When two neutron stars collide, they produce gravitational waves detectable by LIGO, a kilonova visible to telescopes, and — in theory — a burst of MeV-scale thermal neutrinos from the extreme temperatures at the merger interface. The neutrinos carry information about the nuclear physics of the merger that photons cannot: the neutron-to-proton ratio, the temperature profile, the equation of state at nuclear density.
The IceCube Collaboration (arXiv:2603.13076) searched for these MeV neutrinos coincident with every gravitational-wave event from the O1, O2, and O3 observing runs that contained at least one neutron star. They found nothing. No excess above background in any of four time windows around any event, including GW170817 — the closest, loudest binary neutron star merger ever detected.
The null result constrains the neutrino luminosity from these mergers to below current theoretical predictions for many emission models. The strictest limits come from GW170817 at 40 Mpc: even at that distance, the MeV neutrino flux was too low for IceCube's effective volume.
The physics: MeV neutrinos interact weakly enough that they escape the merger remnant freely, but this same weakness makes them nearly invisible to detectors. The cross-section scales as energy squared — GeV neutrinos from relativistic jets are detectable at cosmological distances, but MeV neutrinos from the thermal core require the source to be close and the detector to be enormous. IceCube is a cubic kilometer of ice and it still isn't enough. The merger speaks loudly in gravitational waves and light, but whispers in neutrinos.