Pair Fireball Precursors of Neutron Star Mergers

Brian D. Metzger, Charles Zivancev

Published 2016-05-03Version 1

If at least one neutron star (NS) is magnetized in a binary NS merger, then the orbital motion of the conducting companion through its dipole field during the final inspiral induces a strong voltage and current along the magnetic field lines connecting the two objects. If a modest fraction eta of the electromagnetic power extracted during the inspiral is used to accelerate relativistic particles, the resulting gamma-ray emission in such a compact volume will result in the formation of a thermal electron-positron pair fireball. Applying the steady-state pair wind model of Paczynski (1986), we quantify the luminosities and temperatures of the precursor fireball and its detectability with gamma-ray satellites. Under the assumption that eta ~ 1, the gamma-ray detection horizon of Dmax ~ 20(Bd/1e14 G) is much closer than the Advanced LIGO/Virgo horizon of 200 Mpc, unless the surface magnetic field of the NS is very strong, Bd > 1e15 G. Given the quasi-isotropic nature of the emission, a sub-population of mergers with somewhat weaker NS fields could contribute a nearby population of short gamma-ray bursts, providing a possible explanation for the correlation between the sky positions of BATSE short bursts and local cosmic structure. The fraction 1-eta of the power which is not dissipated close to the binary is carried to infinity along the open field lines by a large scale Poynting flux. Reconnection within this flow, well outside of the pair photosphere, provides a potential site for non-thermal emission, such as a coherent millisecond radio burst.