Shock-powered radio precursors of neutron star mergers from accelerating relativistic binary winds

During the final stages of a compact object merger, if at least one of the binary components is a magnetized neutron star (NS), then its orbital motion substantially expands the NS's open magnetic flux - and hence increases its wind luminosity - relative to that of an isolated pulsar. As the binary orbit shrinks due to gravitational radiation, the power and speed of this binary-induced inspiral wind may (depending on pair loading) secularly increase, leading to self-interaction and internal shocks in the outflow beyond the binary orbit. The magnetized forward shock can generate coherent radio emission via the synchrotron maser process, resulting in an observable radio precursor to binary NS merger. We perform 1D relativistic hydrodynamical simulations of shock interaction in the accelerating binary NS wind, assuming that the inspiral wind efficiently converts its Poynting flux into bulk kinetic energy prior to the shock radius. This is combined with the shock maser spectrum from particle-in-cell simulations, to generate synthetic radio light curves. The precursor burst with a fluence of similar to 1 Jy center dot ms at similar to GHz frequencies lasts similar to 1-500 ms following the merger for a source at similar to 3 Gpc (B-d/10(12) G)(8/9), where B-d is the dipole field strength of the more strongly magnetized star. Given an outflow geometry concentrated along the binary equatorial plane, the signal may be preferentially observable for high-inclination systems, that is, those least likely to produce a detectable gamma-ray burst.

Automated summary

The merging of magnetized neutron stars can increase their wind luminosity and potentially lead to observable radio emission precursors. Through simulations, synthetic radio light curves can be generated with precursor bursts lasting around 1-500 milliseconds. For systems with high inclinations, the outflow concentrated along the binary equatorial plane may make the signal more readily observable.