Nuclear Physics with Neutron Star Mergers: The Spectacular Case of GW170817
Abstract: On August 17, 2017, the LIGO/Virgo collaboration detected gravitational waves from a new type of system: a binary neutron star merger. Two seconds later, a short gamma-ray burst was observed, allowing accurate localization and follow-up observations across the entire electromagnetic spectrum, from high-energy gamma rays to radio. Optical and infrared observations of this event detected a new type of transient: a kilonova, powered by radioactivities of freshly synthesized r-process elements. This event and potential similar events in the future will allow us to see the r-process "in-the-making" and constrain nuclear physics in extreme regimes not accessible in the laboratory or via pulsar observations. With more detections, we might be able to finally resolve the mystery of the main r-process production site.
I will present an outline of a new multiphysics model, developed at Los Alamos National Laboratory, which links the kilonova to the properties of ejected material, such as mass, velocity, orientation, and, most interestingly, nuclear composition. The key sensitivity in kilonova modeling is the nuclear heating and thermalization of generated energy, which in turn depends on the nuclear structure in the neutron-rich regime, potentially accessible to FRIB. The composition and heating are computed with the WinNet r-process network and passed to the multidimensional radiative transport code SuperNu to synthesize optical/infrared spectra with detailed composition-dependent atomic opacities.
For the case of GW170817, the ejecta clearly separates itself into two distinct components, in line with predictions of numerical relativity. The two-dimensional character of our model allows us to infer the approximate orientation of the system. Using the nucleosynthesis network, we can robustly constrain neutron richness of the ejecta.
In the future, advances in FRIB together with more detections will tighten such constraints and impact current uncertainties in nuclear structure, for example models of nuclear mass and fission. Finally, I will briefly discuss other potential implications, such as those for the nuclear equation of state.