We propose a possibility of ultrarelativistic electromagnetic counterparts to gravitational waves from binary neutron star mergers at nearly all the viewing angles .
Our proposed mechanism relies on the merger-shock propagation accelerating a smaller mass in the outer parts of the neutron star crust to a larger Lorentz factor \Gamma with smaller energy \sim 10 ^ { 47 } \Gamma ^ { -1 } erg .
This mechanism is difficult to resolve by current 3D numerical simulations .
The outflows emit synchrotron flares for seconds to days by shocking the ambient medium .
Ultrarelativistic flares shine at an early time and in high-energy bands , potentially detectable by current X-ray to radio instruments , such as Swift XRT and Pan-STARRS , and even in low ambient density \sim 10 ^ { -2 } \mathrm { cm } ^ { -3 } by EVLA .
The flares probe the merger position and time , and the merger types as black hole–neutron star outflows would be non-/mildly relativistic .