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Gravitational Wave Memory from the Relativistic Jet of Gamma-Ray Bursts

The gravitational wave (GW) memory from a radiating and decelerating point mass is studied in detail. It is found that for isotropic photon emission the memory generated from the photons is essentially the same with the memory from the point mass that radiated the photons so that it is anti-beamed. On the other hand, for anisotropic emission the memory from the photons may have a non-vanishing amplitude even if it is seen with small viewing angles. In the decelerating phases of gamma-ray burst (GRB) jets the kinetic energy of the jet is converted into the energy of gamma-ray photons. Then it would be possible to observe a variation in the GW memory associated with GRB jets on the timescale of the gamma-ray emission if the emission is partially anisotropic. Such an anisotropy in the gamma-ray emission has been suggested by the polarizations detected in recent observations of GRBs. The GW memory from GRB jets would provide clues to clarifying the geometry of the jets and the emission mechanism in GRBs. Thus it will be an interesting target for the next generation detectors of the GWs.