University of Pennsylvania Physics & Astronomy Colloquium

Roughly half of the elements heavier than iron in the universe are forged in extreme, neutron-rich environments where nuclei capture neutrons faster than they can undergo beta decay—the so-called r-process. The discovery of "kilonova" emission following the gravitational wave event GW170817 established binary neutron star mergers as an important r-process site, but several observations suggest that additional sources may be required, particularly to explain the abundances of low-metallicity stars.

In neutron star mergers, heavy elements form in outflows from the accretion disk that feeds the newly formed black hole. Broadly similar neutron-rich accretion flows are created in "collapsars"—the explosions of massive, rotating stars.  Particularly massive collapsar disks can become gravitationally unstable and fragment, forming swarms of low-mass neutron stars that coalesce via gravitational waves hierarchically within the same disk environment, potentially triggering a "multi messenger symphony".  The recent discovery of candidate sub-solar mass neutron star merger events by LIGO/Virgo in association with supernovae may support this scenario. 

A different pathway to heavy-element nucleosynthesis arises in the giant flares from highly magnetized neutron stars (magnetars), where crustal material can be ejected into space.  I will present evidence, in the form of a previously unexplained MeV gamma-ray signal, that the famous Galactic magnetar giant flare from December 2004 synthesized ~1e-6 solar masses of r-process nuclei.  Radioactive decay of these ejecta also powers a very short-lived (~minutes) UV/optical transient—a “mini-kilonova”, which could be detected following extragalactic flares with upcoming missions such as ULTRASAT and UVEX.