An intrepid team of astrophysicists predicted that gravitational waves from double neutron stars — crucial to our understanding of the lives and deaths of all stars — might be detected by LISA, a next-generation space telescope.
Gravitational Waves and Double Neutron Stars
The team — led by Mike Lau, a Ph.D. student at the ARC Centre of Excellence in Gravitational Wave Discovery (OzGrav) — presented their results at the 14th annual Australian National Institute for Theoretical Astrophysics (ANITA) science workshop 2020. The paper compares his team to paleontologists: "Like learning about a dinosaur from its fossil, we piece together the life of a binary star from their double neutron star fossils."
Neutron stars are the hot, extremely radioactive "corpses" of a gigantic star after undergoing a cataclysmic explosion called a supernova. A double neutron star is two neutron stars orbiting one another in one system, which disturbs the surrounding space-time, like cosmic waves jamming through the universe.
These ripples are called gravitational waves and have made headlines in the last several years — notably the 2015 detection of waves made by the LIGO/Virgo Collaboration. Gravitational waves like these happen when pairs of black holes spiral in too close together and merge into one.
This is incredible, but scientists have yet to find a way to measure the gravitational waves created when two black holes or neutron stars still have a relatively high orbital distance. Their waves are weaker, but they also contain crucial data on the lives of stars, and could even unveil the existence of entirely new phenomena in the Milky Way.
Bending space-time with binary neutron stars
The new study shows how the Interferometer Space Antenna (LISA) might one day record the gravitational waves from a pair of double neutron stars. LISA is a space telescope set to launch in 2034, and is a critical part of a larger mission that the European Space Agency (ESA) will lead. The space telescope is made of three satellites synced by lasers in a triangle, orbiting the Sun.
The gravitational waves will squeeze and stretch the 40 million-kilometer laser arms of LISA's triangle. Meanwhile, a highly-sensitive detector will monitor the slowly-oscillating waves — which LIGO and Virgo aren't currently capable of detecting.
The team used computer simulations to recreate the conditions for a set of double neutron stars to predict that in four short years of operation, LISA will have monitored the gravitational waves of dozens of double neutron stars in synchronous orbit. The team's findings were published in the Monthly Notices of the Royal Astronomical Society.
Supernova explosions kick the circular orbits of neutron stars they form into oval-shaped, elliptical orbits. Generally, gravitational wave emissions round off the orbit — which is what happened for double neutron stars previously detected by LIGO and Virgo. LISA, however, will detect double neutron stars when they're still at a great distance from one another, allowing astronomers to look at the initial oval orbit.
How elliptical the orbit is — which depends on the eccentricity of the orbit — will tell astronomers a lot about what kind of stars the two were before they became a system of double neutron stars. One of the most interesting epistemic objects will come from looking at the distance between them, which will show how strongly each was "kicked" by the supernova that created them.
The study of binary stars — stars born as a pair — is riddled with unknowns. This is why — once LISA is up and running in the 2030s — astronomers should expect to record the activity of double neutron stars, which will reveal the secrets of stars across the galaxy.