Discovery of a neutron star merging with a mass-gap black hole!
GW230529 Discovery Paper.
The LIGO Scientific, Virgo, and KAGRA collaboration discovered a spectacular signal during its fourth observing run, dubbed GW230529. The compact binary system that created this signal was likely a neutron star merging with a ‘mass-gap’ black hole; the black hole had a mass greater than the heaviest neutron stars and less than the lightest black holes in the Milky Way. I was fortunate to co-lead the discovery paper on behalf of the collaboration!
Before we started observing the universe in gravitational waves, the properties of compact objects like black holes and neutron stars were indirectly inferred from electromagnetic observations of systems in our Milky Way. The idea of a gap between neutron-star and black-hole masses, an idea that has been around for a quarter of a century, was driven by such electromagnetic observations. GW230529 is an exciting discovery because it hints at this ‘mass gap’ being less empty than astronomers previously thought, which has implications for the supernova explosions that form compact objects and for the potential light shows that ensue when a black hole rips apart a neutron star.