Colliding neutron stars – Gravitational waves, neutrino emission, and gamma-ray bursts – by
M. Ruffert1 and H.-Th. Janka2

This document discusses the collision of neutron stars in the context of their potential to generate gravitational waves, neutrino emissions, and gamma-ray bursts. Here are the key points:

  1. Hydrodynamical Simulations: The study uses three-dimensional hydrodynamical simulations to analyze the direct head-on or off-center collisions of neutron stars. These simulations employ a Newtonian PPM code and include the emission of gravitational waves and their impact on the hydrodynamical flow​​.
  2. Gravitational Waves and Neutrino Emissions: The simulations predict gravitational wave signals, luminosities, and waveforms. They show an extremely luminous burst of neutrinos with a peak luminosity of more than 4×10544 \times 10^{54} erg/s for several milliseconds, leading to an average energy deposition rate of more than 105210^{52} erg/s and a total energy of about 105010^{50} erg deposited in electron-positron pairs around the collision site within 10 milliseconds​​.
  3. Gamma-Ray Burst Scenarios: Despite the favorable conditions for gamma-ray bursts (GRBs) in terms of energy release, the study finds that the pollution of the electron-positron pair plasma cloud with dynamically ejected baryons is five orders of magnitude too large. This baryon pollution prevents the formation of a relativistically expanding fireball necessary for GRBs, thus ruling out colliding neutron stars as sources of GRBs powered by neutrino emission​​.
  4. Detailed Collision Dynamics: Upon collision, a strong shock wave is generated, causing significant temperature increases and entropy changes. Matter is squeezed out perpendicularly to the collision axis, expanding behind a strong shock and emitting large numbers of electron antineutrinos. The study also describes the formation and eventual dissipation of a thin “pancake” layer of high-temperature matter at the collision interface​​.
  5. Summary and Implications: The simulations demonstrate that while the energy deposition from neutrino-antineutrino annihilation is substantial, the resulting baryon loading is too high to allow for the relativistic expansion needed for gamma-ray bursts. Thus, colliding neutron stars are unlikely to be the central engines of GRBs due to this baryon pollution problem​​.

These findings provide significant insights into the behavior of colliding neutron stars and their observable consequences, particularly regarding gravitational waves and neutrino emissions. However, they also highlight the challenges in associating these collisions with gamma-ray bursts due to the issues with baryon loading.