A system consisting of two neutron stars is called a binary neutron star or a double neutron star system. Numerous of these systems exist and have been observed through electromagnetic observations. Among the best well-known binary neutron star system is the Hulse-Taylor Pulsar. The observation of the increase in the orbital frequency of the Hulse-Taylor Pulsar has been the first indirect evidence for the emission of gravitational waves. In 2017 it was for the first time possible to detect directly gravitational waves emitted from the merger of two neutron stars, the system GW170817. In addition to the emission of gravitational waves, neutron star mergers are also bright sources of electromagnetic counterparts. In particular, the ejected material that can trigger a kilonova or the remnant system, which consists of a central compact object and an accretion disk.

Stages of the Binary Neutron Star Coalescence:

The coalescence consists of numerous stages as shown in the previous image (credit: T. Dietrich et al., Gen.Rel.Grav. 53 (2021) 3, 27)

1. Inspiral: Due to the emission of gravitational waves, the system `loses' angular momentum such that the two stars approach each other. To leading order, the emitted gravitational wave system depends on the quadrupole moment of the system and can be computed following the quadruple formula:
2. Merger: Typically, the merger is defined as the time when the gravitational-wave amplitude shows its maximum. Interestingly this is after the outer layers of the stars come already into contact.
3. Postmerger: After the merger, there are numerous possibilities on how the system can evolve. If the system is massive enough, it is possible that a black hole is directly formed, otherwise, there is the possibility of forming a HMNS (hypermassive neutron star), a SMNS (supramassive neutron star), or a MNS (massive neutron star) as shown in the image below (credit T.Dietrich et al, Gen.Rel.Grav. 53 (2021) 3, 27).

For an overview about the binary neutron star coalescence and a visualization of a numerical-relativity simulation mimicking the binary neutron star merger GW170817, please see this video.