Inspiraling and merging binary neutron stars ( BNSs ) are important sources of both gravitational waves and coincident electromagnetic counterparts .
If the BNS total mass is larger than a threshold value , a black hole ensues promptly after merger .
Through a statistical study in conjunction with recent LIGO/Virgo constraints on the nuclear equation of state , we estimate that up to \sim 25 \% of BNS mergers may result in prompt collapse .
Moreover , we find that most models of the BNS mass function we study here predict that the majority of prompt-collapse BNS mergers have q \gtrsim 0.8 .
Prompt-collapse BNS mergers with mass ratio q \gtrsim 0.8 may not be accompanied by either kilonovae or short gamma-ray bursts , because they unbind a negligible amount of mass and form negligibly small accretion disks onto the remnant black hole .
We call such BNS mergers “ orphan ” .
However , recent studies have found that { 10 ^ { 41 - 43 } ( B _ { p } / 10 ^ { 12 } G ) ^ { 2 } erg s ^ { -1 } } electromagnetic signals can be powered by magnetospheric interactions several milliseconds prior to merger .
Moreover , the energy stored in the magnetosphere of an orphan BNS merger remnant will be radiated away in { \mathcal { O } } ( 1 ms ) .
Through simulations in full general relativity of BNSs endowed with an initial dipole magnetosphere , we find that the energy in the magnetosphere following black hole formation is E _ { B } \sim 10 ^ { 40 - 42 } ( B _ { p } / 10 ^ { 12 } G ) ^ { 2 } erg .
Radiating \sim 1 \% of E _ { B } in 1 ms , as has been found in previous studies , matches the premerger magnetospheric luminosity .
These magnetospheric signals are not beamed , and their duration and power agrees with those of non-repeating fast radio bursts ( FRBs ) .
These results combined with our statistical study suggest that a non-repeating , precursor FRB may be the most likely electromagnetic counterpart of prompt-collapse BNSs .
Detection of a non-repeating FRB coincident with gravitational waves from a BNS merger may settle the extragalactic origin of FRBs and can place constraints on the nuclear equation of state .
FRBs can also initiate triggered searches for weak signals in the LIGO/Virgo data .