The heaviest neutron star, or the lightest black hole?

Gravitational wave detection brings us a new way to explore the universe. The waves may contain information from massive objects such as black holes, or tiny objects such as neutrons, protons, or even quarks inside neutron stars. Recently, the Laser Interferometer Gravitational Observatory (LIGO)/Virgo Collaboration measured the gravitational wave  signal from the coalescence of a black hole and an unknown compact object, known as the GW190814 event, raising a debate within the community about the true identity of the secondary compact object.

In their GW190814 discovery paper, the LIGO/Virgo Collaboration concludes that a black hole is a more favorable candidate than a neutron star for the unknown compact object. However, in a paper published in Physical Review Letters on December 30, 2020, we demonstrate that the secondary object could have been a neutron star. 

In most cases, scientists have reliable ways to identify how compact a celestial objects is, but those methods don’t apply in the case of GW190814. The first method scientists use is simply looking at the mass of the object. The Ligo/Virgo Collaboration showed that one of the compact objects that made GW190814 had to be 2.5 - 2.67 solar masses, which falls into the mass gap of ~2.5 - 5 solar masses between known neutron stars and black holes. Therefore, we cannot tell whether the object was a neutron star or a black holes based on its mass alone. 

Another way to identify a compact object is by measuring its tidal deformability. When in a binary system, a neutron star can be deformed by the strong tidal force produced by its companion, but a black hole cannot. This deformability is a vital clue encoded in the gravitational wave signal. However, if the companion is a very massive black hole, then even a neutron star would not be significantly deformed. In the case of GW190814, the black hole is about ten times heavier than the unknown compact object, which implies that the tidal force is not very strong. Therefore, the fact that we cannot measure the tidal deformability of the secondary object from the gravitational wave signal may be because the companion black hole was too heavy. 

A third method to identify a compact object is to observe an electromagnetic signal produced by the same binary that made the gravitational waves. When such an observation occurs, we can deduce that there had to be a neutron star in the binary, because black hole binaries of this mass are not expected to produce electromagnetic radiation. Unfortunately, there was not an electromagnetic counterpart in the detection of GW190814 to provide additional evidence of a neutron star. This could be either because the unknown object was not a neutron star, or simply because the binary was too far away to produce radiation detectable from Earth. 

In the GW190814 event, the signal did not contain enough information to measure the tidal deformability, and there was not an electromagnetic counterpart. The mass of the object was slightly higher than the expected mass of a neutron star. Data from previous observations and numerical simulations suggest that the maximum mass of a neutron star should be no more than 2.3 solar masses. The LIGO/Virgo Collaboration relied on these arguments to draw their conclusions about the identity of the secondary object in GW190814.

However, the simulation used by the LIGO/Virgo Collaboration relies on the assumption that the equation of state is smooth and slowly-changing, i.e. that the star’s internal pressure changes continuously and slowly with the star’s density. The equation of state for a neutron star describes how its internal pressure depends on its energy density and temperature, which in turn depends on the star’s composition. Because the densities and pressures inside a neutron star are so high, no one knows the equation of state or the internal composition of a neutron star. One of the most burning questions in the field is whether the core of a neutron star is filled with protons and neutrons or with something more exotic like quarks. Therefore, strong assumptions about the equation of state, like those made in simulations, can be questioned.

In our paper, we point out that the assumptions made by the LIGO/Virgo Collaboration about the equation of state may have been too strong. Neutron stars with quarks inside their cores can have sound wave velocities that change rapidly as one approaches the center of the star. If so, this violates the assumption that the equation of state changes slowly with density. We show that when one relaxes this assumption and includes a rapid change in the velocity of sound waves, neutron stars can have masses higher than 2.5 solar masses. Therefore, we argue that the second object detected in GW190814 could have been a neutron star, potentially with quarks inside its core.