10.3. Binary Neutron Stars vs. Black Hole - Neutron star Mergers

We have grouped together binary neutron star mergers and black-hole neutron star mergers. At present several neutron star binaries are known while no black hole - neutron star binary was found. Still on theoretical grounds one should expect a similar rate for both events [50]. Some even suggest that there are more black hole - neutron star binaries than neutron star-neutron star binaries [282]. There is a lot of similarity between the two processes which are both driven by gravitational radiation emission and both result in a single black hole. First, unless the mass of the black hole is of rather small the neutron star will not be tidally disrupted before it is captured by the black hole. Even if such a tidal disruption will take place then while in the binary neutron star merger we expect a collision, in the black hole - neutron star merger we expect at most a tidal disruption followed by infall of the debris on the black hole.

This could lead to a situation in which one of the two events will produce a GRB and the other will not. Presently it is too difficult to speculate which of the two is the right one. One should recall, however, that there is a marked difference between the gravitational signature of those events and thus hopefully when we discover a coincidence between a GRB and a gravitational radiation signal we would also be able to find which of the two mechanism is the right one.