A Piecewise Model for Gravitational Waves from Young Neutron Stars

Abstract

To date over 90 gravitational wave detections have been made. These events have all originated from the coalescence of a binary system of compact objects, the majority of which have been binary black hole systems. Some of these detections have been from systems where one object was a neutron star and the other a black hole. Only two detections have been made where both objects in the binary system were neutron stars.

The gravitational wave event GW170817 originated from the coalescence of a binary neutron star system. It was significant in that it was the first event of this origin, and was paired with an electromagnetic counterpart. Observations of GW170817 have led to new insights into previously unknown physics, such as the neutron star equation of state, formation of heavy elements and the origins of gamma ray bursts.

Despite the significance of this event, little is known about the remnant object left after the collision. One possible object which may have formed after a coalescence event is a rapidly spinning neutron star. This neutron star may be long lived or have a finite lifespan before collapsing into a black hole.

Continuous wave search techniques are designed to target spinning neutron stars as one of their primary sources. These techniques use a matched filtering process with a Taylor expansion signal model. Newly born neutron stars from a coalescence event are expected to be spinning down rapidly over a short time period. For such rapid spin downs, and for a long-transient time scale, Taylor expansions are not suitable signal models.

In this thesis I present a new piecewise model which can accurately follow the frequency evolution of a young neutron star. The parameter space of this model is composed of the frequency, and derivatives in time of frequency, of a neutron star at specific points in time. This model has the flexibility to model spin downs which are outside the scope of Taylor expansions, by being able to model changing braking indices and other neutron star properties.

We use the piecewise model to carry out two $\mathcal{F}$-statistic searches over the 1800~s of data from the LIGO and Virgo gravitational wave observatories following the coalescences of GW170817 and GW190425. No signal was found. We perform sensitivity studies for both searches. Peak sensitivities are achieved for both searches at a starting frequency of 1700~Hz of $1.64 \times 10^{-22}~\text{Hz}^{-1/2}$ for GW170817 and $10^{-22}~\text{Hz}^{-1/2}$ for GW190425. Show more