The visibility of double neutron star binaries to LISA

Abstract

Double neutron star binaries (DNSBs) in the mHz gravitational wave (GW) regime are an important group of objects which the GW detector LISA will be able to observe. The detection and parametrization of these objects will help develop our current understanding of neutron star formation and evolution. In this thesis, we synthesize a group of DNSBs using data from Church et al. (2011) and normalize the population to observed data of DNSB merger rates (Belczynski et al. 2018; Pol et al. 2019). The DNSB’s orbits are evolved in order to determine if their GW’s frequency is within the LISA band at present time. The binaries that are not within the LISA band are subsequently removed. Initial positions are then assigned to the DNSBs using a model for the stellar density in the galactic thin disk (McMillan, 2017), as well as initial birth times on the assumption that the star formation rate is constant to present time and begins at −10 Gyr from present day. We integrate their local and galactic orbits using a simple galactic potential (Repetto et al., 2012), as well as the separation evolution of a binary orbit (Peters, 1964), until present time. We calculate the strain of the DNSB’s GWs (Kupfer et al., 2018) and convert to both power spectral density and characteristic strain (Moore et al., 2014). The LISA sensitivity curve (Amaro-Seoane et al., 2017), including the background contribution of galactic binaries (GBs), is then compared to the characteristic strain of the DNSB population which shows that 265 DNSBs will be visible (SNR> 1) to LISA after a nominal mission time of 4 years. We show that 42 of these systems will also be resolvable (SNR> 5) to LISA. If the mission is extended to 10 years, the number of resolvable systems goes up to 79. We also show that these resolvable DNSB systems will be mostly clustered around the Galactic Center (GC) and our solar system, with the furthest DNSBs not exceeding 20 kpc from LISA.

Popular Abstract

Nearly all celestial objects ever discovered have been detected by observing the light they produce or reflect. If an object in space is emitting light at a bright enough rate, then we should be able to detect them with our telescopes. However, these luminous objects are not the only things roaming around in the vastness of space. Most objects, be it planets or asteroids or even stars, are either not bright enough to be seen or don’t emit light all together. It’s like trying to tell what’s around you in a freezing, pitch black room: impossible (and somewhat terrifying.) This “seeing” problem is one which plagues the astronomical community and keeps us in the dark on many objects in our Universe. There is, however, another way to detect objects in space: gravitational waves!

Gravitational waves are ripples in space and are caused when massive objects get accelerated. These waves are amazing because, as they travel through space, they lose power at a much slower rate than light. This allows us to use these gravitational waves to “see” much further into the universe than we could before. Gravitational waves are detected with detectors that use incredibly precise lasers to measure the compression of space. Two of these detectors, LIGO and Virgo, were able to detect gravitational waves for the very first time back in 2015. Since then, over 50 detections have been made with most of them originating from merging black holes, as well as a few merging neutron stars.

Neutron stars are the focus of this thesis, specifically two neutron stars orbiting around each. The gravitational waves they release causes the neutron stars to spiral into each other. Neutron stars are created when immensely massive stars collapse violently into what is called a supernova. These stars are around the mass of our Sun, but much smaller. If the Sun was the size of the earth, then a neutron star would be size of a single football field --- now THAT’S dense! It is thought that a system made up of two neutron stars will be detectable, through their gravitational waves, to a future detector called LISA.

LISA is interesting since it will be the first gravitational wave detector in space and will be able to detect two-neutron star systems millions of years before they merge. LISA will be able to vastly increase our insight into the way these neutron stars are formed. This thesis delves into the positions and characteristics of these ``double neutron star binary'' systems in our Galaxy which will be detectable to LISA. We take several initial conditions of neutron stars back when they were born such as: position in the Galaxy, distance between the two stars, and the stars' velocity. Using a variety of models, we can figure out what their conditions are at the present day. Finally, we compare these present conditions to how they would look to LISA.

• author: Storck, Anatole
• supervisor: Ross Church
• organization: Lund Observatory - Undergoing reorganization
• course: ASTK02 20211
• year: 2021
• type: M2 - Bachelor Degree
• subject: Physics and Astronomy
• keywords: gravitational waves, LISA, neutron stars, orbital simulations, Runge-Kutta method
• publication/series: Lund Observatory Examensarbeten
• report number: 2021-EXA176
• language: English
• id: 9049959
• date added to LUP: 2021-06-16 12:32:29
• date last changed: 2021-06-16 12:32:29