17th August 2017: a date marked down in the history books—the day the LIGO/Virgo collaboration made the first detection of gravitational waves from the death spiral of two neutron stars. Just 1.7 seconds later, astronomers observed a short burst of high-energy gamma rays known as a gamma-ray burst (GRB). Global efforts by thousands of astronomers later identified the host galaxy and a supernova-like thermal transient called a kilonova. This event gave astronomers insight into several fundamental and important questions, including an unprecedented understanding of where gold and other heavy elements are produced in the Universe, as well as our best measurement of the speed of gravity. Among other things, it confirmed that neutron star mergers originate from short-duration GRBs. Despite the numerous observations, an important question remains unanswered. What was the outcome of this merger?
Typically, one expects the merger of two neutron stars to immediately produce a black hole—an object so dense, that light itself cannot escape; however, observations of other GRBs show evidence for the immediate formation of a massive, rapidly-spinning neutron star. Such merger remnants, if they exist, have important implications for the physical composition of neutron stars.
Neutron stars are the only place in the Universe where we can study the behaviour of matter at temperatures up to 100 billion times hotter than on Earth and densities greater than an atomic nucleus—these conditions could never be reproduced on Earth. Nikhil Sarin, Paul Lasky, and Gregory Ashton—three researchers from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) at Monash University—recently published a study analysing all short-duration GRBs observed by NASA’s Neil Gehrels Swift Satellite. Out of 72 GRBs analysed, 18 show evidence for the immediate formation of a massive neutron star which later collapses into a black hole. Combining information from all 18 observations, the team were able to accurately describe the physical composition of these neutron stars.
The results indicate that these neutron stars are consistent with having a freely-moving ‘quark’ composition and a composition like regular matter, i.e. composed of atomic nuclei—the building blocks of the Universe. Quarks are elementary particles that contain protons, neutrons and atomic nuclei. In regular matter, these quarks are confined inside protons and neutrons, but in the high density and high-temperature regimes seen in neutron stars, they may move around freely. Scientists must first determine the temperature and density of neutron stars to understand the movement and behaviour of quarks and matter.
OzGrav PhD student and first author Nikhil Sarin says: ‘Our observations show a slight preference for freely-moving quarks. We look forward to getting more observations to definitively solve this puzzle’.
The research also found that, before collapsing into black holes, most neutron stars produce faint gravitational waves which are not likely to be individually detected by LIGO.
‘With the construction of more sensitive gravitational-wave detectors, such as the Einstein Telescope in Europe and the Cosmic Explorer in the US, we’re confident that we’ll eventually detect individual gravitational waves from these systems,’ explains Sarin.