How Neutrinos Shape the Fate of Merging Stars express
The collision of two neutron stars is a cosmic laboratory where matter is compressed to unimaginable densities. In the first moments after the merger, a hot cloud forms and cools by emitting neutrinos. These ghostly particles carry away energy and dictate how much matter is ejected into space and which heavy elements are synthesized.
Until now, computer simulations described neutrinos approximately: their energy was averaged, and complex reactions were simplified. The authors of the study made the model more realistic step by step. They allowed neutrinos to scatter off electrons — much like a billiard ball changes its trajectory after hitting another ball. And they calculated more precisely how neutrino-antineutrino pairs turn into radiation.
The result was surprising: even small corrections lowered the average energy of heavy neutrinos and increased the ejected mass by 50%—though from a very modest initial level (less than 0.005 solar masses). When the scientists slightly varied the total mass of the binary system, the ejecta either strengthened or nearly vanished, like a faucet being turned on and off.
This approach, though it adds noise to the calculations, for the first time allowed tracking fine details of the neutrino spectrum without excessive computational cost.
Original abstract
as written by the authors arXiv:2606.27425
Multimessenger observations of neutron star mergers are unique opportunities to constrain the properties of dense matter and the production site of heavy nuclei. To leverage these observations, we require reliable models of the electromagnetic signals powered by mergers. An important limitation to our ability to develop such models is the use of approximate neutrino physics in simulations. Here, we present simulations using an improved version of our Monte Carlo transport algorithm specifically designed to allow for more advanced on-the-fly calculations of reaction rates that use the simulated energy distribution of neutrinos, including in blocking factors, while still relying on approximations for the angular distribution of neutrinos. We use these new methods to include in simulations inelastic scattering of neutrinos on electrons, and to improve our treatment of neutrino-antineutrino pair annihilation. We find that, without increasing the cost of simulations, we can marginally get to the point when the addition of a single packet represents a change $$\Delta f_ u<1$$ in the angle-integrated distribution function, at the cost of increased shot noise in the coupling to the fluid. With inelastic scattering and a better treatment of pair processes, we find a reduction in the average energy and total luminosity of heavy-lepton neutrinos, and an increase in the amount of mass ejected -- here by $$50\%$$, although on a relatively low amount of total ejected mass $$<0.005M_\odot$$. In a separate set of simulations varying the total mass of the binary away from its prompt collapse threshold, we find rapid variations in the amount of ejected matter and in the geometry and composition of the outflows with the total mass of the system. Finally, we use the simulations with our more advanced transport scheme to study in more detail the energy spectrum of neutrinos across the merger remnant.
In a single second, a neutron star merger produces more neutrinos than our Sun will emit over its entire lifetime.
In Peter Watts's novel Blindsight, neutrino communication is used to talk with an alien spacecraft, so ubiquitous and penetrating are these particles.