The matter and light radiated away from black hole (BH) mergers will all come from the accretion discs of each BH. The accretion discs of the BH masses for whi

The matter and light radiated away from black hole (BH) mergers will all come from the accretion discs of each BH. The accretion discs of the BH masses for which LIGO (and Virgo) is most sensitive will generally be fairly sparse, so the emissions will generally be fairly dim. For a pair of similar-mass neutron stars (NS) that merge into a BH, you can treat region of spacetime close to the merger as being filled with extremely dense accretion discs. The density of matter leads very bright emissions, and the available geodesics produce radiation that will escape to infinity rather than being quickly absorbed close to the source (including within the dense matter around the newly formed BH as it settles into an accretion disc). The picture is slightly different where the NS masses are highly unequal. The Max Planck Institute for Gravitational Physics at Potsdam Germany has done many numerical simulations of NS mergers. NASA's animated one [1] where the NS are of substantially different masses: https://www.youtube.com/watch?v=vw2sLcyV7Vc https://www.youtube.com/watch?v=vw2sLcyV7Vc Systems of mass that are barbell-shaped (a pair of heavy masses connected via an arbitrarily thin bar; for orbiting stars and BHs we take the limit as the bar goes to zero volume and zero mass) will radiate gravitational waves when they are spun about an axis on an axis perpendicular to the bar. These spinning NSes will be radiating gravitational waves with increasing amplitude, and these radiated GWs remove angular momentum from the rotating system, allowing the NSes to move closer to each other. By the start of the video, the GW radiation being emitted should eventually be detectable by instruments at enormous distances; that radiation has allowed the two NSes to reach a critical proximity. At this point the smaller NS disintegrates under tidal stress and its additional mass-energy-momentum that then begins falling onto the larger NS causes the larger NS to collapse into a BH. A large proportion of the smaller NS remains in the region near the new BH and is swept up into an accretion structure alsong with s small proportion of the larger NS that did not get trapped behind the horizon. The accretion structure is briefly very bright, especially in gamma rays, as the matter from the smaller neutron star self-collides until it is entrained into a dense disc. Those early gammas will be visible at enormous distances (e.g. we can pick up extragalactic NS mergers with sky-scanning instruments searching for gamma ray bursts [2]). By comparison, the smaller of a pair of mass-mismatched black holes cannot disintegrate (everything is stuck within each BH's horizon), and the larger of the pair is likely to have a sparser accretion disc, so the amount of disc-disc collision will be relatively low. The reshaping of the accretion material around the merged BHs may be driven principally by the dynamical spacetime around the BH, with only occasional collisions. While such collisions can be arbitrarily energetic (at the point where their geodesics intersect bits of matter may be moving ultrarelativistically with respect to each other), there are unlikely to be enough such emissions to be reliably detectable at large distances. - -- [1] https://arxiv.org/abs/1001.3074 https://arxiv.org/abs/1001.3074 [2] There is a timing-coincidence argument about a short gamma ray burst detected in this way and a detection by LIGO & Virgo that is circulating around the rumour mill. Peter Coles blogged some detail https://telescoper.wordpress.com/2017/08/23/ligo-leaks-and-ngc-4993/ https://telescoper.wordpress.com/2017/08/23/ligo-leaks-and-n... This may be an NS-BH merger, in which the picture is again somewhat different, and depends on the density of the BH's accretion disc both in terms of its matter content and in terms of the momentum (e.g. if the NS and BH are counter-rotating, collisions will be more frequent and more energetic).