Magnetized Outflows from Neutron Star Mergers and Collapsars award

The merger of two extremely dense stellar remnants, or neutron stars, was observed for the first time through gravitational waves and electromagnetic light in 2017. Although this event, named GW 170817, has been studied extensively, there are aspects of the explosion, called a 'kilonova,' which are still not well understood. Using detailed computer modeling, a research team at Columbia University will investigate in detail how the merged stars may have produced a new neutron star remnant (before ultimately succumbing to gravity and collapsing to a black hole). They will explore how explosive ejecta flowed out from the merger site, producing heavy atomic nuclei, whose radioactive decay powered the kilonova. In parallel they will study similar physical conditions that occur following the collapse of a massive star at the end of its nuclear burning life when the star is rotating very rapidly (a so-called 'collapsar'). This work will probe extreme physical processes such as relativistic spacetime around black holes and the extreme densities found in the cores of massive neutron stars. The project also has implications for the origin of the heaviest elements in the Universe. Cataclysmic events, such as neutron star mergers, capture the imagination of students, researchers, and the public alike. Research and educational goals will be integrated in three ways: undergraduate student research, a week-long international school on time-domain gravitational wave astrophysics, and a website disseminated to local teachers for use as classroom educational tools.

The binary neutron star merger GW170817 was accompanied by thermal emission ('kilonova'), powered by the radioactive decay of heavy nuclei synthesized via the rapid neutron capture process (r-process). However, the quantity, composition, and velocity of the ejecta needed to match the observations disagree with those predicted by numerical simulations of the dynamical merger phase. Instead, the bulk of the ejecta is best explained as originating in outflows over longer timescales during the post-merger phase, from either a strongly-magnetized neutron star remnant ('millisecond magnetar') or the neutrino-cooled accretion torus surrounding the compact object. The researchers will explore the properties and kilonova signatures of neutrino-heated magnetized outflows from millisecond magnetar remnants of neutron star mergers and collapsars, both in isolation as well as surrounded by magnetized accretion disks, using three-dimensional general-relativistic magnetohydrodynamical (GRMHD) simulations. The numerical code will be upgraded to include a realistic equation of state (bridging the interior of the neutron star to the lower density wind region) and a neutrino transport scheme to capture the neutrino-driven mass-loading of the winds and the evolution of its electron fraction. A suite of simulations, performed for different neutrino luminosities and mass accretion rates, will cover different epochs in the post-merger evolution. The simulations will address the relative importance and interplay between outflows from the star versus the disk, and their respective baryon-loading.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.