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. 

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. 

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]. We have developed data analysis algorithms [3–6] to consider various signal models of continuous gravitational waves. 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)