  The merger of two neutron stars results often in a rapidly and differentially
rotating hypermassive neutron star (HMNS). We show by numerical-relativity
simulation that the magnetic-field profile around such HMNS is dynamically
varied during its subsequent evolution, and as a result, electromagnetic
radiation with a large luminosity ~ 0.1 B^2 R^3 Omega is emitted with baryon
(B, R, and Omega are poloidal magnetic-field strength at stellar surface,
stellar radius, and angular velocity of a HMNS). The predicted luminosity of
electromagnetic radiation, which is primarily emitted along the magnetic-dipole
direction, is ~ 10^{47} (B/10^{13} G)^2(R/10 km)^3(Omega/10^4 rad/s) ergs/s,
that is comparable to the luminosity of quasars.
