Ultrarelativistic electromagnetic counterpart to binary neutron star mergers

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 $\sim 10^{-2}$ 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.