The interior of a neutron star is perhaps the strangest state of matter in the universe. The material is compressed so tightly that atoms collapse into a sea of nuclear material. We are still not sure whether nucleons will retain their integrity in this state or whether they will dissolve into quark matter. To really understand the matter of neutron stars, we have to pull them apart to see how they work, and that takes a black hole. For this reason, astronomers are delighted with the recent discovery of not one, but two mergers between a neutron star and a black hole.
The behavior of a material is determined by its equation of state. For neutron stars, this equation of state is the Tolman-Oppenheimer-Volkoff equation (TOV). But without a better understanding of neutron star nuclei, its uses are limited. For example, the best TOV computation we have puts an upper limit on neutron star mass at around 2.16 solar masses, but the limit could be up to 2.6 solar masses. To make the TOV equation more accurate, we need to understand whether quark matter forms in the nucleus of a neutron star or whether extreme neutron stars become quark stars.
Observed mergers with the highlighted neutron star events. Source: LIGO-Virgo / Frank Elavsky, Aaron Geller / Northwestern University
Our best chance to learn this comes from observing neutron stars colliding with black holes. When two black holes collide, they don’t emit light directly, just gravitational waves. When a neutron star collides with a black hole, only the neutron star matter emits light when the star is torn apart. By combining optical and gravitational wave observations of such a merger, we can better understand neutron stars.
In January 2020, astronomers discovered two gravitational wave events named GW200105 and GW200115. The first was a fusion of a body of 9 solar masses with a body of 1.9 solar masses, while the second was a fusion of a body of 6 solar masses with a body of 1.5 solar masses. The smaller mass is in both cases too large to be a white dwarf, but well below the mass limit for neutron stars. This makes them the first confirmed mergers of black holes and neutron stars. This is a big deal and will allow a deeper understanding of neutron stars.
Unfortunately, when astronomers searched for optical events that matched the gravitational events, they didn’t find any. So it is not possible to combine optical and gravitational data for these mergers. But the team was able to calculate the likelihood of finding similar mergers in the future. If the two successive events were not a rare coincidence, then we can expect around 50 events per year.
The next observation run for LIGO and Virgo will take place in summer 2022. If we’re lucky, it should give us a first detailed look inside 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.