The energy density of gravitational waves is only large enough to create matter in the vicinity of the collision. The energy density dissipates by spreading out

The energy density of gravitational waves is only large enough to create matter in the vicinity of the collision. The energy density dissipates by spreading out quickly. Astronomers are looking for such photon signatures simultaneous with collision. With two LIGO detectors we can determine the collision time, but the sky location to within an arc. The third detector will give more location precision. Several more are under construction.