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The Aftermath of Binary Neutron Star Mergers: What Remains Behind?

Schematic representation of binary neutron star merger outcomes. Panels A and B: Two neutron stars merge as the emission of gravitational waves drives them towards one another. C: If the remnant mass is above a certain mass, it immediately forms a black hole. D: Alternatively, it forms a quasistable ‘hypermassive’ neutron star. E: As the hypermassive star spins down and cools it can not support itself against gravitational collapse and collapses into a black hole. F, G: If the remnant’s mass is sufficiently low, it will survive for longer, as a ‘supramassive’ neutron star, supported against collapse through additional support against gravity through rotation, collapsing into a black hole once it loses this support. H: If the remnant is born with small enough mass, it will survive indefinitely as a neutron star. Schematic from Sarin & Lasky 2021. Credit: Carl Knox (Swinburne University)

On August 17th, 2017, LIGOThe Laser Interferometer Gravitational-Wave Observatory (LIGO) is a large-scale physics experiment and observatory supported by the National Science Foundation and operated by Caltech and MIT. It’s designed to detect cosmic gravitational waves and to develop gravitational-wave observations as an astronomical tool. It’s multi-kilometer-scale gravitational wave detectors use laser interferometry to measure the minute ripples in space-time caused by passing gravitational waves. It consists of two widely separated interferometers within the United States—one in Hanford, Washington and the other in Livingston, Louisiana.”>LIGO detected gravitational wavesGravitational waves are distortions or ripples in the fabric of space and time. They were first detected in 2015 by the Advanced LIGO detectors and are produced by catastrophic events such as colliding black holes, supernovae, or merging neutron stars.”>gravitational waves from the merger of two neutron stars. This merger radiated energy across the electromagnetic spectrum, light that we can still observe today. Neutron stars are incredibly dense objects with masses larger than our Sun confined to the size of a small city. These extreme conditions make some consider neutron stars the caviar of astrophysical objects, enabling researchers to study gravity and matter in conditions unlike any other in the Universe.

The momentous 2017 discovery connected several pieces of the puzzle on what happens during and after the merger. However, one piece remains elusive: What remains behind after the merger?

In a recent article published in General Relativity and Gravitation, Nikhil Sarin and Paul Lasky, two OzGrav researchers from Monash University, review our understanding of the aftermath of binary neutron starA neutron star is the collapsed core of a large (between 10 and 29 solar masses) star. Neutron stars are the smallest and densest stars known to exist. Though neutron stars typically have a radius on the order of just 10 – 20 kilometers (6 – 12 miles), they can have masses of about 1.3 – 2.5 that of the Sun.”>neutron star mergers. In particular, they examine the different outcomes and their observational signatures.

The fate of a remnant is dictated by the mass of the two merging neutron stars and the maximum mass a neutron star can support before it collapses to form a black holeA black hole is a place in space where the pull of gravity is so strong not even light can escape it. Astronomers classify black holes into three categories by size: miniature, stellar, and supermassive black holes. Miniature black holes could have a mass smaller than our Sun and supermassive black holes could have a mass equivalent to billions of our Sun.”>black hole. This mass threshold is currently unknown and depends on how nuclear matter behaves in these extreme conditions. If the remnant’s mass is smaller than this mass threshold, then the remnant is a neutron star that will live indefinitely, producing electromagnetic and gravitational-wave radiation. However, if the remnant is more massive than the maximum mass threshold, there are two possibilities: if the remnant mass is up to 20% more than the maximum mass threshold, it survives as a neutron star for hundreds to thousands of seconds before collapsing into a black hole. Heavier remnants will survive less than a second before collapsing to form black holes.

Observations of other neutron stars in our Galaxy and several constraints on the behavior of nuclear matter suggest that the maximum mass threshold for a neutron star to avoid collapsing into a black hole is likely around 2.3 times the mass of our Sun. If correct, this threshold implies that many binary neutron star mergers go on to form more massive neutron star remnants which survive for at least some time. Understanding how these objects behave and evolve will provide a myriad of insights into the behavior of nuclear matter and the afterlives of stars more massive than our Sun.

Reference: “The evolution of binary neutron star post-merger remnants: a review” by Nikhil Sarin and Paul D. Lasky, June 2021, General Relativity and Gravitation.
DOI: 10.1007/s10714-021-02831-1

Written by PhD student Nikhil Sarin, University of Adelaide

Source: SciTechDaily