FIG. 1: Rapidly rotating neutron stars may be “humming” continuous gravitational waves. Credit: K. Wette. 

Neutron stars are the ultra-dense, collapsed cores of exploded stars. They are of particular importance as sources of gravitational waves; two neutron stars orbiting each other will eventually collide and merge, emitting a short “chirp”-like gravitational wave signal. A single neutron star may also radiate gravitational waves; if the neutron star is rotating rapidly, and not perfectly spherical about its rotation axis, it will emit a faint “hum” – a continuous gravitational wave signal (Fig. 1). 

Continuous gravitational waves have not yet been detected. They are more difficult to find than gravitational waves from binary neutron stars (or binary black holes), for a few reasons. Because neutron stars are very dense, it’s hard to squeeze them into a non-spherical shape that emits continuous gravitational waves, therefore the “hum” is expected to be very quiet, and thus very difficult to find. We need to “listen” for continuous gravitational waves for a long time – in other words, we need to analyse lots of data from the LIGO and Virgo detectors to be sensitive to these quiet signals. 

FIG. 2: The frequency of continuous gravitational waves changes over different timescales. Credit: K. Wette. 

A further difficulty is that the “hum” of continuous gravitational waves doesn’t stay at the same pitch, or frequency, but changes over different timescales (Fig. 2). Over the course of a day (blue curve), the LIGO/Virgo gravitational wave detectors are rotating with the Earth, which means that they are sometimes moving towards, and sometimes moving away from, the far-away neutron star. This means that the continuous gravitational waves will be Doppler modulated, and the frequency of the “hum” will be changing with a period of a day. (Think of how the sound of an ambulance siren changes when it is moving towards you, versus when it is moving away.) Similarly, over the course of a year (red curve), the orbit of the Earth around the Sun also causes Doppler modulation, and hence a change in the gravitational wave frequency, with a period of a year. Finally, as the neutron star radiates continuous gravitational waves, it will lose energy, and therefore spin more slowly – which means that, over many years, the gravitational wave frequency will also decrease (green curve). 

The Centre for Gravitational Astrophysics is actively involved in searches for continuous gravitational waves. We pioneered the first search for continuous gravitational waves targeting young neutron stars in supernova remnants [1,2]. By studying the metric – a concept borrowed from general relativity – on the signal parameter space [3], we have developed sophisticated data analysis algorithms [4] which let us tune in to the exact frequency of continuous gravitational waves. Taking a complementary approach, we have also developed robust algorithms [5,6] which can track all the variations of the continuous gravitational wave frequency – which may include effects that cannot be easily modelled. We are using these algorithms to analyse the latest gravitational wave data from LIGO and Virgo, and will be listening out for a faint “hum”... 

[1] Wette et al., Class. Quant. Grav. 25, 235011 (2008) 

[2] Abadie et al. (LIGO Scientific Collaboration), Astrophys. J 722, 1504 (2010) 

[3] Wette & Prix, Phys. Rev. D 88, 123005 (2013) 

[4] Wette, Walsh, Prix & Papa, Phys. Rev. D 97, 123016 (2018) 

[5] Suvorova, Sun, Melatos, Moran & Evans, Phys. Rev. D 93, 123009 (2016) 

[6] Sun, Melatos, Suvorova, Moran & Evans, Phys. Rev. D 97, 043013 (2018) 

Contacts: Prof Susan Scott, susan.scott@anu.edu.au; Dr Lilli (Ling) Sun, ling.sun@anu.edu.au; Dr Karl Wette, karl.wette@anu.edu.au