New study reveals possible formation of massive neutron stars from stellar collision

neutron stars
Artist’s impression of two tiny but very dense neutron stars at the point at which they merge and explode as a gamma-ray burst.
Credit: University of Warwick/Mark Garlick

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.

First author Monash School of Physics and Astronomy, and OzGrav PhD student Nikhil Sarin said: “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,” Nikhil said.


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Background

The ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) is funded by the Australian Government through the Australian Research Council Centres of Excellence funding scheme. OzGrav is a partnership between Swinburne University of Technology (host of OzGrav headquarters), the Australian National University, Monash University, University of Adelaide, University of Melbourne, and University of Western Australia, along with other collaborating organisations in Australia and overseas.

LIGO is funded by NSF and operated by Caltech and MIT, which conceived of LIGO and led the Initial and Advanced LIGO projects. Financial support for the Advanced LIGO project was led by the NSF with Germany (Max Planck Society), the U.K. (Science and Technology Facilities Council) and Australia (Australian Research Council-OzGrav) making significant commitments and contributions to the project. Nearly 1300 scientists from around the world participate in the effort through the LIGO Scientific Collaboration, which includes the GEO Collaboration. A list of additional partners is available at https://my.ligo.org/census.php.

The Virgo Collaboration is currently composed of approximately 350 scientists, engineers, and technicians from about 70 institutes from Belgium, France, Germany, Hungary, Italy, the Netherlands, Poland, and Spain. The European Gravitational Observatory (EGO) hosts the Virgo detector near Pisa in Italy and is funded by Centre National de la Recherche Scientifique (CNRS) in France, the Istituto Nazionale di Fisica Nucleare (INFN) in Italy, and Nikhef in the Netherlands. A list of the Virgo Collaboration members can be found at http://public.virgo-gw.eu/the-virgo-collaboration/.

The Kamioka Gravitational Wave Detector (KAGRA), formerly the Large Scale Cryogenic Gravitational Wave Telescope (LCGT), is a project of the gravitational wave studies group at the Institute for Cosmic Ray Research (ICRR) of the University of Tokyo. It will be the world's first gravitational wave observatory in Asia, built underground, and whose detector uses cryogenic mirrors. The design calls for an operational sensitivity equal to, or greater, than LIGO. The project is led by Nobelist Takaaki Kajita who had a major role in getting the project funded and constructed.