Merging Black Holes and Neutron Stars

'Compact binaries’ - stellar binary systems consisting of compact objects (neutron stars or black holes) - were elusive to astronomers until the Hulse-Taylor binary (a pulsar-neutron star system) was discovered in 1975. While over twenty more radio pulsar-neutron star systems have been detected since then, the first binary black hole system was detected at its merger through gravitational waves by Advanced LIGO in 2015. Since then,  ~O(100) confirmed double black holes, two double neutron stars, and three neutron star-black hole systems have been observed by LIGO/Virgo, dominating the dataset of current gravitational wave detectors.

Predicted by theoretical astrophysicists for a long time, these compact binaries provide a wealth of information on various topics: stellar evolution, binary physics, alternative theories of gravity, magnetohydrodynamics, and, more recently, even the expansion of the universe!

I will guide you through the story of compact binaries, from their birth and evolution to their mergers. Specifically focussing on their masses and spin signatures, I will show how uncertainties in theoretical stellar binary physics can be constrained and how the details of their birth environments, can be uncovered by comparing aspects of simulations with combined radio and gravitational wave observations. Furthermore, I will discuss how these binary black hole mergers can reveal the long-sought link between stellar-mass black holes and supermassive black holes.

With more sophisticated detectors being added in the near future, compact binaries are likely to become a dominant source of multi-messenger astronomy, revealing even more secrets of the observable universe.