Investigating the lives of double neutron stars

Scientists have been seeing the Universe through electromagnetic observations, and since 2015 we are listening to it too -- using gravitational waves. With a growing catalogue of gravitational-wave events under our belt as interferometers have advanced, we can now start to study the population properties of compact objects.

By studying populations of neutron stars, we can learn more about how they formed and evolved - but why do we care about neutron stars? Well, if you asked me, I would simply say, because they are cool! From a more scientific standpoint, one of the key questions in physics is understanding how matter acts under the extreme conditions of temperature and pressure. These conditions are not possible to replicate on Earth, hence observations of astrophysical sources with these conditions need to be studied; and a neutron star is a perfect candidate to study matter under these extreme conditions. 

They are what I like to call ‘high energy physics labs in space’. The structure of a neutron star depends on the nuclear Equation of State, and to constrain this equation we need precise measurements on the masses and radii of these neutron stars and the maximum mass of neutron stars. By analysing the population we can probe the upper end of this mass distribution to help constrain this upper mass limit. 

Neutron stars are extremely dense objects that are formed after the explosive deaths of massive stars. These objects have masses greater than our Sun but with radii, the size of a city (~10km) in fact, just a teaspoon of neutron star matter would be like trying to fit Mount Everest into a usual cup of morning tea. 

So what could be better than a neutron star? Two neutron stars in a binary! 

There are thought to be two main ways in which you can form a binary neutron star system: either through an isolated process or via dynamical interactions. The isolated scenario is thought to be the most common way to form these binaries, which involves a binary system of stars collapsing through supernova explosions, one after the other, both becoming neutron stars. 

In the dynamical case, this requires an environment where neutron stars are densely populated, allowing for many stellar interactions. In such cases, you can have a binary system with a single neutron star swap its companion for another neutron star.

Within these channels, several factors influence the formation and evolution of the double neutron star system. The key thing that determines whether a binary will go on to merge is how wide the orbit is; the wider the orbit, the longer the system takes to merge. 

Many researchers are actively working to understand these processes - they do so by modelling populations of binaries and evolving them through time in simulations to see what kind of double neutron star systems they produce. There are still a lot of uncertainties surrounding the underlying physics that are used in these models but these studies have already provided some interesting outcomes.

To date, from all the gravitational-wave observations made, only two binary neutron stars have been detected: GW170817 (which contained a 1.36 - 2.26 solar mass and 0.86 - 1.36 solar mass neutron star) and GW190425 (which contained a 1.60 - 2.52 solar mass and 1.12 - 1.69 solar mass neutron star). However, many more of these binary systems have been observed in radio astronomy. 

By combining radio and gravitational waves observations (so, we study these objects in two different spectrums) we can get a better picture of the population – but this can be tricky. The two populations at a glance do not seem to be the same. In fact, GW190425, the most massive binary neutron merger to date with a total mass of 3.4 times that of our Sun, is not typical of double neutron stars we have observed in the Milky Way. 

So how do we combine these observations?

The key is to understand how these observations are connected. When you observe double neutron stars in radio, you are seeing the mid-life stage of their systems' evolution. With gravitational waves, we witness the final moments of these systems. What connects these observations is the birth population of double neutron stars.

The time between the birth and the merger is important to consider - this is called the delay-time of the system. If the delay-time is short, we can expect to see these systems merge and hence visible in the gravitational-wave population. Some binaries will not be visible in gravitational waves since they will take longer than the age of the Universe to merge.

One of the questions we continue to investigate: Why are there a lack of heavy double neutron stars in radio observations? We use the hypothesis that these heavy double neutron stars systems are fast merging systems, resulting in short delay-times. This would mean that these systems would merge too quickly to be observed in radio.

Let’s consider the standard formation scenario where a double neutron star consists of two neutron stars: a first-born “recycled” neutron star sped up from accretion and a second-born “slow” neutron star. A possible scenario in which a binary can be fast-merging is via a process called “unstable Case BB mass transfer”. Previous studies have considered this channel as the formation mechanism for GW190425. 

In this scenario, we have a neutron star and a helium star with a carbon-oxygen core. Following unstable mass transfer in the common envelope phase, we have a tight binary of neutron stars with orbital periods of less than one hour. This tightened binary becomes a fast-merging system.

In our work, we consider systems with heavy slow mass neutron stars to form fast-merging systems via unstable Case BB mass transfer. Using this assumption we can extract the birth distribution for these slow mass neutron stars.

We find mild support for the fast-merging hypothesis - suggesting we may not need a fast-merging scenario to explain the lack of heavy double neutron stars in our radio populations. This may mean that systems like GW190425 are considered rare. 

With future gravitational wave observations, we can expect to have a larger population of double neutron star mergers to work with. Assuming that future radio and gravitational observations of these systems reveal distinct populations, our model provides a natural explanation for why such massive double neutron stars are not observed in radio.