Neutrinos in colliding neutron stars and black holes

In this chapter, we provide an overview of the physics of colliding black holes and neutron stars and of the impact of neutrinos on these systems. Observations of colliding neutron stars play an important role in nuclear astrophysics today. They allow us to study the properties of cold nuclear matter and the origin of many heavy elements (gold, platinum, uranium). We show that neutrinos significantly impact the observable signals powered by these events as well as the outcome of nucleosynthesis in the matter that they eject into the surrounding intergalactic medium.

💡 Research Summary

This paper provides a comprehensive overview of the critical role played by neutrinos in the extreme astrophysical environments created by the collisions and mergers of compact objects, specifically binary neutron stars and neutron star-black hole systems. It establishes that understanding neutrino physics is not optional but essential for interpreting the multi-messenger signals from these events and for unraveling their broader cosmological significance.

The merger process begins with two compact objects inspiraling due to energy loss through gravitational wave emission, culminating in a violent collision. For neutron star binaries, this results in the formation of a hypermassive neutron star remnant or a prompt black hole, surrounded by a hot, dense accretion torus. Matter is ejected through various channels: tidal tails pulled off during the final orbits, shock-heated material from the collision interface, and later winds from the accretion disk. This ejected material is neutron-rich, originating from the neutron star interiors.

Immediately post-merger, the remnant and disk reach temperatures of tens of billions of Kelvin and immense densities. In these conditions, photons are trapped, and neutrino emission becomes the dominant cooling mechanism, influencing the thermal evolution and stability of the remnant. Beyond cooling, neutrino-matter interactions via weak processes (like neutron capture and positron/electron interactions) actively modify the composition of the matter. Specifically, they change the electron fraction (Y_e), which is the net ratio of electrons to baryons and a proxy for neutron richness.

The paper emphasizes that this change in Y_e is the primary link between neutrino physics and the merger’s observable consequences. The original ejecta is extremely neutron-rich (Y_e < 0.1). Neutrino interactions can increase Y_e, making the material less neutron-rich. The value of Y_e in the ejected matter is the master key determining the outcome of rapid neutron capture process (r-process) nucleosynthesis. Very low Y_e (≲0.25) allows the production of the heaviest elements, including gold, platinum, and uranium. Moderately low Y_e produces lighter heavy elements but not the actinides. To explain the observed abundance pattern of r-process elements in the universe, a mix of ejecta with different Y_e values is required, a diversity made possible by neutrino interactions.

Furthermore, the composition of the ejecta directly shapes the electromagnetic counterpart known as a kilonova. The radioactive decay of unstable r-process nuclei heats the expanding ejecta. As the ejecta becomes transparent, this energy emerges as a transient optical/infrared signal. The opacity, color, brightness, and duration of this kilonova are highly sensitive to the elemental composition of the ejecta, which is itself set by Y_e. Therefore, accurate neutrino physics is crucial for connecting kilonova observations to properties of the merging neutron stars, such as their internal equation of state.

The paper also touches on how neutrino physics connects to other fundamental questions. The amount and properties of the ejected matter depend on the neutron star’s size and mass, which are governed by the poorly understood equation of state of ultra-dense nuclear matter. Thus, neutrinos act as a critical intermediary: their effects must be precisely modeled to use electromagnetic observations (kilonovae) to constrain the nuclear equation of state. The chapter concludes by noting that advanced neutrino phenomena, like collective oscillations, could further influence the ejecta properties, presenting an exciting frontier for future research.

In summary, the paper argues that neutrinos are central actors in compact object mergers. They regulate the cooling and composition of post-merger remnants, control the production of heavy elements, and define the characteristics of the observable kilonova signal. A complete understanding of these cosmic collisions as laboratories for nuclear physics, tests of gravity, and cosmic element factories is impossible without a detailed accounting of neutrino interactions.