Anisotropic mass ejection from black hole-neutron star binaries: Diversity of electromagnetic counterparts

The merger of black hole-neutron star binaries can eject substantial material with the mass $\ensuremath{\sim}0.01--0.1{M}_{\ensuremath{\bigodot}}$ when the neutron star is disrupted prior to the merger. The ejecta shows significant anisotropy, and travels in a particular direction with the bulk velocity $\ensuremath{\sim}0.2c$. This is drastically different from the binary neutron star merger, for which ejecta is nearly isotropic. Anisotropic ejecta brings electromagnetic-counterpart diversity which is unique to black hole-neutron star binaries, such as viewing-angle dependence, polarization, and proper motion. The kick velocity of the black hole, gravitational-wave memory emission, and cosmic-ray acceleration are also discussed.

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