Astronomers Detected a Black Hole-Neutron Star Merger, and Then Another Just 10 Days Later

The interior of a neutron star is perhaps the strangest state of matter in the universe. The material is squeezed so tightly that atoms collapse into a sea of nuclear material. We still aren’t sure whether nucleons maintain their integrity in this state, or whether they dissolve into quark matter. To really understand neutron star matter we need to pull it apart to see how it works and to do that takes a black hole. This is why astronomers are excited about the recent discovery of not one, but two mergers between a neutron star and a black hole.

The behavior of a material is governed by its equation of state. For neutron stars, this equation of state is the Tolman-Oppenheimer-Volkoff (TOV) equation. But without a better understanding of neutron star cores, its use is limited. For example, the best TOV calculation we have puts an upper limit on neutron star mass at about 2.16 solar masses, but the limit could be as high as 2.6 solar masses. To make the TOV equation more accurate, we need to understand whether quark matter forms in the core of a neutron star or even if extreme neutron stars become quark stars.

Our best chance of learning this comes from the observations of neutron stars colliding with black holes. When two black holes collide, they don’t emit any light directly, only gravitational waves. When a neutron star collides with a black hole, only the neutron star matter emits light as the star is ripped apart. By combining optical and gravitational wave observations of such a merger, we can better understand neutron stars.

In January of 2020 astronomers detected two gravitational wave events, named GW200105 and GW200115. The first was a merger of a 9 solar mass body with a 1.9 solar mass body, while the second was a merger of a 6 solar mass body with a 1.5 solar mass body. The smaller mass in both of these cases is too large to be a white dwarf, but well under the mass limit for neutron stars. Thus, they are the first confirmed black-hole/neutron-star mergers. This is a big deal and will unlock a deeper understanding of neutron stars.

Unfortunately, when astronomers looked for optical events to match the gravitational ones, they didn’t find any. So it isn’t possible to combine optical and gravitational data for these mergers. But the team was able to calculate the odds of finding similar mergers in the future. If the two consecutive events weren’t a rare fluke, then we can expect to see about 50 events per year.

The next observation run for LIGO and Virgo will be in the Summer of 2022. If we’re lucky, it should give us our first detailed view of the insides of a neutron star.

Reference: R. Abbott, et al. “Observation of Gravitational Waves from Two Neutron Star–Black Hole Coalescences.” The Astrophysical Journal Letters 915.1 (2021): L5.