansion time, and velocity. Increasing the spin of the black hole leads to longer neutrino irradiation and hence less neutron-rich material, yielding a ligher composition (and thus lower opacity material).
More information here.