New research suggests innovative method for analyzing dense star systems in the Universe

Artist depiction of supernova remnant artist: Pixabay

In a recently published study, a team of researchers led by the ARC Center of Excellence for Gravitational Wave Discovery (Ozygrave) at Monash University suggested an innovative method for analyzing gravitational waves from neutron star mergers. The two wires are distinguished by type (rather than mass), depending on how fast they are rotating.

Neutron stars are extremely dense stellar objects that form when giant stars explode and die in an explosion, their cores collapse, and protons and electrons form one remaining neutron star in another.

In 2017, the two neutron stars named GW170817 were merged for the first time by LIGO and Virgo gravity-wave detectors. This merger is well known because scientists could also see the light produced from it: high-energy gamma rays, visible light, and microwaves. Since then, an average of three scientific studies have been published on GW170817 every day.

In January this year, LIGO and Virgo Collaboration announced a second neutron star merger event called GW190425. Although no light was detected, this phenomenon is particularly intriguing because the two merged neutron stars are much heavier than GW170817, as well as the previously known double neutron stars in the Milky Way.

Scientists use the signals of gravitational waves in the fabric of space and time – to detect pairs of neutron stars and measure their mass. The pair’s heavy neutron star is called the ‘primary’; The lighter is a ‘secondary’.

Recycled-slow labeling scheme of binary neutron star systems

A binary neutron star system usually begins with two ordinary stars, each ten to twenty times larger than the Sun. When these massive stars age out and ‘fuel’ their lives, their lives end in supernova explosions, which leave behind compact remains, or neutron stars. The weight of each remnant neutron star is about 1.4 times the mass of the Sun, but it is only 25 kilometers in diameter.

First-born neutron stars usually undergo a ‘recycling’ process: it accumulates matter from its coupled star and starts moving faster. Second-born neutron stars do not accumulate matter; Its spin speed also slows down. By then, two neutron stars have dissolved – millions – billions of years later – it is estimated that A. Recycled The neutron star may still be spinning rapidly, while other non-recycled neutron stars may be rotating Slowly.

Another method may be the binary neutron star system through a continuous change in dense stellar groups. In this scenario, two unrelated neutron stars, on their own or in other different star systems, meet each other, pair and eventually merge as a happy couple due to their gravitational waves. However, current modeling of stellar clusters suggests that this scenario is ineffective in the merger of neutron stars.

ग्राjagra postdoctoral researcher and lead author of the study, Jingjiang Zhu, says: ‘The motivation of the proposal for a recycled-slow labeling scheme of binary neutron star systems is two-fold. First, it is a common feature expected for neutron star mergers. Second, it may be insufficient to label two neutron stars as primary and secondary because they are likely to be of similar mass and it is difficult to tell which is heavier. ”

A recent Ozygrove study gives both GC170817 and GW190425 a facelift by adopting the recycled-slow scheme. It was found that the recycled neutron star in GW170817 is only spinning lightly or slowly, while GW190425 is spinning rapidly, possibly once every 15 milliseconds. It was also found that both merger events are likely to have two equal-mass neutron stars. Since there is little or no evidence of spin in GW170817, and neutron stars rotate over time, researchers assumed that it took billions of years to merge binaries. This agrees well with the observations of its host galaxy called NGC 4993, where asteroid formation activities have been found over the past billion years.

Gregory Ashton, Ozgrave’s associate investigator and collaborator, says: “Our proposed astrophysics will allow us to answer important questions about the universe, such as the various supernova explosion mechanisms in the formation of binary neutron stars? And what inside dense stars To what extent do interactions occur? Cluster neutrons contribute to forming star mergers? ”

LIGO / Virgo detectors completed their joint third observation run (O3) earlier this year and are currently conducting scheduled maintenance and upgrades. When the fourth run (O4) begins in 2021, scientists will easily anticipate more discoveries of neutron star mergers. The possibility will become even brighter when Japanese underground detectors KAGRA and LIGO-India detectors join the global network in the coming years.

“We are in a golden era of studying binary neutron stars with hyper-sensitive gravity-wave detectors that will deliver dozens of discoveries over the next few years,” Zhu says.

Scientists make puzzles on massive star systems

more information:
Jing-Jiang Zhu et al. Characterization of astrophysical binary neutron stars with gravitational waves. The Astrophysical Journal (2020). DOI: 10.3847 / 2041-8213 / abb6ea

Provided by ARC Center of Excellence for Gravitational Wave Discovery

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